Silvia Flaminio1

Hervé Piégay1

Yves-François Le Lay1

Silvia Flaminio
email: silvia.flaminio@unil.ch

To dam or not to dam? A genealogy of media discourses to highlight human-river relationships in the Anthropocene

Keywords:

Numerous studies have highlighted the negative consequences of dams (long-term alterations of Earth systems and social and economic injustices), and have questioned the sustainability of dams, i.e., the capacity of dams to meet the social, economic, and environmental needs of the current generation without compromising the needs of future generations. After having stalled in the 1990s, dam construction has entered a new period of growth. Recent research has shown that this new phase in dam building can be explained by the promotion of hydropower as clean energy. To clarify the continuing debate on the construction of dams, particularly on their impacts and sustainability into the future, this paper examines the discourses on dams within the public domain. We used a collection of newspaper articles published in Le Monde from 1945 to 2019 on dams that were planned and built throughout the world (n=1,471). Results show a rise of different discourses on water infrastructure, which with time deal less with positive technical appraisals and more with controversies. Since the mid-2000s, new arguments are defended (green energy) and new controversial discourses on the social and economic sustainability of dams (bearability, equitability) have emerged along with environmental questions (greenhouse gas footprint). These discourses reveal different understandings of sustainability which increase the complexity of the debate on dams. Results from this study suggest a need for a stronger dialogue between the scientific and public domains on the impacts of dams. Knowledge transfer and exchange would be particularly beneficial on 'renewable energy' and 'green energy,' and alternative modes of governance regarding dams.

Introduction

During the past century, numerous dams have been built throughout the world [START_REF] Roe | Riverine Environments[END_REF], with purposes that include irrigation, hydropower, water supply, flood prevention and navigation. These hydraulic infrastructures interrupted the flow of water and created reservoirs upstream. Dam construction stalled during the 1990s, leading some to conclude that the era of big dams was over (e.g., [START_REF] Graf | Dam nation: A geographic census of American dams and their large-scale hydrologic impacts[END_REF][START_REF] Mccully | Silenced Rivers: The Ecology and Politics of Large Dams[END_REF]. This decline may be explainable by the reduction of attractive sites for dams, by changing water paradigms [START_REF] Gleick | A Look at Twenty-first Century Water Resources Development[END_REF] and by the progressive adoption by the water sector of the principles of sustainable development [START_REF] Allan | IWRM/IWRAM: a new sanctioned discourse[END_REF], whereby the needs of the current generation are met without compromising the needs of future generations (Brundtland, 1987, p. 37).

Nevertheless, since the end of the 2000s, dam construction has entered a new period of growth.

Plans for 'mega-dams' [START_REF] Best | Anthropogenic stresses on the world's big rivers[END_REF] and for smaller dams [START_REF] Fung | Mapping the social impacts of small dams: The case of Thailand's Ing River basin[END_REF] have multiplied, coupled with schemes for hydropower. Currently, 3,700 major hydropower dams are planned or under construction in the world (Zarfl et al., 2015). Even so, the amount of power produced, given the projected growth demand, is likely insufficient to replace non-renewable electricity resources (Zarfl et al., 2015, p. 167). Such a rebound in dam-building raises environmental questions that are both social and ecological. It feeds the scientific debate on long-term environmental degradation in the context of global environmental change, especially within an anthropocene era of accelerated human interactions with Earth systems [START_REF] Chin | Anthropocene: human interactions with earth systems[END_REF]. The consequences of constructing dams remain diverse (e.g., social, economic, ecological, and geomorphological), at different scales (local, corridor, regional, and even global) [START_REF] Baghel | Discussing Large Dams in Asia After the World Commission on Dams: Is a Political Ecology Approach the Way Forward[END_REF][START_REF] Gross | A global megadam mania[END_REF][START_REF] Skalak | Large dams and alluvial rivers in the Anthropocene: The impacts of the Garrison and Oahe Dams on the Upper Missouri River[END_REF][START_REF] Williams | Humans as the third evolutionary stage of biosphere engineering of rivers[END_REF]World Commission on Dams, 2000). As numerous studies have demonstrated, dams have profoundly affected flow conditions, channel morphology and river ecosystems (e.g., [START_REF] Power | Dams and downstream aquatic biodiversity: potential food web consequences of hydrologic and geomorphic change[END_REF][START_REF] Poff | The natural flow regime: a paradigm for river conservation and restoration[END_REF][START_REF] Rollet | Assessment of consequences of sediment deficit on a gravel river bed downstream of dams in restoration perspectives: application of a multicriteria, hierarchical and spatially explicit diagnosis[END_REF]. They have even been described as an example of the "exploitation of Earth's resources" which characterises the "Anthropocene" [START_REF] Crutzen | [END_REF], and due to the drastic alteration of the world's river systems they have caused, dams have been depicted as characteristic of the human domination of the Earth [START_REF] Nüsser | The emergence of technological hydroscapes in the Anthropocene: Socio-hydrology and development paradigms of large dams[END_REF]. Recent research has shown that dams may not always be an advantageous option considering climate change. The reliance on reservoirs can worsen water and energy shortages [START_REF] Di Baldassarre | Water shortages worsened by reservoir effects[END_REF]. Plans for hydropower dams in Eastern and Southern Africa, for example, will increase some countries' dependence on hydroelectricity and therefore their exposure to the "risk of climate-induced electricity disruption" (Conway et al., 2017, p. 949). Moreover, not only can the resurgence of dam building lead to the disappearance of the last free-flowing rivers [START_REF] Grill | Mapping the world's free-flowing rivers[END_REF][START_REF] Nilsson | Fragmentation and Flow Regulation of the World's Large River Systems[END_REF], the impact of climate change will also be greater on impounded rivers than on free-flowing rivers, which are expected to be more resilient [START_REF] Palmer | Climate change and the world's river basins: anticipating management options[END_REF]. Furthermore, during the past years, restoration ecology and other environmental fields have promoted dam removal to prevent further biodiversity loss and 'reconnect' watersheds [START_REF] Magilligan | River restoration by dam removal: Enhancing connectivity at watershed scales. River restoration by dam removal[END_REF]. Among social scientists, debates regarding dam-building and the impact of dams are also strong, as they have shown how a new discourse on climate change and sustainability [START_REF] Crow-Miller | The (re) turn to infrastructure for water management? Water Alternatives 10[END_REF] is fuelling dambuilding for hydropower or multipurpose reservoirs. Some states and institutions are framing hydroelectricity, for example, as renewable, 'clean', 'green' energy [START_REF] Ahlers | Framing hydropower as green energy: assessing drivers, risks and tensions in the Eastern Himalayas[END_REF][START_REF] Fletcher | When environmental issues collide: climate change and the shifting political ecology of hydroelectric power[END_REF][START_REF] Warner | Old Wine in New Bottles: The Adaptive Capacity of the Hydraulic Mission in Ecuador[END_REF]. Altogether, scholars in different scientific fields are questioning the sustainability of dams [START_REF] Di Baldassarre | The legacy of large dams in the United States[END_REF].

To clarify the continuing debate on the construction of dams, particularly on their impacts and sustainability into the future, this paper examines the discourses on dams within the public domain.

We focus on the public due to the relevance of dams on people's lives, such as in relation to water resource management and energy production, and since the public often exerts political power for decisions concerning dam-building. We also focus on the public discourses on dams since the mid-20 th century, because this period includes the peak in dam building and the development of controversies regarding the negative impacts of dams [START_REF] Roe | Riverine Environments[END_REF]. In this paper, we therefore address the following research questions. First, how have the discourses and debates on dams, and notably on the sustainability of dams, developed within the public domain? Second, how can tracing a 'genealogy' of discourses on dams inform their sustainability into the future?

Approach, material and methods

General Approach

This paper approaches the research questions with a Foucauldian perspective. Foucault promoted a 'genealogical' approach that examines "how series of discourse are formed […]; what are their specific norms, and what are their conditions of emergence, growth and variation" and a critical approach that "distinguishes forms of exclusion, limitation and appropriation" (Foucault, 1971, p. 62-63). Accordingly, we investigate not only the genealogy of discourses relating to dams, but also the controversies at work during different periods. This post-structuralist framework invites us to question and highlight the emergence and the development of new discourses regarding dams. It also helps to reveal how older discourses may still be present and are adaptable to current issues.

We consider a discourse is made of oral or written statements (Adger et al., 2001). It is also "a shared way of apprehending the world. Embedded in language, it enables those who subscribe to it to interpret bits of information and tie them together into coherent stories or accounts" (Dryzek, 2012, p. 9). Following Foucault, previous studies on water and hydraulic infrastructure have illustrated the role and power of discourse, and notably its political ramifications (e.g., [START_REF] Duarte-Abadía | Hydropower, Encroachment and the Repatterning of Hydrosocial Territory: The Case of Hidrosogamoso in Colombia[END_REF][START_REF] Boelens | Hydrosocial territories: a political ecology perspective[END_REF][START_REF] Boelens | Contested Knowledges: Large Dams and Mega-Hydraulic Development[END_REF].

