Early Age Properties of Paste and Mortar Made with Hybrid Binders Based on Portland Cement, GGBFS and Sodium Sulfate

Nowadays, Ground Granulated Blast-Furnace Slag (GGBFS) is increasingly used together with Portland cement to design low carbon footprint binder systems. Such systems with a low clinker factor are generally connected with a slow strength development. Alkaline activation in hybrid binders can provide a suitable solution to cope with this issue. Among the possible activators, sodium sulfate offers the advantage of its low-carbon footprint and good performance in the presence of Portland cement; however, in addition to strength development, other early age properties are also affected and compliance with requirements in the field must be verified. In the present study, hybrid systems based on Portland cement, GGBFS and sodium sulfate were analysed. Sodium sulfate was used in doses between 0 and 10 wt% relative to GGBFS. The GGBFS content amounted to 70 wt% of the binder and water-to-binder ratio was 0.45. Accelerated setting was detected due to the addition of sodium sulfate using the Vicat test and ultrasonic pulse velocity analysis. Increases in early age strength were observed for all the analysed doses of activator. Such increases are attributed to a dual effect: the acceleration of cement hydration and the enhanced dissolution of the slag. On this basis, possible changes in the reaction mechanism in presence of sodium sulfate have been discussed. Upon a comprehensive analysis of results, an optimal content of 5–10% of sodium sulfate in the mixes is suggested.