The sulphate resistance of cements containing red brick dust and ground basaltic pumice with sub-microscopic evidence of intra-pore gypsum and ettringite as strengtheners

This paper presents a laboratory study on the deterioration of blended cement combinations of plain Portland cement (PPC) with red brick dust (RBD) and ground basaltic pumice (GBP). One type of clinker, same Blaine values and two different proportions of additive by mass of clinker, were employed. In addition to these blends, Portland cements without additives were prepared as control specimens.The compressive strength and the sulphate resistance of cements have been experimentally studied in this paper. A series of laboratory tests were undertaken on all specimens. A large quantity of sheet-like C–S–H was found in the mortars incorporating RBD and GBP. The results indicated that the increase in the additive content caused a significant increase in the sulphate resistance of the mortars. Hence, the studied RBD and GBP can be recommended for use as admixtures in cement production. The development of the particular microstructure including the secondary minerals in the plain and blended cements were studied via SEM analysis. SEM images revealed the presence of ettringite and Portlandite minerals, where the former was most probably responsible for the increase (together with the gypsum roses) as well as a decrease of strength based on its formation at different sites and crystal form. Portlandite was responsible for an increase in the specimen strength.