In a nutshell, the relevance of studying discourses is twofold. On one hand, the study of discourses allows documentation of the relationship between society and dams, because discourses materialise this relationship in oral and written statements. On the other hand, since discourses have a performative value, studying discourses also allows better understanding of how planning choices regarding dams are made, and more generally how "political decision-making takes place" (Hajer, 1995: 44). While studies in natural sciences have increasingly highlighted the ecological impacts of dams since the 1960s (e.g., Scope, 1972), social sciences have often focused on controversies related to dam-building and their social consequences (e.g., Kaika, 2006;[START_REF] Hommes | Contested hydrosocial territories and disputed water governance: Struggles and competing claims over the Ilisu Dam development in southeastern Turkey[END_REF].

Such research has focused on case studies situated in specific timeframes and socio-political contexts.

To consider more generally the public debate on dams and build a genealogy of discourses on dams, we used media as a primary source. Media capture the public domain well as places or forums in which individuals "engage each other about subjects of shared concern" (Pezzullo and Cox 2017, p. 20). Media and newspaper accounts also present many biases, as they personalise information, dramatise events, and tend toward novelty [START_REF] Boykoff | Climate change and journalistic norms: A case-study of US mass-media coverage[END_REF]. Nevertheless, media play a role in political agenda setting [START_REF] Downs | Up and down with ecology: The "issue attention cycle[END_REF] and reflect social concerns [START_REF] De Loë | Dam the news: Newspapers and the Oldman River Dam project in Alberta[END_REF]. They are arenas among which new economic and environmental models can be produced [START_REF] Burgess | The Production and Consumption of Environmental Meanings in the Mass Media: A Research Agenda for the 1990s[END_REF].

Previous studies have highlighted the 'centrality' [START_REF] Foxwell-Norton | Saving the Great Barrier Reef from disaster, media then and now[END_REF] of media in the communication of environmental issues and have recognised their value as a valid source to study environmental conflicts and as a stakeholder in environmental politics (e.g., [START_REF] Lester | Media and Environment: Conflict, Politics and the News[END_REF][START_REF] Hansen | News coverage of the environment: a longitudinal perspective[END_REF]. For these reasons, they can be valid for the study of environmental topics and controversies and their evolution over time. In other words, media allow study of the progression of "environmental issues in the public sphere" (Hansen, 2015, p. 217). In the social sciences and environmental studies, media sources have been successful to identify and distinguish different periods in water management on national scales [START_REF] Wei | Evolution of the societal value of water resources for economic development versus environmental sustainability in Australia from 1843 to 2011[END_REF][START_REF] Wei | Evolution of newspaper coverage of water issues in Australia during 1843-2011[END_REF].

In this study, we focus on both national and international news. This paper is based on media narratives from Le Monde, a French newspaper with a strong interest in international news and global socio-political debates (in a similar way to The New York Times or The Guardian). This source is particularly useful as Le Monde has digital archives dating back to the newspaper's foundation in 1944. Although previous studies have largely relied on content analysis [START_REF] Wei | Evolution of the societal value of water resources for economic development versus environmental sustainability in Australia from 1843 to 2011[END_REF], we implemented a method based on textual data analysis (also often called 'text mining').

This study therefore details the evolution of discourses on dams (between 1945 and 2019). It situates in time the integration of social, economic, and ecological issues within the debate on dams.

We then discuss results within the international scientific literature and debates on dams into the future.

Material

We targeted the newspaper Le Monde for the history of the debate on dams, tracing the genealogy of discourses on dams and the framing of dams as sustainable levers. Although a French newspaper may suggest a Franco-centred perspective, Le Monde pays much attention to international news and its reporting considers different and often diverging points of view. It is also one of the few major daily newspapers with archives available in formatted text. Notably, it is accessible through university library subscriptions (through its website, www.lemonde.fr or through online platforms such as www.europresse.com). Le Monde's archives go back to the newspaper's foundation in 1944. Therefore, this source allows a longitudinal approach. Only a few other newspapers provide digitised archives over such a time span. For such reasons, Le Monde has been useful beyond francophone scholarship [START_REF] Brossard | Are Issue-Cycles Culturally Constructed? A Comparison of French and American Coverage of Global Climate Change[END_REF].

Data collection

We combined different queries to avoid collecting articles that did not focus on hydraulic dams.

The terms included "dam & river" ("barrage & rivière", "barrage & fleuve"), "dam & lake" ("barrage & lac"), "dam & reservoir" ("barrage & réservoir"), "dam & licence" ("barrage & concession"). These queries produced over 6,000 articles. Many duplicates were automatically excluded from the corpus. We then read and examined the articles individually. We excluded from the corpus articles that only briefly evoked dams. This process led to inclusion in the corpus of 1,471 articles published between 1945 and 2019.

Data preparation

We collected and tabulated metadata on each article (publication date, author, article title). We also carried out a basic content analysis from the reading of the articles, focusing on information such as the dam site, the river or the river basin, and the country on which the article was centred. When the article only concerned one site, river, river basin or country, we also recorded this information in the metadata table. Moreover, for each article, we determined if the newspaper mentioned a controversy, i.e., an opposition or a disagreement between different stakeholders or communities involving the dam. Finally, using the scale elaborated by de Löe (1999), we determined whether the newspaper adopted a pro-or anti-dam tone, a neutral tone, and whether it presented both the pros and cons or simply whether the dam played a marginal role in the article.

Data analysis

To explore and analyse the data in the metadata table, we firstly summarised and plotted several key characteristics. The number of articles published per year, for example, identified periods of over and under news coverage. We also considered the evolution of dam building in the world between 1945 and 2020, based on the World Register of Dams (ICOLD, 2018), which represents dam builders and managers, and the International Commission on Large Dams (ICOLD), and which contained information on the building date of 51,514 dams throughout the world. After culling the data for the building of dams per year, we also measured the ratio between the dam coverage by Le Monde between January 1945 and December 2019, according to our corpus, and the total number of articles published each year in the newspaper. We further identified the countries and dams that benefited from a broad coverage by Le Monde: we were able to attribute countries to 1,425 articles out of 1,471.

Secondly, we performed a quantitative textual data analysis with the open-source programs Iramuteq [START_REF] Ratinaud | IRaMuTeQ: implémentation de la méthode ALCESTE d'analyse de texte dans un logiciel libre[END_REF] and TXM [START_REF] Heiden | TXM : Une plateforme logicielle open-source pour la textométrie-conception et développement[END_REF]. Textual data analysis, also often called 'text mining', is definable as a set of methods that rest on statistics to analyse text corpora [START_REF] Beaudouin | Statistical Analysis of Textual Data: Benzécri and the French School of Data Analysis[END_REF][START_REF] Heiden | TXM : Une plateforme logicielle open-source pour la textométrie-conception et développement[END_REF]. Iramuteq relies on R software and Python language to perform analyses such as the classification algorithm [START_REF] Reinert | Alceste une méthodologie d'analyse des données textuelles et une application: Aurelia De Gerard De Nerval[END_REF][START_REF] Reinert | Une méthode de classification descendante hiérarchique : application à l'analyse lexicale par contexte[END_REF], which comprises five main phases [START_REF] Cottet | How environmental managers perceive and approach the issue of invasive species: the case of Japanese knotweed s.l. (Rhône River, France)[END_REF]. The classification algorithm proceeds through several steps. First, it segments the corpus into phrases (of 40 words approximately). Second, it lemmatises 1 the words using a grammatical dictionary. Third, it produces a contingency table between the lemma and other forms such as the metadata and the text segments. Fourth, it builds a top-down hierarchical classification based on the contingency table (using χ 2 metrics). Fifth, it leads to the description of different lexical classes that present the degree of association between each class and the different forms (also using χ 2 metrics). Recent studies have used this method to identify discourses on different environmental subjects, such as invasive species [START_REF] Cottet | How environmental managers perceive and approach the issue of invasive species: the case of Japanese knotweed s.l. (Rhône River, France)[END_REF], rivers [START_REF] Comby | Power and Changing Riverscapes: The Socioecological Fix and Newspaper Discourse Concerning the Rhône River (France) Since 1945[END_REF], and water conservation [START_REF] Boyer | Building Consensus? The Production of a Water Conservation Discourse Through Twitter: The Water use it Wisely Campaign in Arizona[END_REF]. In this case, the classification algorithm allowed for the identification of 12 different lexical classes. We then considered the degree of association between the different classes and the decade of publication of the texts. We used the open-source data visualisation framework RAW Graphs [START_REF] Mauri | RAWGraphs: A Visualisation Platform to Create Open Outputs[END_REF], and in particular the 'area graph', to represent the χ 2 for each class according to the different decades.

In parallel, we used the 'progression' function of TXM (which also relies on R statistical libraries) to examine the evolution of specific words and expressions within the corpus, according to the publication year of the newspaper articles. We produced graphs displaying the cumulative frequencies of different expressions such as "wild river" ("rivière|fleuve sauvage") and "living river" ("rivière|fleuve vivant") or "greenhouse gas" ("gaz à effet de serre").

