Boosting early strength in limestone calcined clay cement (LC3) and its phase evolution using waste-derived silica mineral

This study aimed to develop limestone calcined clay cement (LC 3 ) with high early strength through a systematic two-phase investigation. In Phase I, various clays were characterized and evaluated for their reactivity based on mineral composition and heat of hydration, leading to the selection of the most reactive clay with high kaolinite content and an optimal SiO₂:Al₂O₃ ratio. In Phase II, LC 3 was developed using optimized clay, and early-age strength enhancement was pursued by incorporating silica from rice husk ash (RHA) and silica fume (SF) at 1%, 3%, and 5%. Compressive strength and hydration phase development were assessed at different curing ages. Additionally, the electrical resistivity of optimized RHA- and SF-modified LC 3 mixes was compared with conventional LC 3 and cement (OPC). Results indicated that reducing clinker content in LC 3 slowed early strength gain due to lower C-S-H formation. While the incorporation of RHA enhanced the compressive strength, the improvement was most significant at an optimal dosage of 3% RHA. Compared to the control mixture, strength gains of 6.1%, 18.0%, 32.6%, and 12.4% were observed at 3, 7, 14, and 28 days of hydration, respectively. However, higher RHA content (e.g., 5%) resulted in reduced performance due to particle agglomeration, which impeded effective hydration. The SF-modified mixtures outperformed conventional LC 3 at 3 days and achieved strength comparable to OPC by 14 days. Specifically, at a 5% SF replacement level, strength improvements of 33%, 22.1%, 53.8%, and 36.1% were observed at 3, 7, 14, and 28 days of hydration, respectively. FTIR, XRD, and QXRD analyses further confirmed the accelerated hydration kinetics and the formation of additional hydration products in the SF-modified systems. Electrical resistivity tests showed that LC 3 with 5% SF achieved the densest microstructure and highest strength. Although SF proved most effective, RHA remains a sustainable alternative, providing the dual benefit of renewable energy and enhanced silica content during clay calcination, promoting a circular, low-carbon cement production approach.