*3.2. Influence of sFCCC on Setting Times of the New Binder*

Figure 5 shows the setting times of the prepared binder pastes. The paste made only with the sFCCC and Portland cement (FCCC70) showed a flash setting (initial setting was observed already after 15 min), while the setting times of the paste prepared with FA and CEM (FA70) were significantly longer than the setting times of the paste prepared with pure cement (CEM). Decreasing percentages of the FA were substituted with the sFCCC in the binder mixes (starting from 50 mass %, Figure 5, binder FCCC35) in order to obtain the new binder with setting times shorter than the setting times of the FA70 paste, but comparable to setting time of the cement paste. Finally, the binder prepared with 21 mass % of the sFCCC (FCCC21) showed the envisaged properties regarding setting times, and was used for further studies. The goal of this approach was to use sFCCC as an activating component of the binder. The setting times of the new binder should be shorter than the setting times of the binder based only on FA. The obtained results confirmed that sFCCC is a very active pozzolanic material [4]. The accelerating effect of the sFCCC in the blended cement pastes has also been attributed to the adsorption of water on the sFCCC particles and to decreased sulfate content in the blended cement [5].

**Figure 5.** Setting times of the new binder and CEM pastes.

#### *3.3. Compressive Strength of the New Binder*

Analyses of the compressive strength of the new binder mortars showed that using sFCCC as the binder component had a positive impact on early (2 days) compressive strength (Table 3). Afterwards, compressive strength of the FA70 binder was higher than the strength of the FCCC21 binder. Relatively high compressive strengths of the FA70 sample were probably achieved due to the fact that mechanically activated FA was used in the preparation of the new binder. Very fine particles of the ground FA acted as nucleation sites and contributed to cement hydration at the early stages [6].


**Table 3.** Compressive strength of the new binder and CEM mortars.

*3.4. Mineral Composition and Microstructure of the New Binder*

Mineral compositions of the binder pastes based on FA and the combination of FA and sFCCC after 28 days of curing were similar (Figure 6). The main hydration products of the new binders were calcium aluminate hydrate (CAH) and ettringite (E). The shape of the peak at 29.3 ◦2*θ* indicated that calcium silicate hydrate (CSH) was probably formed in the new binder pastes too. As seen from Figure 6, portlandite (calcium hydroxide, CH) could not be detected in the XRD graphs of the binders FA70 and FCCC21, cured for 28 days, which could be explained by the low content of cement in the binders and portlandite consumption by the pozzolanic reactions of FA and sFCCC [4]. The XRD analysis of the CEM paste showed a mineral composition typical for the material.

**Figure 6.** Mineral composition of the new binder and CEM pastes cured for 28 days.

SEM analyses of the FA70 and FCCC21 binder pastes cured for 28 days showed that the microstructure of the new binders was heterogeneous and relatively porous, as seen from the BSE micrographs (Figures 7a and 8a). It seems that the FCCC21 paste was slightly more porous than the FA70 paste, which corresponds with the observed differences in compressive strength (Table 3), and could partly be explained by a higher water/binder ratio of the FCCC21 paste (Table 1). SEM micrographs taken at the higher magnifications confirmed the presence of the hydration products identified by the XRD analyses, probably CSH (Figure 7b) and ettringite (Figure 8b).

**Figure 7.** SEM analyses of the FA70 paste after 28 days of curing: (**a**) microstructure; (**b**) reaction products.

**Figure 8.** SEM analyses of the FCCC21 paste after 28 days of curing: (**a**) microstructure; (**b**) reaction products.

The structural characterization of the new binder pastes did not provide an explanation for the observed differences in the strength development of the binder mortars after the first 2 days of curing (Table 3). It is possible that portlandite was consumed in the FCCC21 more quickly than in the FA70 binder due to the high reactivity of sFCCC [4]. The lower strength of the FCCC21 binder after 28 days of curing, compared to the FA70, could also be due to the very fast hydration of the binder containing sFCCC at the beginning of the reaction, and the attachment of the reaction products on the surface of the un-reacted particles [19,20].
