Physical Integrity and Chemical Evolution Mechanisms of Alkali-Activated Slag under Accelerated Sulfate Attack via Thermal Dissolution

Abstract To accurately evaluate the sulfate attack response of alkali-activated slag (AAS) given real service conditions involving density reduction because of drying shrinkage cracks, this study designed an accelerated sulfate attack scheme employing thermal dissolution. The AAS matrices were crushed into fine particles to simulate the internal exposure state postcracking, followed by immersion in a sodium sulfate solution maintained under constant thermostatic vapor heating. The impact of sulfate attack on the physical integrity of the AAS was quantified by periodically monitoring the particle meso-morphology, measuring the particle size distribution, and determining the crushing strength. Concurrently, multidimensional characterization of the time-dependent evolution of AAS mineral phases and chemical composition was performed using microanalytical techniques, including X-ray diffraction (XRD), differential thermogravimetry (DTG), Fourier transform infrared spectroscopy (FT-IR), and solid-state Si 29 nuclear magnetic resonance ( Si 29 NMR). The results demonstrate that AAS exhibits significantly superior physical integrity compared with ordinary Portland cement (OPC) throughout the entire attack period. The AAS particles maintained dense meso-morphology and their initial size range (1,000–2,000 μm), with no crack initiation or particle disintegration observed. The 90-day crushing strength reached 94.7 N, representing a 9.3% increase over the initial value. Within the AAS system, sulfates were primarily physically adsorbed onto the matrix surface as mirabilite, with only trace amounts of ettringite formed during the later attack stages and no significant formation of deleterious expansive products. Although partial reconstruction of aluminosilicate chains occurred in the C─(A)─S─H gel [evidenced by reduced Q 2 (1Al) peak intensity, increased full width at half maximum, and upfield shift in Si 29 NMR], densification of the silicate network was achieved via a reduction in the chain-terminal Q 1 species and an increase in the chain-middle Q 2 species, effectively preserving the three-dimensional structural integrity.