Shrinkage compensation design and mechanism of geopolymer pastes

Abstract

Geopolymers (GI) possess excellent seawater erosion resistance because of stable hydration products and compact microstructures, and therefore, they have broad application prospects in marine engineering if their large volume shrinkage behavior can be mitigated. In this study, active MgO was used to compensate for different GI shrinkage stages and a scheme was proposed for staged compensation. Autogenous shrinkage was controlled by adjusting the content, activity, and combinations of active MgO with particle sizes of 1–100 µm. Based on the effects of MgO on shrinkage, GI paste composition and microstructure, the GI reaction process and mechanism of MgO-compensated volume shrinkage were examined and summarized. The results showed that the GI paste shrinkage decreased with MgO addition, and the higher the MgO activity, the smaller was the paste shrinkage in the early stages of hardening. Late-stage shrinkage of hardened paste was improved with the addition of low-activity MgO. Combining low and high activity MgO addition effectively ameliorated the paste hardening shrinkage at different stages. In the high-alkalinity environment of the liquid-phase GI reaction, the Mg(OH)2 product presented microcrystalline structures, and it was dispersed in the cement matrix, which resulted in uniform volume expansion. Meanwhile, other reaction products containing magnesium were produced, filling pores in the hardened paste and effectively compensating for volume shrinkage during the hardening process. Based on the effects of MgO on the composition structure, reaction process, and volume shrinkage performance of GI slurries, this study proposes a mechanism for MgO to compensate for the volume shrinkage of GI.