Relating Compressive Strength to Heat Release in Mortars

Abstract

Conventionally, isothermal calorimetry and ASTM C186 heat of hydration results are reported on the basis of heat per mass of cement (powder), with typical units being Joules per gram (of cement), for example. Given that it is the filling of porosity with hydration products that is chiefly responsible for strength development in cement-based materials, there might be merit in instead reporting these results in terms of the unit volume of (initial) water. This paper examines a database of well over 200 mortar mixtures to investigate the relationship between heat release and mortar cube compressive strength development. For reasonably low water-to-cementitious-materials ratios (w/cm) (w/cm < 0.43), a single universal straight line relationship with some scatter is obtained. Based on numerous experimental data sets and the accompanying theoretical computations, the effects of the w/cm, sand volume fraction, cement chemical composition, sulfate content, cement fineness, incorporation of a high range water reducing admixture, and curing conditions on this universal relationship are all considered. Fifty data points from the Cement and Concrete Reference Laboratory proficiency sample program were analyzed in order to develop a linear relationship between ASTM C109 mortar cube compressive strengths and ASTM C186 heats of hydration at 7 d and 28 d. The application of this relationship for virtual testing is also evaluated. In this case, computer simulations would be employed to predict the heat of hydration versus time for a particular cement, and the developed equations would be employed to convert this heat release to a strength prediction at the age(s) of interest. In general, it appears that these relationships can be used to predict mortar cube compressive strengths based on measured heats of hydration, within about ±10 % of the experimentally measured strengths. A preliminary analysis of a single dataset for concretes with and without limestone replacement for cement indicates that the linear relationship between strength and heat release likely holds for concretes as well as for mortars.