Hydration Mechanisms of Alkali-Activated Cementitious Materials with Ternary Solid Waste Composition

Considering the recent eco-friendly and efficient utilization of three kinds of solid waste, including calcium silicate slag (CSS), fly ash (FA), and blast-furnace slag (BFS), alkali-activated cementitious composite materials using these three waste products were prepared with varying content of sodium silicate solution. The hydration mechanisms of the cementitious materials were analyzed by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The results show that the composite is a binary cementitious system composed of C(N)-A-S-H and C-S-H. Si and Al minerals in FA and BFS are depolymerized to form the Q⁰ structure of SiO₄ and AlO₄. Meanwhile, β-dicalcium silicate in CSS hydrates to form C-S-H and Ca(OH)₂. Part of Ca(OH)₂ reacts with the Q⁰ structure of AlO₄ and SiO₄ to produce lawsonite and wairakite with a low polymerization degree of the Si-O and Al-O bonds. With the participation of Na⁺, part of Ca(OH)₂ reacts with the Q⁰ structure of AlO₄ and the Q³ structure of SiO₄, which comes from the sodium silicate solution. When the sodium silicate content is 9.2%, the macro properties of the composites effectively reach saturation. The compressive strength for composites with 9.2% sodium silicate was 23.7 and 35.9 MPa after curing for 7 and 28 days, respectively.