Alkaline activation of fly ash was used to improve the mechanical performance of a silty sand, considering this new material as a replacement of soil-cement applications namely bases and subbases for transportation infrastructures. For that purpose, specimens were molded from mixtures of soil, fly ash and an alkaline activator made from sodium hydroxide and sodium silicate. Uniaxial compression tests showed that strength is highly increased by the addition of this new binder. The results described a high stiffness material, with an initial volume reduction followed by significant dilation. All specimens have clearly reached the respective yield surface during shearing, and peak strength Mohr-Coulomb parameters were defined for each mixture. The evolution of the microstructure during curing, responsible for the mechanical behavior detected in the previous tests, was observed by scanning electron microscopy. These results were compared to soil-cement data obtained previously with the same soil at similar compaction conditions. The main difference between both binders was the curing rate, with alkali activated specimens showing a more progressive and long-lasting strength increase. This was analyzed taking into account the chemical process responsible for the behavior of the mixtures.
The compression behavior of an artificially cemented soil was analyzed by means of the adjusted porosity/cement ratio proposed using a correlation established in the recent literature.It was found that for each value of this parameter, defined as the ratio of porosity to the volumetric cement content, there is a unique Normal Compression Line (NCL). The NCLs of the cemented specimens for each adjusted porosity/cement ratio do not converge with the NCL of the uncemented silty sand at large stresses, but reach a line parallel to it, the lowest the adjusted porosity-cement index, the further the NCL of the cemented sand from the NCL of the uncemented sand.3
Alkali activated cements (AAC) have been extensively studied for different applications as an alternative to Portland cement (which has a high carbon footprint) and due to the possibility of including waste materials such fly ash or slags. However, few works have addressed the topic of stabilised soils with AAC for unpaved roads, with curing at ambient temperature, where the resistance to wetting and drying as well as the mechanical properties evolution over time is particularly relevant. In this paper, a silty sand was stabilized with an AAC synthetized from low calcium fly ash and an alkaline solution made from sodium silicate and sodium hydroxide. The evolution of stiffness and strength up to 360 days, the tensile strength, and the performance during wetting and drying cycles were some of the characteristics analysed. Strength and stiffness results show a significant evolution far beyond the 28 th curing day, but still with a reasonable short-term strength. Strength parameters deduced from triaxial tests were found to be very high with stress-strain behaviour typical of cemented soils. Durability properties related to resistance to immersion and wetting and drying cycles were found to comply with existing specifications for soil-cement, giving validity for its use as soil-cement replacement.
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