Mechanical Properties and Behaviour of Stabilised Soils

Coarse sand cannot sustain pozzolanic reaction owing to the small surface-area-to-volume ratio arising from their large particle sizes. However, natural sandy soils often have small amounts of fines, which may contain pozzolanic materials such as silica. Asghari et al. (2003) reported pozzolanic hardening of admixtures of lime and a sandy alluvium containing about 6% fines. In addition, Nakarai and Yoshida (2015) reported that sand–lime admixtures exposed to the atmosphere may undergo long-term hardening well beyond 28 days, owing to carbonation of the lime by carbon dioxide in the air. However, this is inapplicable to deep mixing where the lime is not exposed to the atmosphere. The most commonly used hardening agent for sand is Ordinary Portland Cement (OPC). Unless some pozzolans are also added, the primary hardening reaction is hydration reaction. The microstructural changes of cement-treated sand are not dissimilar to those of concrete. Subramaniam et al. (2018) noted that 1-day cured samples of OPC-treated sand comprised loosely packed assemblages of calcium silicate hydrates with small amounts of needle-shaped ettringite crystal. The ettringite is likely to have been produced by the reaction between tricalcium silicate and the small amount of gypsum (calcium sulphate), which is added to prevent “flash setting.” After 28 days, the hydration reaction is largely completed, and the calcium silicate hydrate assemblages are much more densely packed. By this time, the ettringite has been almost completely converted to monosulphate. Hence, the hardening effect of OPC on sand is derived mainly from the infilling of the large voids by cementitious products, mainly calcium silicate hydrate. This is similar to the gel-space theory ( Powers 1958 ), which states that the strength of hardened cement paste is proportional to the power of the proportion of space infilled by calcium silicate hydrate.