Laboratory–field assessment of concrete exposed to biogenic sulphuric acid: Influence of SCM-based binders on long-term degradation

Sulphuric acid-induced degradation, especially from biogenic sources, remains one of the most severe durability threats to reinforced concrete infrastructure. This study presents a comprehensive, long-term evaluation of the acid resistance of reinforced concrete incorporating fly ash (FA) and crystallising additives—two powdered commercial crystalline admixtures and one sodium silicate solution. A unique 950-day exposure campaign was conducted under both laboratory-controlled sulphuric acid conditions and real-field biogenic sulphate environments within an operating wastewater treatment plant. The evolution of mechanical properties, mass loss, neutralisation depth, and microstructural changes was quantified through compressive strength testing, image-based surface degradation mapping, SEM–EDX, XRD, and thermodynamic modelling using GEMS/Cemdata18. Results reveal that fly ash substantially reduced degradation rates and mass loss, with limited incremental gains from crystallising admixtures under chemical exposure. However, under biogenic conditions, admixture-containing mixes exhibited enhanced resistance. Thermodynamic simulations elucidated the chemical buffering mechanisms underpinning the durability trends, highlighting the role of portlandite depletion, gypsum saturation, and the stabilisation of hemicarbonate and hydrotalcite. The study provides a rare field-laboratory correlation of degradation mechanisms and proposes binder design principles for acid-resistant concretes. • 950-day sulphuric acid and field biogenic exposure of reinforced concrete mixtures. • Fly ash and crystalline admixtures improve acid resistance via phase stabilisation. • Thermodynamic modelling predicts binder-specific degradation and buffering mechanisms. • Laboratory tests correlate well with field degradation trends under biogenic conditions. • Binder design guidelines proposed for sulphuric acid-resistant wastewater infrastructure.