## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
Evidence for this interpretation was based from the combination of multiple techniques: DSC and XRD analyses, which showed reduced CH peaks and stronger C-S-H signatures; FTIR spectra, which revealed stronger bands associated with silicate and aluminate phases; SEM micrographs, which illustrated denser microstructures in SF-containing pastes with fewer visible portlandite crystals and a more homogeneous distribution of hydration products. The microstructural evidence presented (especially from SEM) suggests that the fine particle size of SF may improve packing density and reduce porosity. These observations support the authors' view that SF contributes not only chemically through extended pozzolanic activity but also physically by enhancing matrix densification, although it is unclear whether the observed microstructural densification is primarily due to filler effect of SF or a continuation of pozzolanic activity under limited CH, which are both possible. This dual chemical-physical role may be particularly beneficial in systems where DK has already consumed most of the available CH, allowing SF to continue contributing to long-term strength through slower pozzolanic reactions and microfiller effects. Hydration kinetic measurements further corroborated the overall trends. Chemically combined water (Wn%) increased with both DK and SF content over time, reflecting the progressive formation of hydration products. However, as each mix used a different water content to achieve standard consistency direct comparisons of Wn% across different mixes are difficult to make. Free lime measurements revealed that CH content decreased over time in all DK-containing mixes, with the most pronounced reductions observed in mixes containing both DK and SF. This confirms the active pozzolanic consumption of portlandite to produce C-S-H and aligns with the observed strength development and phase changes. Overall, the study provides a thorough analysis of how DK and SF interact in cement pastes. While some aspects, such as the exact contribution of SF relative to DK in high-replacement mixes, could benefit from further quantitative differentiation between chemical and physical effects, the authors' interpretation is well-supported by multi-technique evidence. The findings point to a potentially valuable synergy between a rapidly reacting pozzolan (DK) and a more slowly reacting one (SF), particularly in enhancing long-term mechanical and microstructural performance that could be explored for DK mix design optimisation. To conclude the section and visualise key findings, Figure 5 presents compiled compressive strength results (for pastes, mortars, and concrete) from five available studies to facilitate a comparison between the results using DK as SCM. Each study is represented with a distinct colour, marking the respective control mixes and those incorporating DK. The figure clearly illustrates that incorporating DK in low percentages (typically between 5% and 15%) consistently enhances compressive strength by an average of 20%, regardless of the tested material (paste, mortar, or concrete), the type of cement used, or the source of DK. However, only one study [43] investigated higher replacement percentage (above 20%), and a notable decline in compressive strength was observed. This explains why most researchers have shifted focus towards multicomponent SCM-DK systems with lower DK content, as compressive strength remains the most important mechanical property in evaluating concrete performance. This study [43] specifically tested higher percentages of DK reaching 50% replacement and reported a high drop in compressive strength of 30% compared to the control mix, unless a small amount of TiO\({}_{2}\) was used. To summarise, the above findings confirm that DK can be used as a SCM, primarily due to its high amorphous silica content and strong pozzolanic reactivity.