## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
At 28 days, submerged-cured specimens with 10% and 15% DK showed compressive strengths of 52.6 MPa and 47.5 MPa, corresponding to increases of 27.7% and 15.3% over the control mix (41.2 MPa). Compressive strength was used as the primary indicator of durability, complemented by tests on resistance to chloride and sulphate attacks, seawater exposure, and elevated temperatures. Concrete specimens were cured in 5% NaCl solution and tested at the aforementioned ages. Strength continued to rise in DK-modified mixes, peaking at 65 MPa (10% DK) and 57.7 MPa (15% DK) at 90 days. This increase was attributed to the active amorphous silica in DK, which reduces chloride permeability and enhances Cl\({}^{-}\) resistance. A slight strength drop (-7%) was observed at 180 days, likely due to Friedel's salt formation (calcium chloroaluminate) from CaCl\({}_{2}\) and calcium aluminate reactions. A similar trend was observed for specimens cured in seawater (salinity = 4.3%), with comparable strength values. Concrete exposed to 5% MgSO\({}_{4}\) solution showed strength gains, albeit lower than those seen under chloride or seawater exposure. The strength increase was attributed to the formation of gypsum, resulting from the reaction between the sulphate ions and calcium ions. Gypsum reacts with calcium aluminate to form ettrinigite mineral which may increase expansion. Regarding heat resistance, the same study [41] reported increased compressive strengths at 300 \({}^{\circ}\)C, followed by reductions at higher temperatures. At 300 \({}^{\circ}\)C, 10% and 15% DK mixes reached 75.5 MPa and 73.8 MPa, respectively, then declined to 33.4 MPa and 25.7 MPa at 700 \({}^{\circ}\)C. The control mix reached 63.6 MPa at 300 \({}^{\circ}\)C and 17.8 MPa at 700 \({}^{\circ}\)C. Thus, the 10% DK mix showed the best thermal resistance at 700 \({}^{\circ}\)C, with a 36.5% strength reduction from its room temperature value (52.6 MPa). Total porosity also decreased notably in DK mixes: at 700 \({}^{\circ}\)C, 10% and 15% DK mixes had porosities of 12% and 13.4%, respectively, compared to 23.4% in the control. This reduces permeability and thus enhances chemical resistance, contributing to improved durability. A related study with a narrower scope [42] assessed the durability of cement mortars with 7.5% and 10% DK exposed to seawater (salinity = 4.3%). Compressive strength was again used as the primary durability indicator, along with expansion and weight change. As in [39], strength increased up to 90 days then declined. At 28 days, compressive strengths of 0%, 7.5%, and 10% DK mixes were 37 MPa, 46 MPa, and 42 MPa, respectively. At 90 days, they were 44 MPa, 52 MPa, and 48 MPa, respectively; by 365 days, they had dropped to 31 MPa, 41 MPa, and 37 MPa. Mortar bar expansion was lowest in the 7.5% DK mix (0.078%) compared to the control (0.12%), indicating enhanced chemical resistance. Weight gain was also monitored: a slight increase was observed in all mixes, likely due to moisture absorption or calcium compound formation. The 7.5% DK mix had the lowest gain (0.62% at 365 days) versus 1.07% in the control. Some studies also investigated the performance of DK-containing cement systems with further chemical additives such as titania (TiO\({}_{2}\)) [43,44]. Three different ratios of cement/DK were tested (90/10, 70/30, 50/50) and an additional mix incorporating 5% TiO\({}_{2}\) (cement 50%, DK 45%, TiO\({}_{2}\) 5%) was specially prepared. The 28-day compressive strength results showed that the inclusion of 5% TiO\({}_{2}\) raised the strength value from 56 MPa (sample with 50% DK) to 88 MPa as compared to the control mix (80 MPa). The study found that a high replacement level of up to 50% DK is feasible in cement systems and can even result in better compressive strength performance with the addition of a small percentage of commercial TiO\({}_{2}\).