Synthesis of a novel one-part alkali-activated material from solid wastes salt sludge and soda residue

Abstract

In this paper, salt sludge (SS), a solid waste from sodium hydroxide production, was activated at high temperature for the first time. It was combined with soda residue (SR), a solid waste from sodium carbonate production, as a solid activator to activate the solid aluminosilicate precursor, blast furnace slag (BFS). This process developed a novel one-part alkali-activated material, termed salt sludge and soda residue co-activated blast furnace slag cementitious material (SRB), which effectively utilizes solid wastes. The effects of ratios of calcined SS (CSS) to SR, fine aggregate types, and curing methods on the physical and mechanical properties of SRB were investigated. The composition of the hydration products was characterized using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and fourier transform infrared spectroscopy (FTIR), while the hydration process during the setting stage was examined with 1H low-field nuclear magnetic resonance (1H LF NMR). The influence of mix ratio, fine aggregate type, and curing method on the microstructure of SRB was analyzed using scanning electron microscopy (SEM). The results showed that the synergistic activation of BFS by CSS and SR was significantly more effective than individual activation. At the optimal mix ratio (CSS:SR:BFS = 15:15:70), the 3/28 days compressive strength of SRB was 14.3/53.7 MPa. The primary hydration products were C-(A)-S-H gels, along with Friedel's salt, ettringite, and hydrotalcite crystals. Using manufactured sand (MS) as a fine aggregate reduced mortar fluidity compared to river sand (RS) but enhanced the interfacial transition zone (ITZ) with the hardened paste, resulting in a denser microstructure. Heat curing promoted hydration product generation, increasing early (3 days) strength by a factor of 2.5. However, the rapid accumulation and insufficient diffusion of hydration products adversely affected the later strength.