Heats of hydration and pozzolan content of portland-pozzolan cements

Abstract

Methods are described for the measurement of the approximate heat of solution in acid of portland-cement^-pozzolan mixtures. Examples are given of the estimation of the heat of hydration of portland-pozz olan cements. Determinations of the amount of material undissolved in the calorimeter, as well as the heats of solution, were applied to the problem of determining the composition of mixtures of portland cement and pozzolan. Data are presented for the calculation of the heat capacity of the vacuum-flask calorimeter with amounts of hydrofluoric acid other than the 8-milliliter quantity used in measuring the heat of hydration of portland cement by the standard method. 1. Introduction In recent years pozzolans have been widely used with portland cement because of their favorable effects on many of the properties of concrete. Many methods of test of cement or of concrete are not affected by the presence of the pozzolan, which contains a large percentage of silica. The measurement of the heat of hardening [1, 2] l of a portlandpozzolan cement by the heat-of-solution procedure however, is less reliable than the measurement for ordinary portland cement because of the presence of material not readily soluble in acid. The heat-ofsolution method depends on the measurement of the heats of solution in acid of a sample of dry cement and of a corresponding sample of hardened paste. The difference between the two heats of solution is the heat of hardening of the cement paste. A mixture of nitric and hydrofluoric acids, in which nearly all portland cements will dissolve rapidly and completely, is used. Considerable portions of many pozzolans, however, are not readily soluble, and for even approximately complete solution far more time would be required than is desirable in the usual procedure for portland cement. The Bureau has occasionally measured the approximate heat of solution or hydration of pozzolan cements in a variety of investigations scattered over a period of nearly two decades. This paper presents the results of tests made by methods devised to reduce the effect of the difficultly soluble material. In addition to the problem of measuring the heat of hydration of portland-pozzolan cements, there also exists, perhaps more urgently, the problem of determining the pozzolan content of such blends. In the course of the calorimetric measurements, it was observed that part of each pozzolan was undissolved. It was also observed that the heats of solution of the cement-pozzolan mixtures, as well as the percentage of undissolved material, were related to the pozzolan content. So that these data might not be lost, they have been included in this paper, and their application to the measurement of the pozzolan content of portland-pozzolan cements is discussed.