Effects of Nano-TiO2 on CO2 Uptake and Polymorphs of CaCO3 in Early CO2-Cured Low-Heat Portland Cement

Low-heat portland cement (LHPC) is characterized by its energy savings and low CO2 emissions during production and low heat release during hydration due to its high C2S content. However, the CO2 uptake capacity of the prefabricated component produced by LHPC under the early-age (within 7 days) carbonation curing with low CO2 concentration is poor due to the limited masses of ettringite (AFt), calcium silicate hydrate (C─ S─ H), and Ca(OH)2. Therefore, nano-TiO2 (NT) was used to accelerate the hydration and carbonation of the early CO2-cured LHPC. The effects of NT on CO2 uptake and polymorphs of CaCO3 were then studied using common testing techniques (thermogravimetric analysis, X-ray diffraction, etc.). Meanwhile, the numerical model between the polymorphs of CaCO3 masses and the compressive strength was established using the projection pursuit regression method. Results showed that taking 12–72 h of CO2 curing as control groups, the addition of NT did not improve the CO2 uptake of samples. The results are related to the inadequate CO32− concentration and NT refining the pore structure. A longer CO2 curing time and a higher NT dosage would accelerate the transformation of C─ S─ H (III) and weakly crystallized calcite to amorphous calcium carbonate (ACC). This process yielded fatal shrinkage cracks due to the evaporation of free water and an increase in the proportion of pores with sizes <50 nm. Consequently, although 168 h of CO2 curing and 1.5%–2% NT added exhibited the optimal CO2 uptake (3.882–4.318 g/100 g), their compressive strength could only achieve the compressive strength of the pure cement pastes. These findings provide novel insights into balancing the CO2 uptake and mechanical strength of the early carbonated LHPC pastes with NT.