## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
Indeed, in PC, the presence of Al\({}^{3+}\) and SO\({}_{4}\)\({}^{2-}\) encourages early ettrinigite (AFt) formation, increasing rapidly the stiffness of the mix (thus quick setting). Conversely, the lime-treated DKt contains increased amounts of calcium sulphate hemihydrate (CaSO\({}_{4}\)\(\cdot\)\(\lambda\)H\({}_{2}\)O), and likely some excess lime (Ca(OH)\({}_{2}\)), which appear to shift the hydration pathway, potentially reducing the formation of ettrinite (AFt) and favouring monosulphate (AFm) as a dominant sulphate-bearing phase. Although the authors do not directly attribute the delayed setting to AFm formation, the observed compositional differences suggest an altered early hydration pathway that may contribute to slower matrix stiffening, as AFm (which is less expansive than Aft) forms more slowly than Aft and is less associated with rapid early crystallisation; this could explain the observed delay in setting. These contrasting behaviours of the two DK types in fresh mixes show how small compositional shifts in DK (alumina form, sulphate) can alter early hydration kinetics and pathways, ultimately determining whether setting is accelerated or delayed. Consistently with [36], the study also reported a considerable drop in the relative flowability (RF) (i.e., the ratio of the flow directly after mixing to the flow after an elapsed period) with the incorporation of either type of DK. After 60 min, the reported RF was 63% and 76% at 15% replacement levels of DKut and DKt, respectively. Finally, the study showed an enhancement in compressive and tensile strengths by 12% and 10%, respectively, at 10% DK replacement. Other studies confirm up to \(\sim\)20% strength gain in concrete with \(\leq\)15% DK substitution of PC [28]. Conversely, [PERSON] [38] observed a 3% improvement in compressive strength of concrete with \(\leq\)15% DK substitution of PC and a 25% reduction in slump at 15% DK replacement, unlike [36]. However, [38] did not use any superplasticiser or water reducers, which may explain the poor workability and potentially the limited strength gain, as the mix could have been too stiff and porous, with insufficient water for effective hydration. Two subsequent studies incorporated DK in cementitious mortars [39; 40], where DK was introduced as a sand replacement rather than cement replacement. Study [39] found that the setting time was broadly unaffected, consistently with [36]; an 18% drop in flowabilityat 10% DK content and a 46% increase in compressive strength with 17.5% DK content were observed. Higher DK contents led to compressive strength reduction. [40] mainly focused on the effect of incorporating different DK particle sizes ranging from under 75 \(\upmu\)m to over 300 \(\upmu\)m--a particle size range larger than expected for DK given the high specific surface areas reported in other studies. No considerable effect of DK particle size on compressive strength was found. At a replacement percentage of 2.5%, DK significantly delayed the setting time, acting as a clear retarder. Higher replacement percentages slightly reduced the setting time. Overall, the presence of DK shifted the setting time curve towards retardation, unlike in [34], where DK acted as a strong accelerator in both low and high DK percentages. However, the particle size range in [40] (75-300 \(\upmu\)m) is unusually coarse for DK (considering particle size ranges reported in most other studies), which may partially explain the observed retardation. A comprehensive study was carried out to assess the durability of concrete incorporating DK in aggressive environments [41]. Cement was partially replaced by DK at 10% and 15%, and concrete samples were tested at 7, 28, 60, 90, and 180 days under various curing regimes. Initially, compressive strength tests were performed using submerged curing, serving as a baseline for strength development. Additional curing methods were then applied, each addressing specific durability aspects.