Swelling Behaviour of Sulfate Soil Treated with Lime–Metakaolin at Different Curing Ages

Sulfate soil stabilisation, while offering technical benefits for infrastructure, is a challenging process, complicated by the nucleation of ettringite, an expansive mineral that can cause soil deterioration. This study was undertaken to elucidate the synergistic effect of lime and metakaolin on the physico-mechanical performance of high-sulfate-bearing soil. The binder content in the stabilised specimens was fixed at 20 wt%, and metakaolin was used to partially substitute lime at different substitution levels. The physico-mechanical investigation revealed that supplementation of lime with metakaolin had a promotional effect on the unconfined compressive strength and swelling potential. The threshold of this effect was obtained by a binary blend of 7.5L–12.5MK, where the UCS was increased fourfold, while the swelling potential was reduced to a near-zero magnitude of 0.33%. This superior performance is due to the fineness and high reactivity of metakaolin, as both limit the nucleation of ettringite and promote the neoformation of further hydrated compounds, thus yielding a denser interlocked system and increasing its resistance to water soaking.