Sustainable construction materials and local solutions are crucial for meeting urbanisation demands in tropical regions like French Guiana. Poured earthen construction, primarily composed of local soil, offers a cost-effective and environmentally friendly alternative. To ensure good consistency for pouring, the use of dispersants to deflocculate clay particles is an effective strategy that reduces water demand while enhancing the material's physical and mechanical properties. However, the variability of soil impacts the effectiveness of dispersants and the earth's material properties. It is currently unknown how the specific properties of tropical soils can influence the mix design of poured earth and how it impacts their resulting physical and mechanical characteristics. This work examines the soil properties that could help select suitable soil for poured earth construction. Four tropical soils of French Guiana were characterised in detail and used to prepare mortars with varying amounts of water and dispersant (sodium hexametaphosphate). The consistency and compressive strength of the mortars correlated with the mineralogical and geotechnical characteristics of the soils. In particular, soils with high levels of metal oxides produced mortars with the highest compressive strength. Using a dispersant consistently increased the compressive strength and dry density of the mortars and enabled a reduction in water content. Importantly, the analysis revealed that two easily measurable soil properties, the passing at 63 mu m and the pH measured in potassium chloride (pH KCl), could predict the mechanical properties of the mortars and the amount of dispersant needed. Specifically, the pH KCl correlated with the proportion of iron and aluminium oxide and showed a positive linear correlation with the compressive strength. Overall, this study identifies easily measurable soil properties, in particular the pH KCl, for selecting and designing high-performance earth materials with tropical soils. It offers promising prospects for industrialising poured earth construction.

Tropical regions like French Guiana need local building materials to cope with high population growth and the high cost of imported cementitious materials. Poured earthen construction could represent a local, cost-effective, and ecologically friendly alternative. To ensure good workability and facilitate pouring, the use of dispersants to deflocculate clay particles is an effective strategy that reduces water demand and increases the material's density and strength. Natural organic dispersants can replace industrial ones while reducing costs and carbon footprint. It is currently unknown how organic dispersants could improve the workability, physical and mechanical properties of the iron-rich lateritic soils present in French Guiana. Here, different potential dispersants were evaluated at constant water content on a lateritic soil-based mortar: citric acid, sodium carbonate, tannins, tannins+sodium hydroxide (tannins+NaOH), tannins+sodium carbonate (tannins+Na2CO3). These dispersants were compared to industrial sodium hexametaphosphate (NaHMP). Three types of tannins were tested: hydrolyzable tannins from oak and chestnut, and condensed tannins from acacia. This study shows that all formulations improved workability and mechanical strength but only tannins+NaOH or Na2CO3 had a strong dispersant effect comparable to NaHMP. Furthermore, tannins+NaOH or Na2CO3 decreased the mortar's density without impacting strength, which may result from reactions between the soil's iron oxides and tannins, as observed by infrared spectroscopy (FTIR). Altogether, these results show that the addition of organic dispersant is an appropriate strategy to improve the fresh and hardened properties of lateritic soils. Particularly, tannins combined with sodium carbonate may represent an eco-friendly dispersant for poured earth in regions with iron-rich lateritic soils.

As a raw material for additive construction, earth offers a multitude of benefits, from environmental and economic to social points of view. However, the fresh-state properties of printable materials and the curing conditions of additively manufactured elements make large-scale 3D-printed earthen structures susceptible to suffering severe cracking from shrinkage during drying. This project investigates the effect of soil composition and water content on the development of drying shrinkage cracking in 3D-printed earthen structures. This article presents two strategies for minimizing those cracks: decreasing the clay content of the soil by adding fine sand and decreasing the required water content for printability by using a clay dispersant agent. Earth-basedmix designs with different soil/fine-sand ratios and sodium hexametaphosphate (SHMP) contents were subjected to flow table, rotational rheology, and shrinkage cracking tests. The results indicate that the clay and water content are determining factors that minimize the appearance of cracks due to drying shrinkage. Two earthen-based formulations with zero cracks due to shrinkage resulted from replacing 50% wt. of the soil with fine sand and the addition of 0.55 and 2.20% wt. of SHMP. Further research is needed to confirm the validity of these findings across diverse soil types and curing conditions.