With the rapid development of assembled buildings, the low-carbon and high-efficient production of precast concrete components becomes a hotspot for the industry. Portland cement (PC), as one of the most important cementitious materials for the preparation of precast components, has the disadvantages of insufficient early strength development and late volumetric shrinkage. To realize the volume stability of non-steam-cured precast components, expansion agents are often introduced into Portland cement. Sulphoaluminate cement (CSA) with a large amount of Ye'elimite and gypsum is often used to improve the early strength and volumetric stability of PC. Different types of CSA have different expansion capacities due to the different contents of Ye'elimite and gypsum, affecting the composition of the hydration products with the saturation of ettringite. In this paper, the mechanical properties and volumetric stability of the specimens were investigated via varying the relative contents of and in Portland cement (i.e., molar ratio and content). The hydration process, phases composition, and the microstructure of system were characterized to analyze the effect of on the hydration of Portland cement.

Portland cement clinker (Tangshan Jidong Cement Sanyou Co., Ltd., China), and anhydrite (Tangshan Polar Bear Building Materials Co., China) were used as raw materials. CaCO₃, Al₂O₃, and CaSO₄-2H₂O mixed in a molar ratio of 3∶3∶1 were heat-treated in a high-temperature muffle furnace for 2 h to obtain the mineral. Four different molar ratios of M, 2, 5, 8 and 11 were designed, while five content levels of 5%, 10%, 15%, 20% and 25% were set up in a total of 20 sets of crossover experiments. The 10%-M2 represents the content of this sample of in Portland cement clinker is 10% at the molar ratio M of 2.

The water-cement ratio was 0.35, and the paste was molded as a net paste test block with the sizes of 20 mm×20 mm×20 mm. The compressive strength at the corresponding age was determined. The phase compositions of the hydration products were analyzed by a model D/Max-RB X-ray diffractometer and a model STA 449F3 thermal analyzer. The pore structure of the specimens was characterized by a model AutoPore IV 9500 V1.09 instrument.

has an effect on the free expansion rate of Portland cement, and it can significantly increase the early compressive strength of Portland cement clinker. When the content of ≤15% in the sample, the free expansion rate is small, the volumetric stability is better. When hydration time is 14 d, the free expansion rate remains stabilized, and the expansion rate of the specimen does not change at 28 d. The free expansion rate of the sample is lower than that of the sample with . The effect of on the expansion properties of Portland cement belongs to the ettringite-type expansion. The key reason of the volume stability and early strength development of the system is affected by the amount of ettringte. Ettringite growth in the molding paste is limited as the crystal grows, the pressure of ettringite on the surrounding paste environment gradually increases, reaching a certain threshold that makes the specimen gradually collapse. As a result, the amount of ettringite generation is strictly controlled. At ettringite content of less than 8.51%, the compressive strength increases with the increase of ettringite content. Ettringite plays a role of dense pore. However, at the ettringite content of more than 8.51%, the expansion increases with increasing ettringite content, and the mechanical properties of the cement begin to decrease.

The most available pore sizes of the specimens 10%-M11 were smaller than those of the specimens 5%-M11, while the specimens 10%-M11 have a narrower distribution of pore sizes and smaller total porosity, which can account for the maximum mechanical properties. The location and the crystallization situation of hydration products both have an influence on the expansion properties of Portland cement. Ettringite fills the pores of the paste, and the C-S-H gel can adhere other hydration products such as ettringite to enhance the performance of the cement, and avoid the ettringite abnormal growth.

The 10% content of appropriately increased the expansion deformation of Portland cement, limited its late volumetric deformation and stablized the expansion rate. The content of also improved the early mechanical properties of Portland cement, having a positive effect on the development of mechanical properties. Under the condition of 10% and the molar ratio of of 11, the compressive strength of Portland cement at 12 h was increased by 102%, and the free expansion rate at 28 d was 0.279%. There were three main reasons of high early strength in the system, i.e., hydration of , hydration of C₃A with , and rapid hydration of C₃S. The increased content of facilitated the formation of ettringite, and hydration consumed Ca(OH)₂ in the hydration product to generate ettringite to cause the expansion, accelerating the hydration rate of the silicate phase and improving the cement hydration degree. The ettringite generated by the hydration of could compensate for the self-shrinkage of Portland cement, and the short and fine needle-like ettringite grew on the surface of the hardened cement paste and in the pores, and the staggered lapping filled the pores, reducing the total porosity of the cement and the most available pores. An appropriate amount of could optimize the distribution of ettringite in the system, and avoid the expansion caused by the abnormal growth of ettringite crystal size.