## 3 Results and Discussion
### Step 4: Optimizing Quaternary Cementitious Matrices
#### 3.3.4 Microstructural Characterization of Quaternary Cementitious Matrices  
Indicatively, four BSC micrographs of PC55 LS20 FA20+5%\(\upmu\)S are shown in Figure 23. As discussed earlier in Portland limestone cement pastes with high FA contents except for the needle-like hydration products (Figure 23 C,D), CaCO\({}_{3}\) flakes and Ca(OH)\({}_{2}\) crystals a number of rims and reacted FA particles can be detected. It is interesting to note that the investigation of different areas did not yield indications of unreacted FA particles. A fact correlating well with the TG measurements showing that there was enough Ca(OH)\({}_{2}\) still present in the hydrated paste after day 28 and 56 should the FA have needed it to react with. Reacted FA (rFA) particles are circled in Figure 23B. Furthermore, in Figure 23B the reaction of a FA particle leaving a characteristic mullite sphere [39] was identified (enclosed in the rectangle). It was suggested by [PERSON] and [PERSON] that the glass phase in FA has been reacted and the crystalline phase is left almost intact. Adding to this, the typically acicular hydration product of FA in the first 28 days can be seen in Figure 23B (enclosed in a rectangle). Lastly, the presence of microcracks is prominent, possibly signaling cracking due to elevated heat of hydration. Therefore, this specific formulation could be enhanced by reducing the silica fume content. Figure 21: Ca(OH)\({}_{2}\) content \(\upmu\)S modified cementitious matrices based on PC43 LS20 FA37. Figure 22: CaCO\({}_{3}\) content of \(\upmu\)S modified cementitious matrices based on PC43 LS20 FA37. Comparing the reference paste PC60 LS20 FA20 (Figure 14) with the 5% \(\upmu\)S (Figure 23) and the 10% \(\upmu\)S (Figure 24) enhanced quaternary formulations and particularly the 200 times magnification there is an indication that the \(\upmu\)S enhanced pastes are less dense. This cannot be attributed to particle packing that is expected to be enhanced, but credited to the high amount of FA present. The FA was already feeding from the Ca(OH)\({}_{2}\), therefore the more reactive pozzolanic particles of \(\upmu\)S may not be able to access the Ca(OH)\({}_{2}\) crystals encapsulated within the reacted FA particles. That is to say, although Ca(OH)\({}_{2}\) is not depleted in these quaternary formulations, the \(\upmu\)S particles may be deactivated by the inaccessibility of Ca(OH)\({}_{2}\) created by the hydration products around the much greater quantities of FA. As depicted in Figure 24, a similar microstructure to the lower \(\upmu\)S content paste can be observed. The only difference being the plethora of needle-like hydration products detected at all magnifications and the many unreacted \(\upmu\)S particles (small, spherical bright elements). The typically acicular hydration product of FA in the first 28 days can be seen in Figure 24B (enclosed in a rectangle). Another FA particle with hydration gel around it, is circled in Figure 23C. Reacted FA particles are circled in Figure 24C. In agreement with [PERSON] et al. [16], the \(\upmu\)S enhanced pastes showed a more refined pore structure, but are expected to deliver higher total porosities. That is to say, capillary pores were expected to be reduced and finer pores slightly increased. This could partly explain the microcracks present in the images taken. The increased heat of hydration could be the other reason for the development of microcracks as discussed for the +5%\(\upmu\)S addition. Knowing that the presence of the cracks is related to lower durability of the formulations, future research on quaternary pastes using this formulation should contain substantially lower \(\upmu\)S contents. In fact, this observations led to the reduction of the \(\upmu\)S contents in the design of the quinary combinations, based on PC43 LS20 FA37.