Thirdly, we conducted a qualitative and interpretative analysis. We read all 1,471 articles comprised in the corpus. Based on the reading of the corpus and keyword searches performed in TXM, we identified quotes reflecting evolutions that we had previously statistically observed.

Results

Characteristics of coverage by Le Monde

The coverage by Le Monde was mostly strong during the main period of dam building throughout the world, i.e., between the 1950s and the 1970s (Fig. 1). The coverage was characterised by various peaks (e.g., 1952, 1959, 1982, 1990). The qualitative analysis allowed us to link some of these peaks to specific dam projects and controversies: the building of Tignes Dam in France received much attention in 1952; in 1959, Le Monde published a series of articles on the Malpasset disaster (Southern France); other peaks such as the 1982 and 1990 ones did not coincide with a single project event or controversy. After 1980, and during a period in which less dams were being built, Le Monde paid less attention to dams (Fig. 1). However, after 2010, the coverage of dams increased again. Dam (with some articles also focused on Sudan) and the Three Gorges Dam (Fig. 2). 259

From economics to livelihoods: Twelve themes relating to dam-building (1945-2019)

Twelve major themes that relate to dams appeared in Le Monde (Fig. 3). Politics and geopolitics represented 1/5 of the newspaper articles' content (classes 3, 5 and 10). Journalists also lengthily described the landscape and the populations affected by dam-building (classes 1 and 4). Technical characteristics of the infrastructures, rivers and lakes, also often appeared (classes 7, 11 and 12). Industry and economy, as well as the funding issue, were also topical issues (classes 2 and 9).

Whereas environmental protests represented one of the smallest classes of the corpus (class 6,

A discourse focused on the materiality of dams and on their economic and energy performances (1945-1960s)

From 1945 to the beginning of the 1970s, three lexical fields were over-represented in Le Monde's articles on dams (Fig. 4). The journalists described in great detail technical aspects of dams, attaching particular importance to the size of the infrastructures.

Energy consumption and production were also a major theme of the articles in the 1940s and 1950s. This issue, coupled with economic questions along with industry, capital, finances, and development, were also frequently evoked. The focus on energy and economics is explainable by the fact that, during this period, whereas France was experiencing economic growth (the 'Trente Glorieuses' period, in connection with the Marshall Plan), the newspaper mainly described dams A qualitative analysis of the newspaper articles showed that, between 1945 and the end of the 1960s, dams were described as solutions to different types of crises such as drought, flood, water supply or energy shortage. The articles presented dams as tools for development and modernisation, most often on a national scale. Although most newspaper articles remained neutral (i.e., they did not explicitly support the dam projects), some articles demonstrated pride towards the infrastructures and engineering technicity. Regarding the Roselend scheme in the French Alps, a journalist asserted France's superiority over the United States or the Soviet Union: "None of the foreigners [who were visiting the dam and its power plant, La Bâthie] claimed to have something better at home. Not even a Soviet. Not even an American" (Le Monde, 10 th August 1961). Dams were tightly linked with a discourse on national interests and on political and economic independence in the post-World War II and Cold War contexts. Some articles highlighted the beauty of the dams by comparing them to temples or cathedrals. In the newspaper articles, water was represented as raw material that does not have a cost, as a resource that could be entirely exploited for human activities by implementing "rational" projects. The Dordogne River (in the southwest of France) was described as having been "rationally equipped" and as one of the "best examples of the complete usage of a river" (Le Monde, 1 st July 1953). The newspaper articles rarely described the source of water and the environment in which water flows. Rather, journalists mentioned head and flow rate, "white coal" ("houille blanche") when they described water, or "liquid stairways" (Le Monde, 25 th May 1964).

Nevertheless, some articles mentioned debates, disputes and controversies before the 1970s. Local negative consequences were evoked, notably those related to land loss (e.g., during the construction of the Tignes Dam in the Central French Alps), and regional oppositions between downstream and upstream interests (e.g., Serre-Ponçon Dam and the Durance valley in the French Southern Alps).

Although some articles focused on the benefits of the Aswan High Dam (in Egypt), others underlined the geopolitical tensions linked to the project and the archaeological consequences of the inundation of Nubia even after the reconstruction of the Abu Simbel temple. The international campaign related to the dam, carefully followed and reported on by Le Monde, influenced French journalists to adopt a more critical perspective regarding the benefits of dams. By the end of the 1960s, a shift had taken place: while economics, energy production and the technical characteristics of dams played an increasingly smaller role in the discourses on dams, the articles began to question the assets of dams.

A discourse increasingly adding value to the socio-natural environment and landscape (1970s-1980s)

During the 1970s and the 1980s, as dam building reached its peak, dams were extremely topical (Fig. 1b and1d). Nevertheless, these two decades did not present major specificities from a discursive point of view (Fig. 3). Although some themes grew during the 1970s-such as landscape (Fig. 3, class 4)-and the 1980s-such as politics and conflicts (class 3) or environmental protests (class 6)-they did not dominate the articles as much as energy or the technical aspects of dam building that dominated the articles in the 1950s.

While the journalists continued to focus on France or on the dams planned in France, the articles also turned towards North and South American dams (the most publicised having been Itaipu, Fig. 2c, on the border between Brazil and Paraguay) and towards some Asian dams. From the 1970s, the negative consequences of dams appeared more and more although some articles continued to assert the benefits of dams in terms of development. The Oil Crises (which were evoked in three articles between 1977 and 1979) might have played a role in the latter case. The debates and controversies were no longer simply local or regional but were national and even international. The opposition to the Silent Valley Dam in India was described, for example, as part of a broad campaign to defend tropical environments and in particular tropical forests throughout the world. The inundation of farmland was increasingly cited (in particular in the Loire valley with Villerest or Naussac, Fig. 2c, in the centre of France but also in Haïti). By raising this topic, the journalists questioned both the type of nature desired by society and the economic model dominant in France and throughout the world. While the foreign dams built with French assistance continued to specifically attract the attention of journalists (for example, Itaipu, Fig. 2c), compared with articles of the 1950s and 1960s, the articles of the 1970s and 1980s were much more critical of French foreign aid. The building of dams on the Senegal River was described as leading to the "disappearance of subsistence agriculture in favour of the development of export crops" (Le Monde, 15 th October 1983). Ongoing and past debates on American dams-the O'Shaughnessy in the Hetch Hetchy Valley or the Marble Canyon on the Colorado River-were evoked.

In some articles, dams became synonymous with environmental degradation and destruction. Since its inauguration in 1971, the Aswan High Dam has remained newsworthy (Fig. 2c) because of its negative impacts: the increase in soil salinity, river bank erosion, the disappearance of certain species such as sardines (Sardina pilchardus), and the regression of the Nile's delta. In recent articles, Aswan has become a symbol of negative consequences of dams, in particular for arid countries. More attention was also paid to rivers in terms of pollution, yield, temperature, and algae development. Subjects such as hydro-peaking, flushing flows, and discontinuity became newsworthy. For some journalists, once the dams were built, the nature of the river changed. The Verdon River (in the Southern French Alps) "has ceased to exist for a third of its distance" (Le Monde, 28 th June 1982), for example, and the Tagus (in Central Spain) was "imprisoned by a series of dams" (Le Monde, 12 th March 1971). During this period, the expressions "wild river" and "living river" appeared in journalistic vocabulary (Fig. 5a), which revealed the growing concern for pristine and rare environments. Starting in the 1980s, some articles focused on or evoked plans for dams, which were cancelled because of environmental considerations in France but also in Germany, Australia, the United States and the Soviet Union.

According to Le Monde, dams became particularly controversial hydraulic infrastructures during the 1970s and 1980s, although no lexical fields particularly stood out in the corpus. During these two decades, while environmental concerns emerged, economic and social issues split apart as some dams were described as economically interesting but socially questionable. Cumulative frequencies of the expressions "wild river" and "living river". b, Cumulative frequencies of the expressions "indigenous|native population|community|people", "sustainable development", "climate change", "global warming", "greenhouse gas". c, Cumulative frequencies of the expressions "hydroelectricity" and "renewable energy".

From social and environmental impacts to controversies on sustainability (1990s-2010s)

A discourse on the impact of dams on water and physical environment and their interactions with society (1990s-mid 2000s)

During the 1990s and the beginning of the 2000s, the coverage of issues related to dams slightly decreased in Le Monde (Fig. 1d). Nonetheless, environmental protests became more topical during the 1990s in relation to specific debates (for example, the Three Gorges project in China, Fig. 2c).

The social and environmental impacts of dams continuously appeared-although the tone of the journalists remained neutral in general. Impact on water (quality and quantity) also became more central (Fig. 4).

The introduction of such topics in the newspaper's accounts were linked to various events. Firstly, in 1992 the French government passed a water law and defined water as a national heritage.

Secondly, at the end of the 1990s, some members of the French government considered implementing a sustainable water development scheme for the Loire River basin. At the end of the 1990s, such a program implied abandonment of dam projects, with an important example being the Chambonchard Dam (Le Monde, 19 th February 1999). It represented a major turn for water development in France, as dam projects were cancelled due to environmental concerns and protests.

