An experimental study on the upgrade of sulfoaluminate—belite cement systems by blending with Portland cement

Blends consisting of sulfoaluminate—belite (SAB) cements and Portland cement (PC) (CEM I 42·5) were tested. The initial set of PC was 3 h 10 min, of SAB-1 cement 10 min and of SAB-2 cement 20 min, whereas those of blends 85 wt% SAB-1 and 85 wt% SAB-2 were 1 h 10 min and 30 min, respectively. It is suggested that differences in the mineralogical composition of SAB cements influence markedly the setting characteristics of blended cements with very similar fineness. Properties of cement mortars (cement:sand = 1:3 by weight, water/cement ratio = 0·5) containing PC/SAB cement in weight ratios of 100/0, 85/15, 70/30, 55/45, 15/85 and 0/100 were tested in more detail. Flexural and compressive strength of mortars made from PC/SAB-1 cement blends are lower than in PC-mortar kept for 90 days at 20°C/100% relative humidity (wet cure) and 20°C/60% relative humidity (dry cure). SAB-1 cement and the 85 wt% SAB-1/15 wt% PC blend have shrinkage-compensating properties opposite to PC in tested mortars. PC-mortar shows excellent protective properties against corrosion of steel reinforcement. Steel in the SAB cement mortar is not passivated but occurs in the nonstable state. The related pH values of the mortar extracts were 12·41 and 11·32, respectively. In the 85 wt% SAB-1/15 wt% PC-mortar the steel is protected against corrosion. This statement is confirmed by the pH value of the mortar extract of 11·88. A 15 wt% replacement of SAB-1 cement by PC is still sufficient for steel passivation.