Acid resistance of calcium aluminate cement–fly ash F blends

The short-term resistance to sulfuric acid at 90°C of four calcium aluminate cement (CAC)–fly ash class F (FAF) blends activated with sodium metasilicate (thermal shock resistant cements (TSRCs)), cured at 300°C, was compared to that of a calcium phosphate cement (CPC) (CAC–FAF blend activated with sodium hexametaphosphate) and a Portland cement class G/silica blend. The mechanical properties and compositions of the acid-exposed samples were evaluated by measuring their compressive strength and by means of x-ray diffraction, μEDX (energy-dispersive x-ray spectrometry), thermogravimetric and Fourier transform infrared analyses. All calcium-containing hydrates were sensitive to the conditions of acid exposure. In the TSRC blends, these hydrates included hydrogrossular, feldspar family minerals and zeolites; in CPC, feldspar minerals and phosphate phases; and in the class G/silica blend, portlandite and tobermorite. Crystalline calcium sulfates formed in the acid-exposed surfaces with the exception of the most aluminium-rich TSRC samples where only potassium(sodium) aluminium sulfate, alunite, was detected. This sample underwent the least changes in weight, compressive strength and had the lowest sulfur permeation into the sample core. Calcium sulfates precipitated on sample surfaces limited sulfur penetration into the core of calcium-rich TSRC, CPC and G/silica blends.