Experimental study on material ratio and strength performance of geopolymer-improved soil

Geopolymer binder is a kind of green cementitious material with fast hardening, high strength, low shrinkage, and acid-alkali corrosion resistance. It is very beneficial to ensure the firmness, stability, and durability of subgrade structure in cold regions. In this study, the unconfined compression strength test of the clay soil improved by metakaolin-based geopolymer binder was carried out. The material ratio of the metakaolin-based geopolymer binder was analyzed, the optimal mixing ratio of the metakaolin-based geopolymer binder in the clay soil was investigated, and the impacts of metakaolin and alkali-activator on the mechanical properties of the geopolymer-improved soil were discussed. Experimental results indicate that the unconfined compression strength of the geopolymer-improved soil increases first and then decreases with the contents of metakaolin and alkali-activator (a mixture of unslaked lime (CaO) and sodium bicarbonate (NaHCO3) with the mass ratio of 1:1). The ideal mixing ratio of metakaolin and alkali-activator in the geopolymer binder is about 2:1, and the cost-optimal mixing ratio of the geopolymer binder in the clay soil is about 12%. By comparing with pure clay soil, lime soil, and ordinary Portland cement soil, the strength performance and stabilization effect of the geopolymer-improved soil were further studied through unconfined compression strength test, direct shear test, and Brazilian splitcylinder test. Outcomes show that the unconfined compression strength, shear strength, and Brazilian splitting strength of the geopolymer-improved soil have good advantages over the other three kinds of soils. The microstructure analysis by SEM with EDS illustrates that the agglomeration and stabilization effects of the geopolymer-improved soil, ordinary Portland cement soil, lime soil, and pure clay soil are weakened in turn, and the polymerization of the geopolymer binders can be in progress in the alkali-activated environment formed from the hydration of CaO and its reaction with NaHCO3. The failure of the geopolymer-improved soil with the increasing contents of metakaolin and alkali-activator shows that there is a tendency from plastic shear failure to brittle split failure. The results of this study can provide a parameter basis for the application and popularization of the soil improved by metakaolin-based geopolymer binder in engineering. (C) 2020 Elsevier Ltd. All rights reserved.