Correlation of microwave and conduction calorimeter measurements on the hydration of cement

Abstract

Ordinary Portland cement (OPC) is one of the most widely used construction materials. The hydration of cement involves very complicated chemical and physical processes. A better understanding of the mechanisms of hydration and microstructural development in the cement paste is important for improving its mechanical strength. There are two methods to study cement hydration in situ. One is conduction calorimetry [1] which is a well-established method, and the other is electrical measurement [2±7] which is not yet well established because of the dif®culty in directly relating the results of the electric measurement to the chemical and physical changes which occur during cement hydration. We have successfully developed a microwave technique to study the early hydration of OPC [8, 9], slag cement [8, 9], mortar [10] and OPC±high-alumina cement (HAC) blend [11] by measuring the change in dielectric constant during cement hydration. The heat evolution rate of OPC in the ®rst 30 h of hydration measured using conduction calorimetry is shown in Fig. 1. There is a large broad peak in the heat evolution rate curve, which corresponds to the chemical reactions that have occurred during cement hydration. At the same time the rate of change in dielectric constant, -da=dt, was monitored using the microwave technique and the result is also plotted in Fig. 1. It is interesting to see that the two curves have similar shapes and features, showing that there is a correlation between them for OPC. Since the traditional de®nition of the four periods of cement hydration is based on the heat evolution rate curve, if we can correlate the rate of change in dielectric constant curve with the heat evolution rate curve for any cement system, then we can apply this welldeveloped de®nition of the four periods of cement hydration to the rate of change in dielectric constant curve and hence provide an accurate interpretation of the dielectric constant results. This paves the way for a novel method in studying the cement hydration process in situ. In this letter, we explore the correlation between the results measured by microwave technique and the results measured by conduction calorimetry. Several samples, namely, OPC, OPC±HAC, OPC± silica fume, OPC±pulverized fuel ash (PFA) and slag cement, have been measured in this study (Fig. 2). We used the water cement ratio, w=c, of 0.4 and measured samples at a microwave frequency of 9.5 GHz. The details of the measurement have been given in [8±11]. For the OPC±30% HAC blend, there are two different peaks in the heat evolution