Abstract: Laterite is very sensitive to water. This drawback can be substantially mitigated by adding alkaline materials like lime, but adding alkaline materials can impair the long-term performance of laterite due to the weak acidity of laterite. In this study, a certain amount of lime (5%) and metakaolin (4%) (La+L+MK) was added into the laterite to improve its water sensitivity and mitigate the deteriorative acid-base reaction. The mixtures with eight different initial water contents were compacted and cured at predetermined periods. After the curing, unconfined compressive strength (UCS), calcium ion concentration, electro conductibility and pH were tested. The results show that the UCS of the stabilized soil approached its peak at the water content of 26%, so it was not conducive to the strength development of the stabilized soil when the water content diverged from the optimal value. The reason is that when lime is not well hydrated due to lack of water, calcium ion cannot be released, thereby inhibiting the pozzolanic reaction. As a result, cementitious hydrates in the inter-aggregates of laterite cannot be generated. Also, the bonding strength increase due to the pozzolanic reaction is lower than the matrix suction loss caused by excessive moisture when the sample is too wet. The measurements of calcium ion concentration and electro conductivity confirmed the above conjectures. Our results clearly show that metakaolin combined with lime significantly improve the laterite strength at the wet state. Even after being saturated, the relative stability of the lime-stabilized soil can still be maintained with metakaolin additives, which indicates that metakaolin can effectively reduce the water sensitivity of lime soil and improve its durability. This phenomenon is due to the fact that metakaolin contains amorphous silicon and aluminum oxides and has edge-surface-contacted structures at the microscopic scale, thereby enabling it to significantly reduce the soil pH to the alkaline range that favors dissolution of silica and alumina oxides, thus accelerating the pozzolanic reaction and slowing down or inhibiting the deterioration reaction.

Key words: metakaolin, lime stabilized laterite, deterioration interaction, water sensitivity, calcium ion concentration