An Innovative Method for the Sustainable Utilization of Blast-furnace Slag in the Cleaner Production of One-part Hybrid Cement

Abstract Hybrid cement (HC) can be defined as alkali activated-blended-Portland cement (PC). It was prepared by the addition of an alkaline solution to high-volume aluminosilicate-blended-PC. Although this cement exhibits higher mechanical performance compared to conventional blended one (aluminosilicate-PC blend), it represents lower commercial viability because of the corrosive nature of an alkaline solution. Therefore, this study focuses on the preparing one-part-HC using dry activator-based blast-furnace slag (DAS). DAS was prepared by mixing sodium hydroxide (NaOH) with blast-furnace slag (BFS) at low water to BFS ratio, followed by drying and grinding to yield DAS-powder. Different contents of DAS (equivalent to 70 wt.% BFS and 1, 2, and 3 wt.% NaOH) were blended with 30 wt.% PC. A mixture containing 70 wt.% BFS and 30 wt.% PC was used as a reference sample. The mortar was adjusted at sand: powder (BFS-PC and/or DAS-PC) weight ratio of 3: 1. The microstructural analysis proved that DAS-powder is mainly composed of sodium calcium aluminosilicate activated species and unreacted BFS. These species can interact again with water to form calcium aluminum silicate hydrate (C-A-S-H) and NaOH, suggesting that the DAS acts as NaOH-carrier. One-part HC-mortars having 1, 2, and 3 wt.% NaOH recorded 7-days compressive strength values 82, 44, and 27 %, respectively, higher than that of the control sample. At 180-days of curing, a significant reduction in compressive strength was observed within HC-mortar having 3 wt.% NaOH. This could be attributed to the increase of Ca (within C-S-H) replacement by Na, forming Na-rich-phase with lower binding capacity. The main hydration products within HC are C-S-H, C-A-S-H, and chabazite as one of zeolite family.