Effect of gypsum content on the compatibility of cement with alkali-free accelerators

Compatibility issues between alkali-free accelerators (AFAs) and cement in shotcrete applications were explored, focusing on the influence of gypsum content. By conducting macro-performance tests (setting time, compressive strength) and micro-analyses (X-ray diffraction, heat of hydration, scanning electron microscopy), the effects of varying gypsum levels on these interactions were identified. The results showed that an increase in gypsum content from zero to 9 wt% progressively extended the setting time for cement pastes containing 5 wt% AFA. For cements with 7 wt% and 9 wt% AFA, the setting time first decreased before increasing. The compressive strengths at 1, 7 and 28 days typically first increased and then decreased. This behaviour was linked to the additional SO42− from gypsum reacting with tricalcium aluminate (C3A) to form significant quantities of ettringite, enhancing the dissolution and hydration of tricalcium silicate. However, excessive gypsum led to excessive SO42− adsorption on C3A surfaces, inhibiting its hydration and thus diminishing the formation of calcium hydroxide and calcium silicate hydrate hydration products. Optimal results for setting time and compressive strength were achieved when the ratio of C3A to sulfur trioxide (SO3) ratio (C3A/ST ratio) was between 0.38 and 0.56. By adjusting the gypsum content, the C3A/ST ratio in the cement–AFA–water system was effectively regulated, offering critical insights for optimising high-performance shotcrete.