Mechanism-based differentiation of dissolution- and crystallization-dominant acid degradation in nanosilica-modified...

Despite extensive studies on acid resistance in geopolymer mortars and nanosilica modification, degradation mechanisms are still largely inferred from isolated performance indicators such as strength loss or mass change, limiting mechanistic interpretation across different acid chemistries. In this study, a multi-parameter, mechanism-oriented evaluation framework is proposed to distinguish acid-type-dependent degradation regimes in nanosilica-modified metakaolin (MK) and fly ash (FA) geopolymer mortars exposed to hydrochloric (HCl, 4%) and sulfuric (H₂SO₄, 3%) acid environments. Mass change (ΔK), dimensional variation (ΔD), amorphous phase reduction quantified by XRD-based amorphous dome integration (ΔAmorphous), and strength retention (SR) were evaluated concurrently and statistically correlated across four acid-binder systems. The results demonstrate that HCl exposure induces a dissolution-dominated degradation regime, in which strength retention is primarily governed by the stability of the amorphous geopolymer phase, as evidenced by strong negative ΔK-ΔAmorphous correlations (ρ = -0.87 to - 0.90) and positive ΔAmorphous-SR relationships. In contrast, H₂SO₄ exposure leads to a crystallization-dominated regime characterized by sulfate-induced secondary phase formation and crystallization pressure, where strength loss shows a weak dependence on amorphous phase degradation and is instead controlled by internally generated microstructural stresses. Nanosilica exhibits a distinct acid-dependent dual role: low dosages (1-1.5%) enhance gel compactness and restrict ion diffusion, whereas excessive content (2%) accelerates microstructural embrittlement by amplifying crystallization-pressure-driven damage in sulfate environments. Overall, the findings reveal that acid resistance in geopolymer mortars is governed by distinct, quantifiable degradation regimes dictated by acid chemistry. Beyond geopolymer systems, the proposed framework offers a transferable, mechanism-based strategy for interpreting degradation processes in amorphous and nano-modified cementitious materials under aggressive chemical environments.