# Effect of Dry-Wet Cycle Periods on Properties of Concrete under Sulfate Attack
## 3 Results and Discussion
### Microstructural Analysis by SEM  
Figure 10 shows the microstructural morphology of the specimens before and after the sulfate dry-wet cycles. Figure 10a shows that the concrete sample without sulfate attack had better compactness and plenty of flocculated fibrous calcium silicate hydrate (C-S-H), and some acicular ettrinigite (Aft) can be observed. In addition, some micropores and microcracks are also observable. Fly ash contains many active ingredients, such as SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\). On the one hand, a secondary hydration reaction occurred between the fly ash and cement hydration product calcium hydroxide (CH), which generated the gel substance calcium silicate hydrate (C-S-H) and improved the composition of the cementitious materials in the concrete and the compactness of the cement matrix [34]. On the other hand, the presence of active Al\({}_{2}\)O\({}_{3}\) might increase the content of sulfoaluminate hydrate in the hydration product.  
After 28 cycles, the attack products were greatly increased in amount compared to the non-attacked specimens. A large number of needle-shaped ettrinite crystals could be found in T3, and hydrated calcium silicate gel (C-S-H) was observed in a specific area. The specimen compactness was enhanced because erosion products filled the concrete's micropores and microcracks. There was a rapid increase in flexural strength, specimen mass, and dynamic elastic modulus. Increasing the dry-wet cycle period significantly changed the number and volume of these products, such as ettrinite, enhancing the effects in the initial stage of concrete attack and the terminal deterioration.  
As shown in Figure 11, many needle-like prismatic ettrinite (AFI) crystals and much plate-like gypsum could be observed inside the shallow area (15 mm), but more so in T7 than T3. This indicates that the erosion intensity continued to increase with the dry-wet cycle period. With an increase in the attack depth (30 mm), the amounts of attack products continuously decreased. The ettrinite was mainly in the shape of needles and had a smaller volume, as shown in Figure 12a. Therefore, the damage to the concrete caused by the dry-wet sulfate cycles gradually decreased. Upon augmenting the dry-wet cycle period, in the deep area (30 mm), large gypsum crystals formed in T21 and ettrinite was distributed in the form of small and fine needles in the flocculent C-S-H that had not been completely eroded (Figure 12b). The cementation became fibrous, the ettrinite (AFt) distribution was uneven, and there were cracks, but the amounts of attack products slightly increased compared to T3. This means that increasing the interval between wetting processes restricted the supply of SO\({}_{4}\)\({}^{2-}\), thereby reducing the generation of erosion byproducts and, consequently, the erosion intensity. Meanwhile, the ability of the dry-wet process to affect the deeper layers of the concrete also decreased. Therefore, the depth of influence inside the specimen was ultimately limited, restricted by the period of the dry-wet cycle.  
Figure 10: The microstructure before and after the sulfate dry–wet cycle for 84 days. (**a**) Before sulfate dry–wet cycle; (**b**) T3.