Thirdly, the newspaper also highlighted newly discovered environmental impacts of dams, in particular the biological consequences of the release of methylmercury after the flooding of forests or boglands in Canada (Le Monde, 27 th December 1991) or the release of greenhouse gas. The newspaper's columns debated the latter, when the main French operator Électricité de France (EDF) was obliged to temporarily shut down its newly inaugurated dam and power plant of Petit Saut in French Guyana. The flooded tropical forest released so much methane that many fish suddenly died. Scientific studies demonstrating that the reservoir released important quantities of greenhouse gas that were quoted by the newspaper led journalists to conclude that "hydroelectric dams are not insignificant in terms of greenhouse effect" (Le Monde, 28 th August 1995). The subject of greenhouse gas was introduced in some articles on dams since the early 1990s (Fig. 5b) but only became topical at the end of the 1990s and even more so after 2004.

When sustainability enters the discourse on dams (from the mid-2000s)

Although international conferences and reports introduced the concept of sustainable development at the turn of the 1990s (with the 1987 report Our Common Future and the 1992 Rio Earth Summit), based on Le Monde's reporting, the sustainability of dams only became central in the discourses on dams after the mid-2000s. The expression "sustainable development" appeared regularly since 2005 (Fig. 5b). Similarly, the use of the expressions "climate change", "global warming" and "sustainability" increased substantially after 2004 (Fig. 5b), possibly in relation to the 2001 Third Assessment Report of the Intergovernmental Panel on Climate Change. Economic development was tightly linked to environmental and social sustainability, as shown in the class 8 (Fig. 3), which gained momentum in the last decade (Fig. 4). Sustainability was shown as a paradigm for development and for water management that can be invoked to either defend or criticise dams.

Livelihoods, living standards, social equity and justice and human rights were more and more reported on after 2000 (Fig. 4). Le Monde participated in giving indigenous communities an international audience (Fig. 5b), in particular in the cases of Southeastern Asian dams (such as Murum in Malaysia) or of South and North American dams (such as Belo Monte in Brazil, Fig. 2b or Agua Zarca in Honduras, or various Canadian projects), which may endanger these communities' livelihoods, according to the newspaper.

The newspaper also explained, however, how different projects may contribute to sustainability. Le Monde reported that the Nam Theun 2 project, planned in Laos, was defended by the World Bankalthough the World Bank had previously refused to finance some other dam projects-and presented by the international institution as an example of sustainable development (Le Monde, 14 th September 2004). The construction of the multipurpose Kandadji Dam on the Niger River in the South of Niger was also said to have raised no opposition "because the dam was conceived according to the standards of sustainability" (Le Monde, 10 th May 2012). Instead, the story of an irrigation dam planned on the Acheloos River in Greece, was reported in the newspaper after the decision taken by the Greek Council of State (i.e., the Supreme Administrative Court) to prohibit the dam's construction as the project contradicted the principles of sustainability recognised by the Greek constitution (Le Monde, 6 th March 2014). In many recent articles focusing on China (which is the second most newsworthy country, Fig. 2a and Fig. 2b), the journalists argued that sustainability has entered the discourse of Chinese officials. This discourse focused on justifying the building of new dams when, in fact, the multiplication of dams and the accompanying growth and climate change "put pressure on hydrology" (Le Monde, 12 th November 2008). The Three Gorges Dam, proposed by the Chinese government who "assured that such construction work is essential for the modernisation of a country where energy shortages are slowing development" (Le Monde, 16 th December 1994), in fact "amplifies the consequences of global warming which has already contributed to diminishing the Yangzi River's yield" (Le Monde, 30 th December 2009). While irrigation represented a controversial topic, as demonstrated by the debates on the Sivens Dam project in Southwest France (Fig. 2c), "sustainable", "development" and "renewable" are amongst words that co-occurred most with the word "hydroelectricity" in the newspaper corpus. Although hydropower remained much less articulated as a theme connected to dam building than it was in the 1940s and 1950s, hydroelectricity became more topical in the newspaper also at the turn of the second millennium (Fig. 5c). The newspaper revealed the complexity of the debate. While indicating the possibility of greenhouse gas release related to certain dam-reservoirs, it also explained that after "the Fukushima accident, the questioning on nuclear power and the struggle developing countries. While reporting on a meeting between the African Union and ICOLD, journalists explained that hydropower is presented as a clean "alternative to fossil energy resources" and the organisations' "intentions are clear: large dams must become once again instruments for the development of nations" (Le Monde, 27 th November 2008). The journalists also highlighted, however, that this return to large infrastructures can be a source of major political tensions, for example in the case of the Great Ethiopian Renaissance Dam, which the newspaper has very often reported on in recent years (Fig. 2c). By 2019, with the rise of an environmental discourse on economic, social and environmental sustainability, and the emergence of themes such as climate change, global warming and greenhouse gas emissions, the reporting on dams has therefore become more ambiguous and associated with uncertainty.

Discussion

Despite the fact that Le Monde does not specialise in environmental issues, the results of the analysis showed that the newspaper documented changes in discourses on hydraulic infrastructures such as dams. It provided insights on a continuing dilemma over the period and into the future: to dam or not to dam? Although dams have been controversial infrastructures as early as the 1940s, it was mostly after the 1970s that that they were portrayed as disrupting social and environmental configurations, the daily living environments of the local inhabitants. From this point of view, the results showed a time gap between the scientific controversy on the impact of dams, which emerged during the 1960s at conferences such as the Royal Geographical Society symposium on "Man-made lakes" (in 1965), and the media and political debate. The necessity to take into account the local population and conduct impact assessments, which was proposed by the scientific community at the beginning of the 1970s (Scope, 1972), was not advanced in newspaper articles of Le Monde until later years of the same decade. The characterisation of dams and hydropower as 'clean' and 'green', which has been debated within the scientific community since the 1990s [START_REF] Rosenberg | Environmental and social impacts of large scale hydroelectric development: who is listening?[END_REF], only appeared in the media in the mid-2000s. Moreover, while Allan (2003, p. 2) underlined that "the water sector in the North adopted the principles of sustainable development during a protracted discursive struggle from the late 1950s to the mid-1970s", the issue of sustainability only progressively emerged in newspaper articles in the 1970s and only became a prominent topic, conceptualised as such, in the past decade.

These results are similar to those of other studies on media coverage of environmental issues. In a study of the coverage of climate change in the mass-media of the United States, [START_REF] Boykoff | Climate change and journalistic norms: A case-study of US mass-media coverage[END_REF] explained that "Even though scientists had warned for years that emissions of carbon dioxide and other greenhouse gases would lead to changes in the Earth's climate, this did not capture large-scale media attention in the US until 1988US until " (p. 1195)). The time gap is also explainable by events not directly connected with dams, but which triggered a sudden interest in such infrastructure and its sustainability. In France, in the mid-2000s, the government organised a multi-party debate on ecology and sustainable development ('Grenelle Environnement'), whose aim was to elaborate a list of measures to implement. In 2002, the name of the Ministry for Environment ('Ministère de l'Environnement'), which had kept the same name since 1971, was changed to 'Ministry for Ecology and Sustainable Development' ('Ministère de l'Écologie et du Développement Durable'). These political events could potentially explain the growing interest in the sustainability of dams in the mid-2000s. As [START_REF] Boykoff | Climate change and journalistic norms: A case-study of US mass-media coverage[END_REF] underlined, increases in the media coverage of environmental topics are often attributable to "a combination of circumstances that [are] far from purely scientific in nature" (p. 1196).

From both a genealogical and critical perspective, results of this study showed that, until the 1970s, Le Monde produced and conveyed a discourse highlighting human domination over nature. This discourse mostly promoted development and paid much attention to the infrastructures themselves.

From the end of the Second World War to the 1970s, the media's discourse can essentially be described as 'Promethean' or 'Modernist' (Kaika, 2006), i.e., a discourse about taming and controlling a 'savage' nature [START_REF] Boelens | Contested Knowledges: Large Dams and Mega-Hydraulic Development[END_REF]. With a strong focus on industrial development, the newspaper also highlighted the dependency of capitalist States and companies on the building of new dams and hydraulic infrastructure. In this perspective, dams appeared as 'hydraulic fixes'-in other words, hydraulic projects branded by their supporters as tools for growth, social and economic development, but which in fact contribute essentially to the longevity of hydraulic companies and capitalist States (Swyngedouw, 2015). Evidenced in articles published after the 1970s, the newspaper had apparently distanced itself from the Promethean discourse and adopted a more critical tone. The media began to pay more attention to human livelihoods but also to other nonhuman living beings. The evolution of the newspaper's discourses illustrates how dams have become the 'symbol of an exogenous development model that is imposed on vulnerable local societies' (Barraud, 2008: 3)

2 .
The genealogical approach utilised in this study also revealed how sustainability has entered the discourse on dams and shows a coexistence of different conceptions of sustainability. Although different stakeholders seemingly agree today that sustainability involves three constituent elements-ecology, economy and society-the newspaper articles showed changing prioritisations of these three elements (equitability, viability, bearability). The articles of the 1970s and the 1980s, for example, highlighted the opposition between stakeholders who favoured economy and who expressed few concerns regarding ecology-putting forward dams as equitable development levers-and stakeholders concerned with social changes related to dams, and finally an emerging group of stakeholders concerned with ecological issues. By the 2010s, sustainability was apparently central to stakeholders on all sides-i.e., against or in favour of dams-and the environmental impact of energy infrastructures in particular was brandished by both dam proponents and dam opponents. For some stakeholders-in particular for some governments or hydraulic institutionsthe articles showed that sustainability was more connected with social and economic aspects than with environmental ones. More generally, the tension between the different pillars of sustainability and the consequences of dams at different scales (e.g., local, national, and global) underlines the importance of deliberation in order to move towards concerted decision-making. It also reveals the extent to which the sustainability of dams, and notably the criteria we use to define viability (the opposition between river ecology conservation and carbon emission control) and equitability, has today become extremely controversial in the public domain.

