Clinoptilolite zeolite as a substrate for alkali-boosting cement hydration

Clinoptilolite-Ca zeolite exhibited limited pozzolanic activity at early ages in portland cement systems, resulting in slower initial hydration and strength development. Conversion of Ca-zeolite to Na- or K-forms, combined with increasing the pH of the blended system through pH engineering of the treated zeolite, substantially enhanced its reactivity. Calorimetry revealed accelerated C₃S and C₃A hydration, with higher peak intensities and shorter dormancy periods for Na-or-K zeolites, whereas bulk portlandite analysis indicated that Ca-zeolite inhibits clinker reactions. Phase-resolved quantitative EDS confirmed these trends, showing retardation of hydration for Ca-zeolite and promotion of both silicate and aluminate phases reactions for Na-and K-zeolites, along with the formation of relatively higher amounts of AFm phases. Chemical shrinkage and mortar strength measurements further supported these findings. Notably, pH-engineered zeolites increased early-age strength by up to 10 %, with 7-days mortar strength remaining equivalent even at 20 % cement replacement, highlighting their potential as sustainable supplementary cementitious materials.