Calcite precipitation is a promising bio-mediated technique for liquefaction mitigation. Its influence on early-cycle dilative behaviour, particularly between 0.2 and 0.5% double-amplitude (DA) axial strain, remains poorly quantified. This study investigates the undrained cyclic response of Toyoura sands and Keisha No. 4 treated with 0.4 or 0.8% calcite (by dry weight), cured at 30% or 97% saturation (Sr), and tested at confining pressures (CP) of 50, 100, and 200 kPa (relative density DR = 50%). The number of cycles (N) to three strain thresholds: \({\text{N}}_{3}\) , \({\text{N}}_{2}\) , and \({\text{N}}_{1}\) have been tracked. Results show that low Sr = 30% and fine grain size ( \({\text{D}}_{50}\) = 0.17 mm) synergistically enhance cyclic resistance: for Toyoura sand at \({\upsigma}_{\text{c}}^{^{\prime}}\) = 100 kPa and 0.8% calcite, \({\text{N}}_{3}\) increased from 9 (untreated) to 52 cycles (+ 478%), whereas Keisha No. 4 ( \({\text{D}}_{50}\) = 0.825 mm) showed only a + 222% increase under identical conditions. Critically, calcite bonding delays the phase transformation only up to ~ 0.5% DA; beyond this, relative angularity, defined as the ratio of calcite crystal size to grain size, governs post-bonding dilation and dominates \({\text{N}}_{1}\) – \({\text{N}}_{2}\) gains. At \({\upsigma}_{\text{c}}^{^{\prime}}\) = 200 kPa and 0.8% calcite, lowering Sr from 97 to 30% increased \({\text{N}}_{1}\) by sixfold (from 10 to 61 cycles) in Keisha No. 4, directly linked to more contact-localized calcite deposition (SEM-verified). These findings demonstrate that optimizing microstructural efficiency, not just calcite content, is key to extending cyclic life in the critical pre-liquefaction window.
