Influence of metakaolin on the hydration and microstructure evolution of cement paste during the early stage

The incorporation of metakaolin into cementitious materials could improve the compressive strength and durability of cement paste, which are correlated well with the hydration behavior of cement-metakaolin blended mixture. The aim of this work is to reveal the coupled effects between the cement hydration and the pozzolanic reaction of metakaolin during the early stage, the methodologies including fluidity, setting time, compressive strength were employed to measure the properties of cement paste, while another methodologies, such as conductivity, Mercury Intrusion Porosimetry (MIP), Thermogravimetric (TG), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were adopted to investigate the hydration behavior and microstructure characterization. The results indicated that the fluidity of fresh cement paste decreased sharply with the increase of metakaolin, and the addition of superplasticizer could improve the fluidity of fresh cement paste effectively. The setting times of fresh cement paste were delayed slightly with the incorporation of metakaolin. The conductivity of cement suspension dropped gradually with the increase of metakaolin during the early stage, meaning the hindrance to the ion dissolution from cement-metakaolin blended mixture. The compressive strength increased slightly with the addition of metakaolin at 1 d, while it increased remarkably at 3 d. The results of MIP presented that the pore structure was refined with the increase of metakaolin at 3 d. The TG results showed that the incorporation of metakaolin promoted the formation of hydration products at 3 d. The XRD results presented that calcium hydroxide (CH) decreased with the addition of metakaolin, which may be consumed by the pozzolanic reaction of metakaolin. SEM observation confirmed that the microstructure of cement paste became much denser with the increase of metakaolin, which was in good agreement with the results of compressive strength and MIP measurement.