Conclusions

In conclusion, analysis of media discourses held on dams since the mid-20 th century revealed how different generations of debates on dams, water planning and more generally on development paradigms have emerged and evolved. Results of the genealogical study of the newspaper Le Monde provided answers to the research questions posed in this paper as follows:

First, how have the discourses and debates on dams, and notably on the sustainability of dams, developed within the public domain?

Our analysis shows that between 1945 and 2019, different discourses have been developed on dams.

Until the 1960s, the dominant discourse focused on the materiality of dams and their economic and energy performances. Essentially Promethean, this discourse was occasionally opposed to a discourse presenting dams as controversial and having negative social and cultural impacts. During the 1970s and the 1980s, i.e., a decade after the beginning of the scientific controversy on dams, the discourse on dams increasingly added value to the socio-natural environment and landscape affected by dam projects; dams tended to be presented as economically interesting but socially and even environmentally questionable. After the 1990s, the impacts of dams were more and more highlighted, in relation to society and to the physical environment. Finally, our results showed the extent to which the debate on the sustainability of dams has gained momentum during the past ten years.

However, the notion of sustainability was addressed in newspaper articles on dams in an ambivalent way. On one hand, negative social and environmental impacts of dams were listed to demonstrate that dams are incompatible with sustainability. On the other hand, the newspaper articles explained how in some cases hydropower projects could contribute to sustainable development.

Second, how can tracing a 'genealogy' of discourses on dams inform their sustainability into the future?

Analysing past and present discourses on dams can help inform future discussions on dams and their sustainability. Our results have shown that controversies about dams are not new in the public domain, and that the notion of sustainable development has to some extent reactivated the Promethean discourse in which dams are presented as solutions to a range of socio-environmental problems. Commonalities between past and present discourses can be found, even though the Promethean discourse is no longer dominant in the media we analysed.

The analysis of media discourses on dams also showed the complexity of the public debate regarding the knowledge of the social and ecological impacts of dams, of both short-term and longterm environmental alterations they may cause, and of the political choices to be made in the future.

To gain more complete insights and continue informing discussions on dams and sustainability, future work can extend to other media sources, as archives are becoming more and more digitised.

Additional sources may reveal different trends in the evolution of discourses on dams and the integration of sustainability within these discourses. Other avenues may include exploration of the sustainability of dams as producing 'green energy', as well as the study of other sources, notably the advertisements (e.g., leaflets, posters, etc.) produced by hydropower companies. Results from this study further suggest a need for a stronger dialogue between the scientific and public domains.

Knowledge transfer and exchange would be beneficial on topics including the meaning of 'renewable energy' and 'green energy,' and alternative modes of governance regarding dams.

Fig. 1 .

 1 Fig. 1. Dam building in the world and dam coverage in Le Monde (1945-2019). a, Number of dams 249 built between 1945 and 2020 according to the World Register of Dams database (n=51,514). b, 250 Number of articles in the corpus on dams published between 1945 and 2019 in Le Monde 251 (n=1,471). c, Total number of articles published in Le Monde between and 2019 252 (n=2,594,606). d, Ratio between the number of articles on dams published in year in Le Monde, and 253 the total number of articles published per year by the newspaper (per mille). 254 255 Le Monde's coverage was both international and Franco-centred. The corpus included 582 articles 256 focused on France (40,1%). It also encompassed articles on 105 other countries (Fig. 2). Much 257 attention was paid to dams planned or built in Egypt and China, and in particular to the Aswan High 258

Fig. 2 .

 2 Fig. 2. Spatial characteristics of Le Monde's coverage of dams (1945-2019). a, Articles on dams per country (excluding France). b, Cumulative frequency of the number of articles published on the ten main countries based on the corpus (excluding France). c, Cumulative frequency of the number of articles published on the ten main dams based on the corpus.

Fig. 3 .

 3 Fig. 3. The 12 lexical classes of Le Monde's articles on dams (1945-2019). The figure is the result of the hierarchical classification of the textual segments performed with Iramuteq. The authors added the keywords, established through a qualitative analysis, underneath the class numbers to facilitate reading the figure.

Fig. 4 .

 4 Fig. 4. Evolution of the 12 lexical classes between 1945 and 2019. The figure, made with RAW 321 Graphs, shows the degree of association (χ 2 ) between the different classes and the decade of 322 publication of the newspaper articles. The degree of association can be positive or negative (above 323 or below the horizontal lines). The scale representing the degree of association is shown in the top-324 left corner of the figure.325

Fig. 5 .

 5 Fig. 5. Significant keywords and expressions and their evolution between 1945 and 2019. a,

In linguistics, a lemma is generally defined as "a set of lexical forms having the same stem and belonging to the same major word class, differing only in inflection and / or spelling" (Francis and Kucera, 1982: 1). In general, a lemma is the standard dictionary entry of a word. To lemmatise means to tag words of a corpus according to their stem. In this case, the software we use matches each word of the corpus to a dictionary entry.

linked to France (colonies or ex-colonies, such as Morocco or the Ivory Coast), and dams

Translated from French by the authors.

Acknowledgements

The authors would like to thank their colleagues from the University of Lyon and from "Environnement Ville Société" for many interesting discussions on methods for analysing discourse.

The authors would also like to thank the reviewers and the editor-in-chief for their insightful comments and suggestions. This work was performed within the framework of the EUR

d'Avenir" operated by the French National Research Agency (ANR).

Funding

This study was supported by a PhD grant from the French Ministry of Research ("Contrat doctoral unique fléché normalien") from 2014 to 2017.

Competing interests

The authors declare no competing interests.


Alban Pinel

Elodie Prud'homme

Aurelia Charlot

Etienne Fleury

Yves Jorand
email: yves.jorand@insa-lyon.fr

Earthen construction : demonstration of feasibility at 1/2 scale of poured clay concrete construction

Poured earth construction is a very promising alternative to cement-based construction, however it requires a large increase of productivity. Poured earth concrete can meet this issue, but is currently stabilized with cement leading to a high environmental impact. One possible solution is the use of alginate, a biopolymer, to provide a transient liquid-solid transition before the wall is effectively hardened by the long process of drying. Using design experiment, the impact of four parameters (water to binder ratio, dispersant ratio, alginate ratio and level of acidifier) has been investigated on the consistency and the mechanical properties. In this study the feasibility of the use of poured earth concrete stabilized with biopolymer is demonstrated through low wall manufacturing.

Introduction

In order to reduce the carbon footprint related to housing construction, a large part of the scientific community considers the replacement of cement concrete by unfired earth-based materials as a promising route. Indeed, it is a building material with very low embodied energy [START_REF] Van Damme | Cement and Concrete Research[END_REF][START_REF] Melià | Journal of Cleaner Production[END_REF][START_REF] Pacheco-Torgal | Earth construction: Lessons from the past for future eco-efficient construction[END_REF][START_REF] Morel | Building houses with local materials: means to drastically reduce the environmental impact of construction[END_REF], with many advantages in terms of energy consumption and comfort resulting from its hygrothermal performance [START_REF] Van Damme | Cement and Concrete Research[END_REF][START_REF] Mcgregor | The moisture buffering capacity of unfired clay masonry[END_REF][START_REF] Allinson | Energy and Buildings[END_REF][START_REF] Taylor | Evaluating rammed earth walls: a case study[END_REF][START_REF] Paul | A comparison of occupant comfort and satisfaction between a green building and a conventional building[END_REF][START_REF] Singh | Thermal performance study and evaluation of comfort temperatures in vernacular buildings of North-East India[END_REF], and, lastly, the recycling of the earth material is particularly simple with no significant harmful impact on the environment [START_REF] Van Damme | Cement and Concrete Research[END_REF][START_REF] Morel | Some observations about the paper 'earth construction: Lessons from the past for future eco-efficient construction' by F. Pacheco-Torgal and S. Jalali[END_REF][START_REF] Gallipoli | A geotechnical perspective of raw earth building[END_REF].

In order to be able to build productively with earth, and thus replace cement concrete in the construction of small and medium sized houses, the development of a poured earth construction technique is desirable. Many advantages could be gained from this. In addition to drastically increasing productivity compared to traditional techniques, the process would make it possible to conserve the tools and methods of cement concrete technology and, above all, to reduce the labour intensity.

