Preparation and characterization of an alkali-activated cementitious material with blast-furnace slag, soda sludge, and industrial gypsum

The production of conventional cement not only consumes a great deal of energy and natural raw materials but also emits CO2. This work prepared an alkali-activated cementitious material with blast furnace slag (BFS), soda sludge (SS), and industrial gypsum (IG). First, an SS-activated-BFS precursor was prepared by investigating the use of SS as an alkaline activator for BFS activation; the effect of the addition of IG on the compressive strength, pH, and the hydration phases and microstructure of the SS-activated-BFS precursor was then investigated. Results indicate that SS provides an alkaline environment for the activation of BFS by the dissociation of Ca(OH)2; the OH− ions reacted with the active SiO2 and AlO-2 ions dissolved from BFS, forming the tetrahedral [H3SiO4]-, tetrahedral [H3AlO4]2- and octahedral [Al(OH)6]3- ions, which further combined with the Ca2+ ions, Cl- ions, and SO2 − 4 ions provided by SS to form the block gismondite, lamellar calcium chloroaluminate hydrates, and rod-like ettringite in the SS-activated-BFS precursor. Results also show that IG can promote the formation of the ettringite in the alkali-activated cementitious material, increasing the compressive strength. The alkali-activated cementitious material prepared with 37.60 wt% BFS, 56.40 wt% SS, and 6 wt% IG, is the highest with 12.07 MPa at 2 days, 25.35 MPa at 7 days, and 43.02 MPa at 30 days, which can be regarded as a supplementary to the 42.5 grade conventional cement. This work proposes a reference for the management of BFS, SS, and IG and provides an eco-friendly and energy-conserving supplementary to conventional cement.