Understanding the effect of alkali content on hydration, hardening, and performance of Portland cement – A comprehensive review

Abstract

Understanding the impact of alkali content in Portland cement on cement hydration is crucial for optimizing cement quality from the source and enhancing cement performance. This paper systematically elucidates the sources and content of alkalis in cement, as well as the effects of alkalis on hydration properties, strength, and durability. The aim is to comprehensively understand the mechanism of alkalis in cement, facilitating the rational use of cement with different alkali contents. The results indicate that alkalis in cement exist in both solid-solution and soluble forms. The effect of alkalis in cement systems is double-edged. An appropriate alkali content promotes the early hydration of Portland cement, enhancing early strength. Moreover, alkali with an appropriate content regulates the hydration rate of mineral phases. High alkali content inhibits the dissolution of initial C3A and the formation of AFt. With increasing alkali content, the hydration rate of cement accelerates, resulting in a shortened induction period that favors the development of early strength in cement. However, higher levels of alkali content inhibit the development of C3S crystals, with increasing alkali content resulting in reduced size and aspect ratio of C3S crystals. Secondly, high alkali content leads to an increased degree of supersaturation required for hydration product formation, slowing down the precipitation of hydration products, which is detrimental to the later-stage growth of hydration products. Especially prone to alkali-silica reaction (ASR), the expansion of the paste leads to the appearance of cracks and a decrease in porosity. This provides pathways for harmful ions such as chloride ions and sulfate radicals to enter the environment, thereby compromising durability. This review provides better theoretical guidance for improving cement quality and enhancing stability.