However, to achieve such an objective, the earth based concrete must meet four main requirements : (i)-to have a high fluidity so as to be pumpable, ideally self-compacting (ii) -to present, once placed in the frameworks, a liquid -solid transition that allows the wall to support its own weight in a much shorter delay than its drying time. Then the minimal mechanical resistance to be reached in the wet state can be estimated at about 0.07 MPa [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF], this should be obtained in less than 24 h to approach the common practice of cement concretes, (iii) -to be able to be implemented with a robust formulation compatible with the large variability of earth-based raw materials. To address this, in addition to the classical practice of granular correction, the use of additives with little or no detrimental effects on the hygrothermal and recycling properties can be considered, (iv) -to be available at a cost-price close to that of cement concretes.

The fluidification of clays is a well known process [START_REF] Van Damme | Cement and Concrete Research[END_REF][START_REF] Jouenne | Traite de Ceramiques et Materiaux Mineraux (Treatise on Ceramics and Mineral Materials)[END_REF][START_REF] Van Olphen | An Introduction to Clay Colloid Chemistry[END_REF] and several solutions exist. However, since the control of pH value or ionic strength appears to be difficult on construction sites, the addition of dispersant (superplasticizers) appears to be the simplest solution. It has been shown that the required fluidification of clays based binders, the first condition mentioned above, can be achieved by the addition of synthetic dispersants [15][START_REF] Garrido | OH-Al polymers and HMP adsorption on kaolinite: experimental conditions and viscosity of deflocculated suspensions[END_REF][17][START_REF] Andreola | Effect of sodium hexametaphosphate and ageing on the rheological behaviour of kaolin dispersions[END_REF][START_REF] Moevus | Earthen construction: an increase of the mechanical strength by optimizing the dispersion of the binder phase, Materials and Structures[END_REF][START_REF] Guihéneuf | Addition of bio based reinforcement to improve workability, mechanical properties and water resistance of earth-based materials[END_REF][START_REF] Landrou | Lime as an antiplasticizer for self-compacting clay concrete[END_REF]. Another important feature of dispersant 3 / 27 addition is that it significantly improves the mechanical resistance of the clay binder based material [START_REF] Landrou | Lime as an antiplasticizer for self-compacting clay concrete[END_REF]. An extrapolation of these results to various earth-based products was also presented by Moevus-Dorvaux et al. [START_REF] Moevus-Dorvaux | Environmental-Clay-based Concrete -Terra[END_REF] and Ronsoux et al. [START_REF] Ronsoux | Poured earth as concrete[END_REF].

The second point is a major technological barrier. It has been addressed by Pinel et al. [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF] and an innovative formulation has been proposed at the mortar scale. It allows an adjustable and robust liquid-solid transition such as the one obtained with cement. It is based on the addition of alginate, a polysaccharide, which is a bio-sourced, non-toxic, biodegradable and cheap product. This transition is induced by the cross-linking of the alginate chains via bonds incorporating Ca 2+ ions. It allows the mortar to form a gel in a given time. The release of the calcium ions required for this gelling, is obtained by an addition of an acidifying agent, whose action is progressive, the glucono-δ-lactone (GDL). Hence, clay based mortar specimens offer mechanical compressive strengths of up to 0,09 MPa after 24 hours without any water being released. Also, an important aspect is that these formulations allow to set the working times from 1 to 2 hours [START_REF] Pinel | Transition liquide-solide dans des dispersions d'argiles contrôlée par un biopolymère: Application à la construction en terre[END_REF].

The objective of this study is to extrapolate this polymer-based setting method to a concrete formulation and to test it on a scale intermediate to that of the mortar specimens (lab scale) and the wall (scale 1), i.e. at 1/2 scale. Since the main objective is to obtain a formulation that can be easily poured in frameworks, a focus was made on the consistency and the working time. For this purpose, we use the clay-based binder phase from the above-mentioned study [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF] in the same way as cement in conventional concrete. The influence of four formulation parameters -the ratio of water to binder, dispersant, alginate and GDL -on consistency and mechanical properties 24 h after pouring is investigated.

Experimental methods

Materials

Since the clay based binding phase used here is the same as in [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF], only its main characteristics will be mentioned here. This is a clay-limestone by-product, AC 0-100 from Carrières du Boulonnais (F-59250 Ferques, France). X-ray diffraction qualitative analysis shows the presence of clays (19 % wt., mainly kaolinite and illite), quartz, dolomite, calcite (60 % wt) and goethite. The median particle size D50 is 10 µm with 100% of grain sizes under 100 µm. Its specific surface is estimated to 21,5 m 2 /g using methylene blue adsorption . Before use, the product is systematically dried in an oven at 105°C for more than a week in order to avoid any correction due to the water absorbed by the product during storage.

The aggregates used are 0/4 mm sand and 4/10 mm gravel, supplied by CEMEX (01340, Montrevel en Bresse, F). The aggregates are alluvial silico-limestone products, rounded and washed. The moisture content of the sands is measured according to NF P94-049-2 [25].

Controlled gelation is achieved by using an alginate selected from a wide range of commercially available products during a preliminary study [START_REF] Pinel | Transition liquide-solide dans des dispersions d'argiles contrôlée par un biopolymère: Application à la construction en terre[END_REF]. This alginate, Protanal LF 200 FTS supplied by FMC Biopolymer, appears to be the best combination in terms of viscosity in aqueous solution and ability to set in the presence of calcium ions. It is commercialized in a powder form with a molecular mass (Mw) of around 230 000 g/mol and a high guluronate ratio (67%) likely to promote significant gelling. The other materials used in this study are sodium hexametaphosphate (SHMP) from Sigma-Aldrich (ref. 71600) as dispersant and glucono-δ-lactone (GDL) also from Sigma-Aldrich (ref. G4750) as acidifying agent. Tap water, with a temperature comprised between 12 and 14 °C, is used in all the experiments.

Concretes preparation

Concrete formulations are based on the standard Dreux-Gorisse formulation rules, considering AC 0-100 as cement. This clay binder phase is used at a rate of 300 kg/m 3 in all formulations. Also a constant sand to gravel ratio of one to one is used to promote high consistency index.

Firstly, a solution of alginate in water is prepared 24 hours before concrete processing with concentrations ranging between 12 and 15 g/l, depending on requirements. If necessary, additional water is added to achieve the desired water/binder ratio. The dispersant (SHMP) is then solubilized under vigorous agitation. When processing concrete, a dry pre-mixing of aggregates and binder phase is conducted in a "Collomix" mixer during 1 min. The alginate-SHMP solution is then added and the whole is mixed for 3 minutes. At this point, the consistency of the concrete is evaluated with the Abrams cone according to NF EN 12350-2 slump test [26]. Then the GDL acidifier is added to the batch and a further 3 min mixing period is carried out to ensure a homogeneous distribution of the GDL acid. The consistency is then measured after 0, 20 and 45 minutes of rest times.

The concrete compositions tested are given in table 1 and 2. The range of variation of each component is chosen with the objective of obtaining at least an S4 consistency. This is done on the basis of study of Pinel et al. [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF]. For the presentation of the different compositions, the following criteria are considered here :

-the water to binder ratio (W/B), equivalent to the classic water to cement ratio, considering AC-100 as the binder, -the dispersant ratio (SHMP) in % g/g, -the alginate ratio in % g/g, -the level of GDL in % g/g.

These percentages are in respect to the clayey binding phase.

Mechanical tests

Compression test specimens ( 11 cm / h 22 cm) are made by pouring the concrete in three layers, each compacted with a vibrating needle, in standard cardboard moulds. The moulds are hermetically sealed to prevent any water evaporation and stored for 24 hours. At the end of this time, the mechanical compression resistance is measured. It corresponds to the mechanical resistance in the wet state, i.e. only due to the gelling of the polymer without any possibility of water release. All compressive strength values are obtained by averaging the resistances of three specimens.

Mechanical compression tests are conducted on a Schenk Trebel testing machine at imposed crosshead displacement rate of 1.0 mm.min -1 . To compensate for non parallelism, a system consisting of two metallic plates separated by a steel half-sphere is inserted between the sample and the upper piston of the machine. The engineering fracture strength is deduced from the load-strain curve.

For the half scale tests, the concretes are poured into small formworks measuring 38 x 45 x 10 cm and vibrated with a needle. After levelling, the formwork is covered with a plastic film to prevent any drying. As with the mechanical tests on humid samples, formwork removal is carried out after 24 hours.

