*3.5. Scanning Electron Microscopy*

In the SEM images, crystalline structures were observed in the pores of the nonattacked mortars containing the crystalline admixture (Figure 8a,b). These structures had very similar shapes (needlelike crystal) to those that the admixture manufacturer assures are formed when the crystalline chemical reacts with the calcium hydroxide and other by-products of cement hydration. These crystalline formations are insoluble in water and fill and plug pores, capillaries and microcracks of the cementitious materials [62]. In addition, in the mortar manufactured with the Xypex admixture, the typical hexagonal shapes of the portlandite were observed (arrows in Figure 8d). Figure 8c,d shows images of the mortar with the crystalline admixture after being exposed to the sulphuric acid solution for 56 days (8 weeks). It is also possible to observe crystalline formations in the pore but with lower density than for the case of the nonattacked C mortar. The decrease in crystal density may be due to the acid attack that could be reducing the crystalline formations. In the SEM images corresponding to the nonattacked control mortar (Figure 9a,b), the needlelike crystalline shapes described above did not appear. After the acid attack, crystalline forms appeared in the control mortar (Figure 9c,d), which were different from those observed in the mortar with the admixture, and they could correspond to bassanite (hemihydrate phase of gypsum). The bassanite was the result of the effect of the sulphuric acid on the products of cement hydration and was detected in the diffractogram (Figure 5a).

(**a**) (**b**)

**Figure 8.** *Cont*.

**Figure 8.** Scanning electron microscopy (SEM) images performed at 84 days (12 weeks) from the mortar manufacture. Left (**a**,**c**): images at 500× magnification. Right (**b**,**d**): zoom at 1500× magnification on the left image square. Top (**a**,**b**): mortar type C (with 1.5% of crystalline admixture) kept in a nonaggressive environment. Bottom (**c**,**d**): mortar type C exposed to a sulphuric acid attack for 56 days (8 weeks).

(**a**) (**b**)

**Figure 9.** SEM images performed at 84 days (12 weeks) from the mortar manufacture. Left (**a**,**c**): images at 500× magnification. Right (**b**,**d**): zoom at 1500× magnification on the left image square. Top (**a**,**b**): mortar type A (0% of crystalline admixture) kept in a nonaggressive environment. Bottom (**c**,**d**): mortar type A exposed to a sulphuric acid attack for 56 days (8 weeks).
