Study on the effect of gypsum content in cement on the adaptability between cement and alkali-free accelerator at different temperatures

Abstract

The setting time and early strength of shotcrete is greatly affected by the gypsum content of cement. In this paper, setting time and compressive strength, and microscopic methods such as XRD quantitative analysis, thermogravimetric analysis and scanning electron microscope observation were used to determine the effects of changes in gypsum content on the hydration and mechanical properties of cement mixed with accelerator at different temperatures. The results showed that an increase in the gypsum content of cement promoted the formation of ettringite (AFt) and decreased the setting times of cement pastes at 0°C, 20°C and 40°C. Mortar compressive strength test revealed that after curing at 0°C, 20 °C and 40°C, the highest compressive strength was exhibited with final molar C₃A/SO₃ ratio of 0.84, 0.84 and 1.18, respectively. The mechanical properties of hardened cement pastes were adversely affected by C₃A/SO₃ ratios that were too high or too low. When the C₃A/SO₃ ratio was too high, which facilitated conversion of AFt to monosulphate(AFm) and slowed C₃S hydration. Conversely, when the C₃A/SO₃ ratio was too low, which enabled large numbers of AFt to occupy the space where C-S-H gels was formed, resulting in decalcification of C-S-H gels.