Comparing Phase Development and Rheological Properties of OPC Paste Within the First Hour of Hydration

Abstract

In this paper the very early hydration of ordinary Portland cement at 20 °C and 30 °C is investigated, focussing on the first hour after the addition of water. The development of the rheological behaviour of cement paste during hydration is compared with the kinetics determined by quantitative in-situ X-ray diffraction (QXRD) and heat flow calorimetry. For both temperatures the cumulative heat correlates very well with the evolution of ettringite content over the first hour of hydration. The precipitation rate of ettringite is strongly influenced by temperature. This results in a higher formation rate of ettringite already in the first minutes after mixing at 30 °C. Analogously the temperature affects the rheological behaviour of the cement paste. The measured torque values are also higher for 30 °C from the first point of investigation on. Furthermore, the increasing torque can be linked to the rise in the ettringite content at both temperatures. The formation of other hydrate phases influencing the development of the rheological behaviour within the observed time frame cannot be proven with the data at hand. Therefore, it can be assumed that ettringite content is the determining mineral for the early workability of cement pastes.