6 / 27

Results and discussion

Preliminary tests

As mentioned in the introduction, a clay based mortar formulation with a setting time of around a day has been developed in a previous study [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF]. In terms of function, this formulation, in addition to the aggregate, can be decomposed into (i) A mineral binder for the long term dry state cohesion, this is the clay part; (ii) an organic binder for short-term cohesion in the wet state, this is a biosourced polymer (sodium alginate) which can transform into a gel due to the presence of calcium cations (calcium being commonly present in mineral components); (iii) an acidifier whose role is to release these divalent ions homogeneously within the mortar, i.e. to allow an "internal gelation", the GDL; (iv) a key additive which combines the role of fluidifier and setting retarder, the SHMP. The latter has a particularly important role to play, as it reduces the demand for water, increases the mechanical resistance of the dry product and controls workability due to its chelating action. It should be noted that the SHMP rate is higher than the optimum value for fluidification, since this over-dosage allows workability to be adjusted. An optimisation of this formulation has led to the mortar composition given in table 1 and the first concrete formulations tested are directly based on it (see also table 1).The W/B ratio range starts at 0.5, which corresponds to the ratio used in mortars.

a) prior the onset of gelation (without GDL addition)

A necessary condition for achieving the intended objective is to check that a consistency higher than class S4 (fluid) can be obtained at the end of the first mixing phase, i.e. before adding the acid (GDL) to the fresh concrete. As alginate has a strong impact on the viscosity of the interstitial fluid, two alginate ratios (0,15 and 0,3% g/g), lower than the optimum observed on mortar formulations (0,6% g/g), against one slightly higher (0,75% g/g) were selected. Similarly, two W/B ratios 0.5 (same as the mortar optimum) and 0.56, are selected. However, the obtained mixture presents little or no slump, far from expectations. To obtain the required consistencies, new formulations with higher W/B ratios were prepared. Then, it was observed that S4 consistencies could be reached only in the case of the two lower alginate ratio (0.15 and 0.3% g/g) and for W/B ratios of between 0.64 and 0.73. For the highest alginate content, 7 / 27 0.75 % g/g, even the unrealistic W/B ratio of 0.88 did not allow a consistency S2 (plastic) to be exceeded (table 1 and fig. 1).

b) after the onset of gelation (with GDL addition)

On this basis, low content alginate concretes with the addition of GDL were tested (table 2).

Surprisingly, in all cases the gelation became too important as soon as the homogenization mixing (3 min) is completed and almost zero slump was observed. It was then noted that the aggregates used for concrete formulations are described as "silico-calcareous" while a silicious sand was incorporated to the reference mortars admixtures. Then, samples of each of these sands were immersed in pure de-ionised water or with the addition of 200 mM of GDL, with a sand/water ratio of 82/18, which is close to the environment achieved during the preparation of the mortars. The mixtures were maintained for 24 hours with three periods of agitation, then pH values measurements and ICP analyses were carried out on the supernatant (table 3). The [Ca 2+ ]/[H + ] molar ratio is obtained by considering that one mole of GDL releases one mole of H + . It can be observed that in the absence of GDL, the two sands do not show any noticeable difference, they both release very few calcium ions in this configuration. On the other hand, we note that a significant quantity of calcium ions are released in the presence of GDL and that the "silico-calcareous" sand releases twice as much as the "siliceous" sand (90 vs. 48 mM). The molar ratio [Ca 2+ ]/[H + ] indicates that it is close to the theoretical maximum of 1 mol of Ca 2+ released for every 2 mol of H + introduced. Moreover, the pH value of the supernatant is slightly acidic, it reaches 6.1, unlike the case of "silicious" sand, for which it is 3.5. These observations suggest that the "silico-calcareous" sand is indeed more calcareous than silicious one, it releases the theoretical quantity of calcium ions expected under acidification and its pH value is much more buffered, confirming the presence of a basic species such as calcite. These observations confirm that the chemical nature of the aggregates can have a marked effect on the quantity of calcium ions available for gelling and therefore on workability.

Of course, imposing a particular type of aggregate for concrete production is not a realistic option.

Therefore, it appeared necessary to adapt the concrete formulation to make it more robust to this type of variation, or at least to propose simple corrections to the formulation that can be easily implemented on site.

Concrete formulation optimisation (to produce a feasibility demonstrator)

In order to limit time and resources, this optimisation step uses a fractional factorial experimental design based on the Taguchi method. This type of experimental approach should make it possible to achieve more readily a formulation that will enable the demonstrators (low walls) to be produced. The Taguchi method simplifies the use of the experimental designs, details on this method can be found for example in [27]. Also, an example of its use has already been presented in [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF]. Preliminary tests have identified four determining parameters and null or weak interactions between these parameters were suspected. The parameters considered are (i) the W/B ratio, and the concentration of (ii) alginate, (iii) GDL and (iv) HMP. The preliminary study also defined the range of viability for the different manufacturing parameters of an alginate-based PCC (Poured Clay Concrete), i.e. the minimum and maximum acceptable levels for each of the parameters. Table 4 summarizes the values tested by the design of experiments. The minimum SHMP ratio has been chosen at 2% since the initial reference ratio of 1.63% was too low to avoid early setting of the concrete. The selected W/B ratios are rather high in order to ensure that a high fluidity concrete would be achieved in all cases.

A first-order evolution, i.e. without curvature, between the minimum and maximum levels is assumed, the study will therefore be limited to two levels per parameter. Then an orthogonal Taguchi array of type L8 (2^4) is employed. In such arrangement each parameter tested has the same weight. No replication of experiments has been carried out. The response factors should allow an evaluation of the feasibility of casting then dismantling 24 hours after the formwork removal. The first ones are measurements of consistency and workability via slump tests at different times during the use of the concrete: before adding the acid, then after the last mixing at times t = 0 min, 20 min and 45 min. The last response parameter is the mechanical compressive strength of the concrete after 24 hours with no water loss.

Figures 2 to 4 give the main effects plots for each selected response, they show how each factor affects the selected response in comparison to the mean value. Figure 2 shows the effect of the different parameters on slump after the first mixing, i.e. before the addition of GDL acid. These results confirm that the chosen values are now suitable in achieving a high workability of the mix. They are all of S5 class. At this step, it seems that only the W/B and alginate ratio have an incidence. Their effect is weak and at a level close to the uncertainty of the test [START_REF] Larrard | Concrete Mixture-Proportioning -A Scientific Approach[END_REF].

After acid addition (figure 3a), i.e. at the beginning of the use of the mix, all the tested conditions show that an S5 consistency is retained. This contrasts with the observations made during the preliminary experiments and it is a confirmation that the chosen SHMP ratios are able to ensure a high initial workability. The slight effect of the water content is not surprising due to the high level of the chosen W/B ratios. Moreover, the high slumps value obtained, with average values ranging from 220.4 (without GDL) to 217.5 mm (with GDL), are close to the limit of the method for an evaluation of the behaviour of fresh concrete [START_REF] Larrard | Concrete Mixture-Proportioning -A Scientific Approach[END_REF]. More surprising is the predicted effect of the alginate ratio. Indeed, an increase of the slump when the alginate ratio raises from 0.3 to 0.5 % is anticipated, as in the preliminary tests, a strong degradation of the slump for an increase of 0.3 to 0.75% g/g of alginate is observed.

After 20 min the slump average value is clearly degraded and then reaches 100 mm. However, all the parameters show much more marked effects. The strong setting retarding effect of the SHMP is now the predominant parameter even before the W/B ratio. The higher ratio of this chelating agent (2.5%) is then required to maintain a high workability. The alginate ratio always shows an inverse tendency to what was observed in the preliminary tests. Moreover, this goes in contradiction with what can be anticipated by considering the effect of alginate which clearly increases the viscosity of the interstitial fluid [START_REF] Palluault | Nouveaux traitements de surface respectueux de l'environnement par des gels polymères réticulables, Application à la préparation des surfaces d'usage dans le secteur aéronautique[END_REF]. The effect of the GDL ratio, although noticeable, is small compared to the other parameters. It shows a logical evolution, a high GDL ratio allows a greater release of Ca 2+ , which leads to a decrease of the fluidity of the concrete. At 45 min, the average slump has dropped to 78 mm. This coincides with a further increase in the effect of SHMP and GDL ratio. On the contrary, the alginate and the W/B ratios show a lower incidence. At this point, as the setting caused by the alginate physical cross-linking becomes the major limiting phenomenon for slump, the two parameters (SHMP and GDL ratio) directly related to it become logically predominant. On the contrary, water no longer becomes a major parameter because at 45 min the setting by cross-linking of the alginate can begin the formation of a 3D network.

Since the main objective is to obtain a high workability, an important prediction of the design of experiment is that it appears possible to obtain a workability of over 45 min with a S5 consistency class. This is a theoretical result obtained by considering the sum of the favorable evolution of each parameter. SHMP ratio is the determining parameter for this, due to its Ca 2+ sequestrating action.

The compressive strengths predicted on moist specimens are given in figure 4. The average value, 35 kPa, is lower than the targeted value. This is to be compared with the value of 92 kPa previously obtained on mortar [START_REF] Pinel | Towards poured earth construction mimicking cement solidification: demonstration of feasibility via a biosourced polymer -Materials and Structures[END_REF]. The effects observed on the mechanical strength after 24 hours led to small variations, of the order of only a few kPa, which limits the relevance of the analysis. As expected the resistance increases with the alginate ratio and decreases with SHMP and W/B ratios. The effect of an increase in the SHMP ratio to 2.5% shows that the capture of Ca 2+ ions, if it increases workability, has a negative effect on the cross-linking after 24 h of the polymer and therefore on the mechanical strength. The effect of GDL is negligible here in view of the error bars. The low range of strength obtained on the concrete scale derives from less favorable value of the W/B, alginate and SHMP ratios which were respectively of 0.50, 0.60 and 1.63 % g/g in the optimal mortar mix.

