Zinc Oxide as a Standard Substance in the Solution Calorimetry of Portland Cement

Abstract

Zinc oxide is the standard substance specified for calibrating the heat-of-solution calo­ rimeters used in determining the heat of hydration of portland cement in several American and foreign cement specifications. The heats of solution of zinc oxide samples from different sources and after different heat treatments have been determined in the standard mixture of nitric and hydrofluoric acids. It is concluded that the value given in the specifications is low, that heat treatment in the range 310 to 950 °C is not critical, that small variations in the ZnO/acid weight ratio are without significance, and that analytical-reagent zinc oxide from various sources may safely be used. The mean value obtained for the heats of solution of zinc oxide from twelve sources determined in triplicate in 1.00HNO 3,0.284HF,26.38H2O at a ZnO/acid weight ratio of 7/425 at 25 °C was 257.82 cal/g with a standard error of 0.015 cal/g. The thermochemical calorie of 4.1840 joules is used. The temperature coefficient based on least-square fitting to 16 data points obtained in an earlier study was —0.087 cal/g-deg, with a computed standard error of 0.013 cal/g-deg. The corresponding values for heat of solution and temperature coefficient given in the specifications are 256.6 cal/g and — 0.1 cal/g-deg, respectively.