*3.2. Concrete Durability*

Figure 8 shows the compressive strength values of concrete specimens cured in standard conditions and immersed in sulfate solutions, also giving the strength loss. All the concrete specimens were manufactured with a W/C of 0.4. The compressive strengths of the concrete samples with the addition of 10%, 20%, and 30% BA decreased from 30.5 MPa, 27.7 MPa, and 26.0 MPa to 25.7 MPa, 25.6 MPa, and 24.9 MPa, respectively. Meanwhile the strength loss decreased from 15.6% to 7.5%, then to 4.2%. Thus, the more BA is added to the concrete, the smaller the strength loss will be. The BA can thus improve the performance of concrete against sulphate attack. To some extent, this is due to the very high surface area of the analyzed BA, which can fill in the pores of the concrete and prevent the sulphate solution from seeping into concrete. A similar phenomenon was found in cement concrete mixed with fly ash and other recycled micro powders [36–38].

**Figure 8.** Strength loss in Na2SO4 solution.

The porosity values of cement concrete samples at different W/C and addition levels of BA are shown in Figure 9. The total porosity, coarse capillary porosity, and fine capillary porosity all increased with increasing W/C. The addition of BA was able to efficiently decrease the porosity of the concrete, especially the coarse capillary porosity. The pore structure and porosity are the key factors that affect the strength of cement-based materials [39]. They are also the decisive factors in the resistance of cement-based materials to invasive media [40]. The attack resistance of cement-based materials is poor when the porosity is large and the pores are interconnected. On the contrary, better performance of concrete against attack can be obtained. This explains why the concrete with BA had better sulfate resistance.

**Figure 9.** Influence of (**a**) W/C and (**b**) BA content on porosity of cement concrete.

#### *3.3. Intrusion of Concrete by Sodium Sulfate Solution*

Figure 10 shows the heights of capillary rise with time at different W/C values and BA contents. The height of capillary rise increased with increasing W/C. The capillary rise height of the concrete with BA was obviously lower than that of the concrete without BA.

**Figure 10.** Height of capillary rise at (**a**) coarse and (**b**) fine capillary porosity.

The correlations of the capillary rise height versus the coarse and fine capillary porosity are shown in Figure 11. There was a good correlation between the height of capillary rise and the coarse capillary porosity (pore size of ≥30 nm) of the concrete, with a correlation coefficient of 0.819. However, the height of the capillary rise had little correlation with the fine capillary porosity (pore size of <30 nm), with a correlation coefficient of only 0.0669. Thus, the capillary porosity types with various pore sizes have different effects on capillary rise in concrete. The coarse capillary porosity plays a key role in the capillary transport of a solution in concrete [41].

**Figure 11.** Correlations of capillary rise height vs. (**a**) coarse and (**b**) fine porosity.

Figure 12 shows the correlations of the crystallization rate of capillary transmission versus the coarse and fine porosity. The crystallization rate of capillary transmission increased with increasing W/C. A shorter time for sodium sulfate solution to reach the surface of concrete results in a larger area of crystallization on the concrete surface. The crystallization rate of the concrete with BA decreased significantly. In addition, the crystallization rate decreased with increasing BA content. There was a good correlation between the crystallization rate of capillary transmission and the coarse capillary porosity, with a correlation coefficient of 0.959. However, the crystallization rate had little correlation with the fine capillary porosity, with a correlation coefficient of only 0.0973. That also fully explained the importance of the coarse capillary porosity in the capillary transport of a solution in concrete.

**Figure 12.** Correlations of crystallization rate vs. (**a**) coarse and (**b**) fine porosity.

The influence of the coarse and fine porosity on the mass of sodium sulfate solution absorption and their correlations are shown in Figure 13. W/C had a significant influence on the mass of sodium sulfate solution absorption, which was largest at a W/C of 0.45, followed by a W/C of 0.35. After the addition of BA, the absorption amount of sodium sulfate solution in the concrete decreased significantly, and it deceased with increasing BA content. There was a good correlation between the absorption amount of sodium sulfate solution and the coarse capillary porosity, with a correlation coefficient of 0.9793. However, a poor correlation between the absorption amount of sodium sulfate solution and the fine capillary porosity was found.

In conclusion, the fine capillary porosity has little influence on the height of capillary rise, sodium sulfate solution absorption, and crystallization rate of capillary transmission, while the coarse capillary porosity plays a key role in the intrusion of sodium sulfate solution into concrete. The concrete samples at various W/C values and BA contents had different porosity properties. Thus, W/C and BA content have an obvious influence on the transfer and crystallization of salt solution in concrete. The height of capillary rise, sodium sulfate solution absorption, and crystallization rate decreased with increasing BA content. All this proves that the addition of BA can improve the performance of concrete against sulphate attack.

**Figure 13.** Correlations of mass of sodium sulfate solution absorption vs. (**a**) coarse and (**b**) fine porosity.
