# Deterioration Process of Cementitious Material Properties under Internal Sulphate Attack
## 3 Results and Discussion
### Strength Measurements  
Figure 13a,b shows the compressive strength and flexural strength of mortar specimens with different SO\({}_{3}\) content in the aggregate when maintained at 20 \({}^{\circ}\)C, respectively. The results show that the higher the SO\({}_{3}\) content in the aggregate, the lower the compressive strength and flexural strength of the mortar specimens, which is because gypsum replaces part of the aggregate and its own low strength leads to the low strength of the specimens. In the mortar specimens with 0% SO\({}_{3}\) content in the aggregate, the compressive and flexural strengths are enhanced with the extension of age. In the mortar specimens with 0.6%, 1.2%, 2.4%, 4.8% and 7.2% SO\({}_{3}\) content in the aggregate, the compressive and flexural strengths grow first and then decrease, which indicates that with the hydration reaction of cement, the early filling of mortar pores by calcium alumina has a certain contribution to the strength. However, after the strength of cement slurry rose and hardened 3 months later, the expansion stress generated by the expansion of ettringite caused cracks in the mortar and damaged the internal structure of mortar.When the SO\({}_{3}\) content in the aggregate was 0.6% and 1.2%, the compressive strength of mortar specimens at 28 d was 60.5 MPa and 46.9 MPa, and the flexural strength was 10.0 MPa and 7.7 MPa, respectively. The compressive strength at 360 d was 69.2 MPa and 57.0 MPa, and the flexural strength was 9.8 MPa and 9.3 MPa, respectively. As can be seen from Figure 10, the expansion rate of mortar specimens with SO\({}_{3}\) content of 0.6% and 1.2% in the aggregate did not exceed 0.1%, but there was a certain tiny expansion relative to the blank sample, so the strength of mortar specimens had loss after 3 months, but the compressive and flexural strength of mortar at 360 d was basically higher than the strength at 28 d. When the SO\({}_{3}\) content in the aggregate was 2.4%, 4.8% and 7.2%, the compressive strengths of mortar specimens were 38.2 MPa, 36.9 MPa and 35.6 MPa, and the flexural strengths were 6.5 MPa, 6.3 MPa and 6.0 MPa at 28 d, respectively. The compressive strengths at 360 d were 36.6 MPa, 32.9 MPa and 29.5 MPa and flexural strengths of 5.0 MPa, 4.6 MPa and 4.3 MPa, respectively. From Figure 13, it can be seen that the swelling rates of mortar specimens with 2.4%, 4.8% and 7.2% SO\({}_{3}\) content in the aggregate at 360 d were 0.33%, 0.55% and 0.61%, respectively, the swelling of mortar was larger and caused greater strength loss, and the mortar compressive and flexural strengths at 360 d were already lower than the strength at 28 d.  
Figure 12: XRD patterns of mortars with different sizes of gypsum aggregate. The 28-d strength of mortar specimens was selected as the initial strength, and the rate of change of strength at different curing ages was converted to obtain the trend of the rate of change of strength, which is shown in Figure 14. It can be seen from the figure that with the increase of curing age, the rate of change of compressive flexural strength of mortar with 0% SO\({}_{3}\) content in the aggregate was increasing, while the rate of change of the strength of mortar specimens mixed with gypsum sand showed different degrees of decreasing due to the internal sulphate attack. When the SO\({}_{3}\) content in the aggregate was 2.4%, 4.8% and 7.2%, the compressive and flexural strengths of the mortar specimens decreased to a level lower than the initial strength at 360 d due to the sulphate attack, with compressive strength losses of 4.2%, 10.8% and 17.1%, and flexural strength losses of 23.1%, 27.0% and 28.3%, respectively; and the swelling rate of the specimens exceeded 0.1%, indicating that the swelling rate of cementitious materials with the introduction of gypsum is harmful when it exceeds 0.1%, similar to the criteria for judging the activity of alkali aggregates [28].