Synergistic Effect and Mechanism of Nano-C-S-H Seed and Calcium Sulfoaluminate Cement on the Early Mechanical Properties of Portland Cement

The combined utilization of mineral accelerators and nano-seeding materials is a novel method to promote the early strength of cement-based materials. In this paper, the effects of nano-C-S-H seed (NCS) on the early compressive strength of the Portland cement (PC)– calcium sulfoaluminate cement (CSA) binder were investigated. The results showed that NCS and CSA synergistically contributed to the early strength of PC. In detail, a 326.3% increase in the 10 h compressive strength of PC paste was obtained through the addition of NCS (2 wt%) and CSA (5%) in common. This was higher than the sum of the increases observed with the single additions of CSA (157.9%) or NCS (87.6%), with the same above dosage, in PC. Meanwhile, the early strength enhancement effects of NCS and CSA, when used together in PC, lasted longer than the effects of either used alone. Moreover, the synergetic effect mechanism was analyzed by isothermal calorimeter, QXRD, TGA, MIP, and SEM techniques. The calorimetry, XRD, and TGA results demonstrated that the synergistic mechanism was associated with the synergistic promotion effects of CSA and NCS on the hydrates. The fast hydration of CSA produced large amounts of ettringite and also consumed partial free water to promote the performance of the seeding effect of NCS which, simultaneously, further accelerated the precipitation of C-S-H gel and CH. The high alkie environment was also beneficial for the continuous generation of ettringite. In addition, the results of MIP and SEM measurements showed that the micro-filling effect of NCS significantly optimized the pore structure of a PC-CSA blend-hardened paste. Thus, the synergistic strength enhancement effects of CSA and NCS on PC were attributed to the matching of the promotion of hydration generation and the optimization of pore structures in hardening cement paste. The results of this article provide a new approach to achieving the rapid development of the early strength of cementitious materials, with potential applications in precast concrete and low-temperature construction.