Hydration Behavior of Portland Cements with Different Hemihydrate/Gypsum Ratios in the Presence of Common Chemical Admixtures

Abstract

Abstract Interactions that take place between hydrating portland cement and chemical admixtures during the early hydration period affect the performance of concrete. Many of the early hydration reactions involve sulfate phases. Cement manufacturers intergrind calcium sulfate with clinker in the finish mill to provide control for the aluminate reactions during hydration. The calcium sulfate is typically added as gypsum, but some of this gypsum can dehydrate to calcium sulfate hemihydrate or soluble anhydrite, depending on the conditions in the finish mill. The final form of the calcium sulfate may affect the hydration characteristics of the cement in the presence of various chemical admixtures. This paper reports the results of studies of the performance of cement paste and mortar. Three cement samples were manufactured from a single clinker with the same amount of interground calcium sulfate but different relative ratios of gypsum and calcium sulfate hemihydrate. The clinker was manufactured to produce low-alkali Type V cement. The total calcium sulfate addition was approximately 3.7 %, and the amount remaining in the cement as gypsum was 47 %, 31 %, or 11 % of the total amount of gypsum and hemihydrate. Common chemical admixtures were studied, including lignosulfonate, naphthalene sulfonate, polycarboxylate, formulated Type A (non-lignosulfonate), and formulated non-chloride accelerator. Cement pastes that were prepared using the cements and admixtures were tested for workability, workability retention, and hydration profile with isothermal conduction calorimetry. Mortars that were prepared from the cements and admixtures were tested for workability, workability retention, air content, setting time, and early and late-age compressive strength. Initial indications are that the initial workability and workability retention of cement paste in the presence of admixtures are affected by the form of the calcium sulfate. The time of setting and strength development of mortar are affected by the form of the calcium sulfate as well.