Effect of gypsum on early hydration of amorphous C₁₂A₇ and its mechanism

Abstract

In order to promote the efficient application of amorphous C12A7 in early strength and rapid setting engineering materials, and understand the interaction mechanism between gypsum and amorphous C12A7, the influence of gypsum on the early hydration of amorphous C12A7 was studied by X-ray diffraction, scanning electron microscopy and isothermal calorimetry. The results show that proper dosage of gypsum can retard the hydration of amorphous C12A7, with the increasing dosage of gypsum, the time of the first exothermic peak is retarded, the second exothermic peak of hydration is reduced, and the duration of exothermic peak is prolonged; After 24 hours hydration, the accumulate heat of hydration of the sample with 50 % gypsum are the highest. In the experiment, ettringite and AH3 gel are the main hydration products. The content of gypsum and hydration ages can change the morphology of ettringite, and affect the compressive strength of the sample. At the same age, with the increase of the dosage of gypsum, the compressive strength of the hydrated pastes increases from 33 % to 50 % and then decreases from 50 % to 64 %. Amorphous C12A7 with 50 % gypsum system has the highest strength at day 1, day 3 and day 7.

Keywords

• gypsum
• amorphous c₁₂a₇
• hydration products
• ettringite
• strength