Study On The Influence Mechanism Of Metakaolin On The Cement Hydration Products

As a kind of mineral admixture, metakaolin has stable components, extensive sources and similar activity with silica fume. Therefore, it exists a great potential of using metakaolin as mineral admixture instead of silica fume in the production of high-strength concrete. In this paper, the cement mortar strength, water demand of cement standard consistency, setting time and carbonation resistance by adding metakaolin were discussed and further study on different systems metakaolin cement samples of the content of hydration products calcium hydroxide, calcium silicate hydrate (C-S-H) gel surface morphology, microstructure formation process and the chemical composition at different hydration age by XRD, TG-DSC, AFM, SEM-EDAX and other testing methods. The main conclusions are as following:(1) Adding metakaolin can improve the strength of cement mortar. When metakaolin dosage at6%, the cement mortar strength after3days increased6.9MPa, when metakaolin dosage at9%, the cement mortar strength after28days increased8.8MPa. Considering the development of early and late strength that the metakaolin content of6-9%is more appropriate.(2) Adding metakaolin can increase normal consistency water demand, the more volume of adding metakaolin the more water demand has also increased. When metakaolin content reaches15%, the normal consistency water demand increased from28.8%to32.2%. Mixing metakaolin can significantly shorten setting time, with the increase of metakaolin content, the setting time are beginning to decrease, metakaolin has accelerated clotting effect, can play a quick-setting effect. Adding metakaolin reduces the total cement paste chemically combined water. Metakaolin admixture had little effect on cements’carbonation resistance.(3) Adding metakaolin has not significantly effect on the composition of the hydration products, the hydration of cement pastes has begun to appear in a large number of needle-like ettringite, hexagonal calcium hydroxide crystals and flocculent C-S-H gel after3days. With the extension of hydration age, the structure of hardened cement paste was more dense, also increased a lot of hydration products. The content of hydration products calcium hydroxide was reduced by adding metakaolin, from19.79%to10.08%when metakaolin content reaches15%after60days. (4) With the extension of hydration age, the particle size of C-S-H gel was smaller, had clear boundaries between the particles, more closely gather. When mixed with9%metakaolin, the particle size of the sample was about50nm nano C-S-H gel together to form150-200nm particle size micron C-S-H gel structure after3days, the particle size of C-S-H gel was about40nm, nano particles formed, the micron structure no longer existed. Meanwhile, the calcium and silicate ratio has declined with the extension of hydration age and adding metakaolin, the calcium and silicate ratio of C-S-H gel from1.53to1.19when metakaolin content reaches15%after60days. The active ingredient SiO2of metakaolin and hydration products calcium hydroxide reacted to higher strength, better stability, low calcium and silicate ratio of C-S-H gel.