Effect of calcium alumina silicate hydrate nano-seeds on the hydration of low clinker cement

The replacement of cement clinker with supplementary cementitious materials (SCMs) is considered a very effective way of carbon reduction. However, the slow pozzolanic reaction of SCMs weakens the early strength, which hinders its application. In this study, the dedicated nucleating agents for Al-rich SCMs, C-A-S-H nano-seeds, were synthesized by co-precipitation using polycarboxylate superplasticizer (PCE) as dispersant. The performances of four nano-seeds with different Al/Si and their effects on hydration in 50% metakaolin cement (MK50) were investigated in detail. The results showed that C-A-S-H nano-seeds had a greater strengthening effect than C-S-H nano-seeds especially at early ages, and also accelerated setting, with Al/Si = 0.10 being the most significant. The Al/Si = 0.10 nano-seeds advanced the second main exothermic peak by 6.7 h, promoting the production of stratlingite and causing the highest contents of CH and bound water in hydrated samples. It was also demonstrated by 27Al NMR that C-A-S-H nano-seeds enhanced the reaction degree of metakaolin by producing more C-A-S-H than AFt or AFm. The pozzolanic activity was further explored in the reaction between metakaolin and CH. This study aims to develop an efficient strengthening agent (i.e., C-A-S-H nano-seeds) for low clinker cement, and to reveal the mechanistic difference between them and common C-S-H seeds from the perspectives of hydration and pozzolanic reactions, which is of great significance for the carbon emission reduction of cement industries and solid waste utilization.