Enhanced pozzolanic reactivity in hydrogen-form zeolites as supplementary cementitious materials

Abstract

Pozzolans rich in silica and alumina react with lime to form cementing compounds and are incorporated into portland cement as supplementary cementitious materials (SCMs). However, pozzolanic reactions progress slower than portland cement hydration, limiting their use in modern construction due to insufficient early-age strength. Hence, alternative SCMs that enable faster pozzolanic reactions are necessary including synthetic zeolites, which have high surface areas and compositional purity that indicate the possibility of rapid pozzolanic reactivity. Synthetic zeolites with varying cation composition (Na-zeolite, H-zeolite), SiO₂/Al₂O₃ ratio, and framework type were evaluated for pozzolanic reactivity via Ca(OH)₂ consumption using ion exchange and in-situ X-ray diffraction experiments. Na-zeolites exhibited limited exchange reactions with KOH and Ca(OH)₂ due to the occupancy of acid sites by Na⁺ and hydroxyl groups. Meanwhile, H-zeolites readily adsorbed K⁺ and Ca²⁺ from a hydroxide solution by exchanging cations with H⁺ at Brønsted acid sites or cation adsorption at vacant acid sites. By adsorbing cations, the H-zeolite reduced the pH and increased Ca²⁺ solubility to promote pozzolanic reactions in a system where Ca(OH)₂ dissolution/diffusion was a rate limiting factor. High H-zeolite reactivity resulted in 0.8 g of Ca(OH)₂ consumed per 1 g of zeolites after 16 h of reaction versus 0.4 g of Ca(OH)₂ consumed per 1 g of Na-zeolite. The H-zeolite modulated the pore fluid alkalinity and created a low-density amorphous silicate phase via mechanisms analogous to two-step C-S-H nucleation experiments. Controlling these reaction mechanisms is key to developing next generation pozzolanic cementitious systems with comparable hydration rates to portland cement.