The role of intrinsic soil properties in the compressive strength and volume change behavior of unstabilized earth mortars

Despite the growing interest in earthen construction, there is critical lack of reliable experimental data on the soil properties which mostly affect the engineering characteristics of the dried building material. Therefore, the main objective of this research was to explore the influence of some of these properties, namely clay fraction content, Specific Surface Area (SSA), Cation Exchange Capacity (CEC), chemical and mineralogical composition and various forms of iron and calcium carbonate on earth mortars. The initial trigger for this research was the extraordinary compressive strength of four earth mortars prepared with different soils. So, these four soils, along with seven others from previous research, were thoroughly examined using soil science techniques to investigate the link between soil properties and compressive strength and linear shrinkage of earth mortars. A relationship between the compressive strength to CEC ratio and dry density was found, highlighting the decisive role of clay activity as expressed by CEC, in earthen materials properties. According to linear regression and dominance analysis, the strongest correlation was exhibited by SSA followed by CEC, demonstrating that compressive strength is largely dependent on these two properties. Less strong correlation was found for clay fraction content, while poorly ordered/amorphous iron oxides were found to correlate with strength and shrinkage, but their contribution requires further research. Regarding the mineralogical properties, it was found that the mortars that achieved the highest strengths contained poorly crystalline smectite clays. Finally, even significant differences in soil chemical composition did not necessarily lead to different mortar properties.