Abstract
AFt is one of the major products at the early stage of cement hydration. It is an important product that influences the performance of the fresh and hardened cement pastes such as the setting time. However, there is a lack of detailed investigation on the growth of AFt in the cement pastes with a long-time scale. In this work, we reported a detailed analysis by using in-situ powder X-ray diffraction (XRD) on the growth of AFt in the cement pastes during hydration. Samples of the hydrated ordinary Portland cement (OPC) and another locally produced Portland cement with very high tricalcium silicate (C3S) content with different water–cement (w/c) ratios were investigated continually till they were hydrated for about 270 days by powder XRD. The work shows that during Portland cement hydration, the AFt reaches its maximum content with very high speed within about 24 h, which is influenced by the content of C3S in the raw cement samples and the w/c ratios of the cement pastes. Once the maximum content of AFt was reached, it decreases very fast within the following couple of days, and then decreases slowly and finally reaches a stable level at the late stage of hydration. The results also present that a lower w/c ratio is beneficial to the formation of AFt and the conversion of AFt to AFm as well. While higher w/c ratios are favorable for the AFt to remain stable in the hardened cement pastes.
Highlights
AFt, the most important phase of which is ettringite, is one of the major determinants of the hydration properties and performance of Portland cement at early stages such as the initial setting time [1] and the early strength [2] of the hardened cement pastes
AFt is a major product generated from the hydration of calcium sulphoaluminate cement, expansive cement and rapid hardening cement [6,7,8]
It is found that AFt forms as long as there is the existence of gypsum, i.e., the presence of sulfate ions, in the cement pastes [17]
Summary
AFt, the most important phase of which is ettringite, is one of the major determinants of the hydration properties and performance of Portland cement at early stages such as the initial setting time [1] and the early strength [2] of the hardened cement pastes. Except for the typical needle-shaped crystals [19,21], AFt could be spherical [22], cubic-shaped [19] or in small massive clusters [3,23] Most of these studies are focusing on the AFt formation mechanism or structure investigation [24]. It provides experimental proof to enrich the formation and growth mechanism of AFt during cement hydration
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