Influence of the Degree of Calcination of Eggshell on the Strength, Hydration, and Microstructure of Cementitious Mortars

<jats:p>Waste eggshell is a biological limestone with the potential to partially replace Portland cement in cementitious mixes, in both calcined and uncalcined states. However, the performance of Calcined Eggshells (CES) is strongly influenced by thermal treatment, as it removes the organic membrane and may alter the calcite-rich phase. This study aims to investigate the influence of calcination temperature and holding duration on the fresh behaviour, 28-day compressive strength, hydration, and microstructure of cementitious mixes containing eggshells as a partial cement replacement. Eggshells were calcined at 400, 600, and 800 oC for 1, 2, and 3 h and replaced with cement at the replacement levels of 5%, 10%, and 15%. The experimental program consisted of two sequential stages, where the first stage aimed to select representative mixtures for detailed hydration and microstructural analysis in the second stage. In the first stage, flow consistency and 28-day compressive strength were measured for all mixes containing CES, and compared with the control mix, mixes with industrial-grade extra pure limestone (LS), and mixes with uncalcined eggshells (ES). Based on phase assemblage, mixtures were classified into calcite-rich (undecomposed) and partially decomposed calcination regimes. Flow consistency and 28-day compressive strength testing identified CES calcined at 600 °C for 2 h at 5% replacement (CES5(600-2)) as the best-performing calcite-rich mixture, and CES calcined at 800 °C for 3 h at 15% replacement (CES15(800-3)) as the best-performing partially decomposed mixture. Thermogravimetric analysis (TGA) showed that CES5(600-2) achieved the highest bound-water content (25.13%) among calcite-rich mixtures, indicating enhanced hydration, while CES15(800-3) exhibited a markedly elevated calcium hydroxide content (21.21%) attributable to the combined contribution of cement hydration and hydration of residual CaO from calcination. Scanning electron microscopy corroborated these findings, revealing a well-developed C-S-H-rich matrix in CES5(600-2) and a highly complex heterogeneous microstructure in CES15(800-3). The results demonstrate that the degree of calcination governs the phase assemblage and hydration pathway of CES, offering guidance for optimising eggshell-derived materials as sustainable supplementary cementitious materials.</jats:p>