Effect of using calcined eggshells as a cementitious material on early performance

This study focuses on the viability of using calcined eggshells (CES) as a partial replacement of cement by analysing early age performance. Eggshells found on the landfill sites are contaminated with smelly organic components. One of the recommended ways for extracting pure stable calcite from waste eggshells is to remove the volatile component and organic components by calcining them. To get the calcined eggshells, raw eggshells were calcined at 400 degrees C, 600 degrees C, and 800 degrees C. Each temperature was held constant for the duration of one hour, two hours, and three hours, respectively. These calcined eggshells were replaced with cement at the rate of 5%, 10%, and 15% in addition to the mixes containing eggshells and limestone with the same replacement level. For early age performance, setting times were estimated using the Vicat method, the heat evolution was monitored using isothermal calorimetry during the first 24 h and the compressive strength after 24 h were measured. Due to the disturbance of sulphate (SO3) balance, a certain reduction was observed in the setting time. The calcined eggshell proved to be a better accelerator as compared to both eggshells and limestone. In the heat evolution curve, the secondary shoulder peak appears in mixes containing high-temperature calcined eggshells which indicates the formation of the AFm phase. The cumulative heat release associated with calcined eggshells is higher as compared with eggshells and raw eggshells when replaced with cement. Moreover, the compressive strength was found higher in the calcined eggshells with undecomposed calcite as compared to the calcined eggshells with decomposed calcite. Also, the reverse trend was observed in heat of hydration versus strength correlation in decomposed and undecomposed calcite condition.