The Insight into Nanocalcium Carbonate in Solid Waste–Based Sulfoaluminate Cement Hydration and Strength Development

Due to its high specific surface area and reactivity, nanocalcium carbonate (NC) can significantly improve the mechanical properties and microstructure of cementitious materials, as well as enhance their durability. In this study, the influence of calcium carbonate with different particle sizes on the setting time, heat of hydration, mechanical properties, phase change, and pore structure of solid waste-based sulfoaluminate cement was investigated. The dosages of NC were 0.5, 1, 2, 3, and 5% by weight, respectively. The addition of micron-sized calcium carbonate (MCC) was 2% by weight. The results indicated that the addition of NC could promote cement hydration, thus shortening the setting time. After a curing period of 90 days, the compressive strength of the cement sample with 2% by weight NC increased by 42.55%, whereas that of the cement sample with 2% by weight MCC increased by 22.02%. After carbonation curing for 28 days, the anticarbonation of the cement samples with 2% by weight NC increased by 28.86%. The chemical shrinkage decreased by 65.02% with the addition of 2% NC. Compressive strength tests after freeze-thaw cycles demonstrated that NC could enhance the freeze-thaw resistance of cement. Scanning electron microscopy and X-ray diffraction analysis showed that NC increases the nucleation sites and chemical reactions of cement, and a large amount of aluminum gel and ettringite, as well as a small amount of hemicarboaluminate and monocarboaluminate, were formed during hydration. The results of the mercury intrusion porosimetry test showed that adding NC could reduce the porosity and specific surface area of cement slurry and make the cement structure denser. Similar studies have also been conducted on MCC to further investigate the size effects. The results imply that MCC does not participate in the hydration reaction of cement but reduced porosity and made the structure more compact through filling effects.