Investigation on the Unusual Hydration and Hardening Process of a Portland Cement with Low Alkali Sulfate Content

Abstract

The infrequent setting behavior and hydration process of a Portland cement with very low alkali content were concerned. Gypsum and K₂SO₄ with different quantities and ratios were added into cement to adjust its sulfate and alkali content. The setting time and compressive strength of cements were tested, the hydration process and hydrates of cements were investigated by isothermal calorimetry, non-evaporable water content and XRD. The results show that gypsum addition is incapable of delaying the quick initial setting of the cement clinker. A moderate substitution of gypsum by K₂SO₄ in same reasonable sulfate content can promote the dissolution of gypsum, suppress the formation of h-AFm, and therefore prolong the initial setting time of the cement. Moreover, the Portland cement clinker shows a very hysteretic hydration exothermic process, accompanying with a hysteretic finial setting and a weak mechanical property in early age. An appropriate dosage of gypsum can promote the hydration of the cement and result in a reasonable final setting behavior and satisfactory strength development. The modification of gypsum on the properties of cement is influenced by its alkali content. The measures which could accelerate the sulfate-supply in cement is necessary to acquire a favorable hardening property when the cement with low alkali content is used in projects.