## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
The gamma-ray shielding power was also investigated, and results showed a significant improvement, as the linear attenuation coefficient (\(\upmu\)) increased from 0.127 cm\({}^{-1}\) to 0.199 cm\({}^{-1}\) at 661.6 KeV for the DK mix with TiO\({}_{2}\) and control (0% DK) mix, respectively. The authors explained that the TiO\({}_{2}\) may act as a nano-filler and active seeds in the formation of gels CSHs, CAHs, and CASHs as indicated in the XRD, TGA, and SEM tests. The N\({}_{2}\) adsorption-desorption isotherms showed texture characteristic changes from macro to mesopores when TiO\({}_{2}\) was added. The increase in compressive strength was also attributed to structure redistribution. The innovative idea of incorporating a small amount of titania could potentially lead to a breakthrough in achieving higher DK contents in the concrete mixes. DK as SCM with the addition of nano-carbon black (NC) was also studied for mortar mixes [35]. Cement mortar samples with up to 20% DK were tested; they all showed an enhancement in compressive strength, with 28-day strength values of 46.2 MPa and 50.7 MPa, respectively, for 5% and 10% DK. This corresponded to a 4.5% and 14.4% improvement, respectively, compared to the control sample (44.3 MPa). The FTiR results indicated the formation of more C-S-H gel when DK was introduced. By adding small percentages of NC (from 1% to 3% by DK weight), a marginal gain in compressive strength of approximately 1.5% was observed in all mortar samples. Regardless of the inclusion of NC, all samples with DK generally suffered a dramatic reduction of over 50% in setting times. Namely, the initial setting time was 58.3 min and 79.5 min for the 20% DK for the control and 20% mix, respectively; the final setting times were 118.6 min and 158 min for the control and 20% mix, respectively. Silica fume (SF) was also incorporated into DK/cement mixes [45]. In this detailed experimental study, DK was used as a partial cement replacement at 20%, 25%, and 30% by weight to produce pozzolanic cement pastes with and without the addition of 5% SF. The experimental programme included evaluations of early and late compressive strengths, as well as hydration kinetic parameters such as chemically combined water (Wn%) and free lime content. The hydration behaviour and performance of these systems were evaluated over a 90-day period using compressive strength testing, Differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The results showed that early-age compressive strengths (1-28 days) were generally lower in DK-modified pastes compared to the control (100% PC), which the authors attributed to unreacted DK, acting as a filler in the matrix until its pozzolanic activity becomes dominant. However, at 90 days, pastes with 20% and 25% DK demonstrated improved or comparable strength to the control, attributed by the authors to the gradual pozzolanic consumption of calcium hydroxide (CH) and the consequent formation of C-S-H. Conversely, the 30% DK blend exhibited the lowest strength across all hydration ages; the authors attributed this to excessive cement replacement resulting in insufficient portlandite (CH) generation to sustain further pozzolanic reactions, and hence strength gain, due to excessive unreacted material. In this case, the unreacted DK is likely to have acted more as an inert filler than as a reactive material. The sharp performance drop at 30% DK suggests a critical threshold for useful replacement, reinforcing the need for cautious mix optimisation. The inclusion of 5% SF in the DK blends led to considerable improvements in compressive strength at both early and late hydration ages, with the greatest enhancement observed in the 30% DK mix. The authors attributed this to a synergistic pozzolanic effect between DK and SF, both of which react with portlandite (CH) to form large quantities of amorphous and microcrystalline C-S-H.