One of the most useful insights from this study is the effect of the alginate ratio. It appears that a ratio of 0.5% is positive both in terms of the workability, and the 24 h moist mechanical strength.

While the effect on mechanical strength is clear, the effect on the rheology of fresh concrete is more difficult to be explained. However, Kuo and Ma [START_REF] Kuo | Ionically crosslinked alginate hydrogels as scaffolds for tissue engineering: Part 1. Structure, gelation rate and mechanical properties[END_REF] made a similar observation on alginate gels also obtained using a similar internal gelling method, in an aqueous medium but without mineral fillers. They suggest that a higher alginate concentration, by inducing a more pronounced viscosity of the solution, leads to a slower arrangement of the polymer chains to form the junction zones required for the interactions with calcium and thus for gelation. Considering the observations of the preliminary tests, it can be assumed that with alginate ratios significantly higher than 0.5 % g/g, this positive effect on workability is ultimately masked by the increased viscosity of the interstitial liquid. An optimum ratio between 0.5 and 0.75 % g/g which would improve mechanical resistance without affecting workability is therefore achievable. An alginate 11 / 27 -SHMP interaction can also be questioned since the only point that really changes is the SHMP ratio which is strongly increased. Further trials to complete the experimental design and to test this hypothesis are necessary. So finally, the optimal formulation predicted by the experimental design corresponds to formulation #3 (table 4).

Realization of low wall demonstrators

As the objective was the realization of a demonstrator in the form of a low wall (38 cm x 45 cm x 8 cm), we have given priority here to obtaining an optimum in terms of concrete working conditions, i.e. to obtain a workability of more than 45 minutes with a consistency of S5. This motivated the choice of formulation #3 (see table 4). No further test for the optimum validation is therefore required. A new batch with a volume of 20 l suitable for pouring low walls has therefore been processed. According to compression tests on standard specimens, the material has a strength of 38 kPa 24 hours after pouring, theoretically allowing a 1.5 m height wall to support its own weight.

The concrete batch was easy to place by pouring and then clamping with a vibrating needle as it could be done on site. After levelling, the formwork was covered with a plastic film to prevent drying from interfering with the gelation of the material. It was possible to remove the formwork from the wall within 24 hours thanks to the liquid-solid transition provided by the alginate. The concrete thus does not adhere to the formwork and has a relatively smooth surface (figure 5). Slight swelling was observed at the level of the wall web before demoulding; this disappears once the wall is removed from the mould. This swelling is due to the release of CO2, probably in the form of microbubble, during the CaCO3 dissociation in the acidified medium. There is legitimate concern about this swelling and its effect on the formworks. However no abnormal deformation of the formworks were observed.

Comparative evaluation of the production costs of alginate based Poured Clay

Concretes

The cost of an alginate stabilisation is evaluated here in comparison to that of a cement stabilisation (table 5) . This is done on the basis of a clay concrete prepared from a natural soil without granular correction. The aim is to simply give a first idea of the price difference between the two solutions.

The estimate for alginate stabilisation is made on the basis of the optimal composition of the experimental design used to cast the low wall. For the comparison with cement stabilisation, we consider an addition of 6 to 10 wt.% in relation to the dry mass of the natural soil, as commonly reported in literature [START_REF] Reddy | Low embodied energy cement stabilized rammed earth building-A case study[END_REF][START_REF] Jayasinghe | Compressive strength characteristics of cement stabilized rammed earth walls[END_REF][START_REF] Morel | Building houses with local materials: means to drastically reduce the environmental impact of construction[END_REF]. We did not consider the addition of SHMP or any superplasticizer in the case of cement stabilisation, as this type of addition is generally not considered required in this case. For the different admixtures, the prices per kg indicated in the table are based on typical 2021 prices, for quantities of the order of one to ten tons. For the alginate, a pharmaceutical grade (HV Protanal) was used in this study, its price is of course very high. However, we have checked that it could be replaced by a low purity grade, I3G80 alginate from Alliance Gums and Industries for example. Mortars using this alginate have shown a workability and resistance to 24 hours equivalent to HV Protanal [START_REF] Van Olphen | An Introduction to Clay Colloid Chemistry[END_REF].

Alginate and GDL are much more expensive than cement. Nevertheless the quantities used for 1 m 3 of earth concrete is low, which limits their price impact. If the lowest cement stabilization ratio is selected, then an alginate stabilisation would increase the total price by only 8 %. On the other hand, considering the higher cement ratio, alginate stabilization would be cheaper by more than 30%.

It should be noted that the cost of HMP appears to be the second highest among the additives. Its overdosage (relative to its "gelling retarder" action for alginate) therefore has a significant impact on the cost of the material.

In any case, if the alginate causes a limited extra cost compared to cement stabilisation, the interest of its use must be evaluated in relation to the overall specifications: the short and medium-term resistance, hygrothermal properties, embodied energy, its renewable and bio-sourced characteristic, etc.

Conclusion and perspectives

Concretes based on clay binders with a setting time similar to that of cement concretes have been developed from a clay mortar with the same characteristic. This setting is linked to the controlled cross-linking over time of a bio-sourced additive, alginate, by calcium ions, naturally present in the mineral products used. This experimental formulation of clay concrete is very fluid (class S5) and can be worked for more than 45 minutes. It has enabled the manufacture and demoulding of a 1/2 scale demonstrator of the low wall type. These results allow a first validation of the concept of PCC.

One of the main lessons of this extrapolation is that a significant contribution of calcium ions can come from the aggregates. We can therefore expect strong variations in the availability of these ions depending on the used products. This has led us to develop a solution that allows us to smooth out the calcium intake and thus ensure a constant setting time. This consists of a simple adjustment of the SHMP ratio, the calcium chelating agent. This correction can easily be done on site. Then, SHMP appears to be a determining parameter for maintaining workability, in particular it offers a significant robustness against composition variations of the raw materials.

These results therefore open up the prospect of carrying out a trial on a scale 1. Nevertheless, to achieve this new objective, it will be necessary to improve the 24 hour compressive strength of the wet concrete. The value obtained here allows a 1.5 m high wall to support its own weight, without any consideration of a safety margin. In order to improve the strength of wet clay concrete, initial guidelines have been given, in particular a reduction in the W/C ratio and the transition to an S4 class, that will be sufficient for a pouring process. A full validation of the PCC concept will then require being able to extrapolate this formulation to natural soils that will only be modified by a granular correction.

Figure captions

Fig. 1 :Fig. 2 :Fig. 3 :Fig. 4 :Fig. 5 :

 12345 Fig.1: Effect of the W/B ratio content on slump of clay concrete formulations without addition of GDL acid at alginate ratios of 0.15, 0.3 and 0.75%.

Figures

  Figures

Fig. 1 :Fig. 2 : 5 S 4 S 3 Fig. 4 :Fig. 5 :

 1254345 Fig.1: Effect of the W/B ratio content on slump of clay concrete formulations without addition of GDL acid at alginate ratios of 0.15, 0.3 and 0.75%.

Table captions Table 1 :

 captions1 Concretes formulations without GDL addition

Table 2 :

 2 Concretes formulations with GDL addition

Table 3 :

 3 The indicated Ca 2+ /H + molar ratio is obtained by considering that one mole of GDL releases one mole of H +

Table 4 :

 4 Table of Taguchi design of experiment with the parameters and levels tested.

Table 5 :

 5 Cost estimation for the production of one cubic meter of clay concrete, stabilized with cement or alginate

Table 3 :

 3 Evolution of Ca 2+ released as a function of sand origin with or without GDL addition (the indicated Ca 2+ /H + molar ratio is obtained by considering that one mole of GDL releases one mole of H + ).

		Without GDL		With 200 mM GDL
		Surnatant pH	[Ca 2+ ]	Surnatant pH	[Ca 2+ ]	[Ca 2+ ] / [H + ]
	CEN sand	8.6	0.4	3.5	48	0.24
	CEMEX sand	8.2	0.8	6.1	90	0.45

Table 4 :

 4 Table of Taguchi design of experiment with the parameters, levels and responses for the 8 experiments. The predictions given in fig. 2, 3 and 4 are calculated on the basis of these results.

	exp.		Parameter & level		Slump observed at t (min)	Moist state strength
	#	W / B ratio	Alginate % g/g	GDL % g/g	HMP % g/g	0	20	45	R24h (kPa)
	1	0.67	0.3	1.35	2.0	200	5	0	37.5
	2	0.73	0.3	1.35	2.0	225	0	0	34.0
	3	0.67	0.5	1.35	2.5	230	225	219	38.2
	4	0.73	0.5	1.35	2.5	228	225	225	34.5
	5	0.67	0.3	2.0	2.5	190	0	0	33.0
	6	0.73	0.3	2.0	2.5	225	222	180	21.6
	7	0.67	0.5	2.0	2.0	224	0	0	41.7
	8	0.73	0.5	2.0	2.0	225	120	0	38.0