Understanding of alkali–silica reaction in lime-pozzolana mortar incorporating pumice and borosilicate

To assess the efficacy of lime-calcined clay binder in mitigating alkali–silica reaction (ASR) deterioration, this study identifies potential reaction product formation mechanisms in 2-year-old alkaline solution-exposed mortar bars using X-ray diffraction, micro-computed tomography, scanning electron microscopy, and evaluates their influence on mortar performance. The bar samples exhibit only 0.02% ASR-induced expansion, ascribed to the high alumina incorporation from the binder matrix into the initially formed sodium silicate hydrate (N-S-H) gel, enhancing silica polymerization and eventually forming zeolites such as phillipsite and chabazite. These zeolites, with a (Na+K)/Si ratio of 0.2–0.5, can uptake alkali, thus reducing their availability for further reactions. The crystalline ASR product Na-shlykovite is also observed, coexisting with the zeolites and forming at their core. Additionally, pumice inclusion in the binder enhances its ASR mitigation capability by supplying additional alumina and accommodating reaction products within its internal voids, thereby preventing the propagation of shrinkage cracks into the binder matrix. Consequently, including pumice with borosilicate in lime-calcined clay mortar increased its compressive strength by 52% compared with using only borosilicate under prolonged alkaline conditions.