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.

Abstract:The treatment of soils with cement is an attractive technique when the project requires improvement of the local soil for the construction of subgrades for rail tracks, as a support layer for shallow foundations and to prevent sand liquefaction. As reported by Consoli et al. in 2007, a unique dosage methodology has been established based on rational criteria where the voids/cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength. The present study broadened the research carried out by Consoli et al. in 2007 through quantifying quantifies the influence of voids/cement ratio on the initial shear modulus ͑G 0 ͒ and MohrCoulomb effective strength parameters ͑cЈ , Ј͒ of an artificially cemented sand. A number of unconfined compression and triaxial compression tests with bender elements measurements were carried out. It was shown that the void/cement ratio defined as the ratio between the volume of voids of the compacted mixture and the volume of cement is an appropriate parameter to assess both initial stiffness and effective strength of the sand-cement mixture studied.

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

Unsaturated cemented soils are frequent both as designed materials and as naturally occurring layers. Both desiccation and cementation act separately as hardening mechanisms, but it is not clear how exactly their effects combine. Do they enhance one another? Are they mutually reinforcing? This study presents results from an experimental campaign aimed at answering these questions. Five different mixtures of soil (a granite saprolite) and cement (with cement contents in the range 0% to 7% on a dry weight basis) are tested in isotropic compression at four different water content levels. Initial void ratio is also controlled, using two initial compaction densities. Loading is performed at constant water content and suction is inferred from a set of water retention curves obtained from parallel psychrometric and pore-size distribution measurements. The range of yield stresses explored in this study covers almost two orders of magnitude and extends up to 7 MPa at suction values of up to 14 MPa. Both desiccation and cementation increase yield stress, but their effects are less marked when both act together, and therefore they are not mutually reinforcing.