The h eat of formation of 3CaO· Ab03 ·CaC03·10. 6SH, O at 25°C was determined by the heat-of-solution method, with 2N HCI as the solvent and 3CaO·Alz0 3·6H20 and CaC03 as the reactants. The heat of solution of CaC03, to form di ssolved CO2, was obtain ed by a new technique and a modifi ed calculation which se rved to include t he heat of vaporization of the gas escaping in t he reaction and res ulted in a higher value than those obtained b.,' Wells and TaYJOr a nd by Bac kstrom in determin ations represe nting only partial so lution of CO2• The res ul ts obtained were: 3CaO· A1z0 3· CaC03·10. 6SHzO H eat of formation from elements, t:.TJ~ fro m r eactan ts a n d H 20 H eat of solu tion in 2N H CI 3CaO·AIzOj · 61-1, 0 H eat of so lu tion in 2N H Cl CaC03 H eat of so luti on in 2N H CI

Studies of binary systems are helpful in the investigation of polycomponent systems formin g industrial products, such as portland cement, dolomite refractories, or blast-furnace slags . Details are given of the application of differential heating and cooling curves to the determination of the temperature of the ab;{J inversion of 2CaO.Si02 in binary mixtures with CaO, SiOz, AIzOa, MgO, FezOa, CaFz, Ti02, BaO, Cr20a, BzOa, MnzOa, V20 6, PZ0 6, Na20, and K 20. These added materials, excepting CaO, lowered the temperature of the a-.=2{J inversion and estimates of the limits of solid solution in a-2CaO.Si02 are given . X-ray patterns of {J-2 CaO.Si02 containing BaO, PZ0 6, NazO, or K 20 indicated the existence of solid solutions of the substitution type. Observations were made of the effectiveness of the added materials in preventing dusting, which results from the {J--+'Y inversion of 2CaO.SiOz. It was found that MnzOa should be added to the list of known chemical inhibitors of dusting. CONTENTS Page

Differential heating curves were used to study the behavior of calcium sulfate at high temperatures. After cooling, the material was examined by means of the petrographic microscope and by study of its X-ray pattern. It was found that the beta-to-alpha transition of calcium sulfate occurs at approximately 1,214° C, and the conclusion of its discoverer that the high temperature form is unstable was confirmed. There appears to exist a eutectic mixture of alpha CaSO, and CaO which melts at approximately 1,365° C. The reported existence of basic sulfates of calcium was not confirmed.

The results of chemical and microscopic analyses and heat-of-solution measurements on samples of commercial portland cement clinker are reported. Differences between quantitative microscopic analysis and compound composition calculated fr om chemical analysis have systematic relationships with chemical composition of the clinker. The heats of solution calculated from compound composition by microscopic and by chemical analysis show fair correspondence with the observed heats of solution. Assumptions on which both methods of calculation are based are shown to be somewhat faulty.

The heats of solution at 25° C in 2.09 molal HCI of gypsum, hemihydrate, natural anhydrite, and anhydrous CaS04, prepared by heating gypsum at various temperatures, have been determined. From these values the heats evolved in the hydration to gypsum of hemihydrate, and of anhydrite prepared at 1,000° C, were calculated to be 4,100 ±30 and 3,990 ±20 cal/mole, r espectively. The heat evolved in the hydration to gypsum of soluble anhydrite prepared by dehydrating gypsum at 75° C is not less than 6,990 cal/mole . In the course of this work X-ray and microscopic studies gave no indication of the existence of more than two forms of anhydrous calcium sulfate: Soluble anhydrite a nd a modification formed rapidly at high temperatures, the latter being identical with natural anhydrite. The heat evolved in the transition of soluble anhydrit e to natural anhydrite is not less than 3,000 cal/mole. I.