## 3 Results and Discussion
### The Effects of PSA Emulsions on the Fracture Properties of Modified Mortar  
The fracture energies of the mortars modified with the series of PSA emulsions can be obtained by testing the load and mid-span deflection curves of the polymer-modified mortars. The results are shown in Figure 9. The fracture energies of both SA1M and SA2M are lower than that of the blank mortar for different curing ages and methods, and the fracture energy reduces with increasing P/C ratios. This is mainly because the content of the soft monomer BA in the SA1 and SA2 emulsions is too high, resulting in a significant influence of the chemical reactions of the PSA emulsions in the cement-based materials. While forming a nested membrane structure, it has an obvious impact on other hydration products, which not only reduces the mechanical properties of the mortar but also decreases its toughness. In addition, it is worth noting that the test results of the compressive-flexural ratios show that the compressive-flexural ratio of SA1M is lower than that of the other types of modified mortars, and thus its toughness should be better. This may be because the five types of PSA emulsions reduce the compressive and flexural strengths, making the compressive-flexural ratio unsuitable for this system. Figure 8: The variation in the modulus of elasticity with P/C ratios for PSA emulsion-modified mortars: (**a**) 7 days standard curing; (**b**) 28 days standard curing; (**c**) 28 days mixed curing. Figure 7: Compressive–flexural ratios for a series of PSA emulsion-modified mortars: (**a**) 7 days standard curing; (**b**) 28 days standard curing; (**c**) 28 days mixed curing. The fracture energy of SA3M is slightly higher than that of the blank mortar at different curing ages and with different methods. The differences in fracture energies between the modified mortars are very small at different P/C ratios. The fracture energy value for SA4M increases with an increasing P/C ratio. When the P/C ratio is 15%, its maximum fracture energy is 211.2 N/m, approximately 2.4 times that of the blank mortar. This is the highest fracture energy among the mortars modified with the five types of PSA emulsions. The results of the compressive-flexural ratio show that the compressive-flexural ratio of SA4M is higher and the toughness is lower compared with the other five types of modified mortars, which further confirms that it is not suitable for this system. In addition, its maximum fracture energy at 7 days of curing is 2.1 times that of the blank mortar, indicating that the SA4 emulsion has formed a film and plays a role even when the hydration is incomplete. At 7 days of curing, the change in fracture energy with the P/C ratio for SA5M is similar to that of SA1M and SA2M, and the fracture energy is significantly lower than that of the blank mortar. The fracture energy of SA5M is slightly higher than that of the blank mortar at 28 days under the standard curing condition. The fracture energy of SA5M increases with the increase in the P/C ratio at 28 days of curing under the mixed curing condition, and the range of the increase is much higher than that under the standard curing condition at 28 days of curing. The highest value is approximately 1.4 times that of the blank mortar. The fracture energy of SA5M is the most affected by the curing method. This is mainly because the content of the hard monomer St in the SA5 emulsion is high, and its T\({}_{\text{g}}\) and MFFT are highest, only slightly lower than the cement curing temperature. The polymer does not form a film under the wet curing condition of standard curing, resulting in a lower fracture energy; however, the polymer forms a film under the mixed curing condition with dry curing, which significantly increases its fracture energy. The influence of the series of PSA emulsions on the fracture energy of the mortar follows a similar pattern at different ages and with different curing methods: SA1 < SA2 < SA3 < SA5 < SA4. The optimal composition ratio of St to BA in the PSA emulsion is 4:6.