Effect of fineness of high-alumina ground granulated blastfurnace slag on magnesium sulphate attack

Abstract

This paper describes the sulphate resistance of hardened mortar and paste specimens incorporating 50% ground granulated blastfurnace slag (BFS). The main variable is the fineness level of the slag materials, which are divided into 445, 600 and 800 m2/kg Blaine surface area. Ordinary Portland cement mortar and paste were also used to compare the degree of deterioration by sulphate attack. All mortar and paste specimens were exposed to 4·24% magnesium sulphate solution at 20 ± 1°C for 540 days. The tests used in this study include visual examination, compressive strength and mass loss for mortar samples, and microstructural observations such as X-ray diffraction (XRD), differential thermogravimetric analysis and scanning electromicroscopy for paste samples. Experimental results indicated that mortar specimens with a high-fineness ground granulated blast-furnace slag at 50% replacement showed a poor resistance to magnesium sulphate attack compared with those of lower fineness. The XRD results confirmed that gypsum formation was primarily responsible for the deterioration in the hardened cement matrices of mixtures with high-fineness ground granulated blastfurnace slag. In addition, the conversion of cementitious C–S–H to M–S–H (or M–C–S–H) also led to the severe deterioration in the mortar specimens with a 800 m2/kg fineness level. This work suggests that care should be taken when using BFS with high-fineness level under magnesium sulphate attack.