## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete  
As noted earlier, DK is chemically characterised by a significant reduction in alumina content compared to metakaolin (MK) and an increase in reactive amorphous silica. This transformation affects its behaviour as pozzolanic material. Most studies (15 publications) focus on DK as partial replacement of Portland cement (PC). An early comprehensive investigation [36] used two different Egyptian DK samples combined with CEM I and II (according to EN 197-1 [37]). Tests studied the setting time, flow, and slump of cement and fresh concrete as well as the compressive strength of mortars and hardened concrete. Samples were prepared using two approaches: partial replacement of cement with DK and DK added to PC. The results showed that 10-15% DK replacement enhanced mortar strength by up to \(\sim\)14% (except for one mix where the strength was not affected) and concrete strength by \(\sim\)10%. The lower strength improvement in concrete mixes was attributed to the difficulty of dispersion of DK particles in concrete mixes compared to mortar mixes. This observation is likely due to surface adsorption effects; DK immobilises water because of its high specific surface area, increasing the water demand and necessitating the use of superplasticisers. The reduction in flow (-50%) was also attributed to the fine particles of DK and the increased porosity, increasing water demand and necessitating superplasticiser use to maintain consistent slump (S4) (170-200 mm), compared to the control mix. The investigation also reported a high specific surface area for both DKs used, i.e., 20.1 m\({}^{2}\)/g and 43.2 m\({}^{2}\)/g, respectively, and high strength activity indices (SAI) at 28 days for both DKs, i.e., 0.94 (94%) and 1.13 (113%), respectively, for a 10% cement replacement by DK. Setting time remained broadly unchanged. The authors do not explain this observation, but it is possible that DK, containing reactive alumina and sulphate, and its interactionwith cement aluminates facilitated early ettrinigite (AFt) formation, thus compensating for potential retardation effects. Moreover, preliminary durability testing on selected concrete samples showed that the carbonation depth was almost 2 mm after exposure for 70 days at 25 \({}^{\circ}\)C using 4% CO\({}_{2}\)[36]. A study by [PERSON] et al. [34] compared acid DK (pH \(\sim\)5, DKut) and lime-treated DK (pH \(\sim\)8, DKt) used as partial PC replacement in mortars. They had, respectively, 42 m\({}^{2}\)/g and 40.1 m\({}^{2}\)/g BET specific surface area; DKut showed \(\sim\)127% pozzolanic activity and DKt \(\sim\)117% pozzolanic activity vs. SF. The mineralogical composition of both DKs consisted mainly of quartz (SiO\({}_{2}\)) with small traces of anatase (TiO\({}_{2}\)). The only difference was that DKt had a significantly higher amount of calcium sulphate hemihydrate (CaSO\({}_{4}\)\(\cdot\)\(\lambda\)H\({}_{2}\)O) compared to DKut. Both DK types required a higher water-to-binder (\(w\)/\(b\)) ratio to achieve a standard consistency as DK dosage increased; this was expected due to the high specific surface area of DK and is consistent with [36]. DKut accelerated setting by \(\sim\)25% at 10% replacement, while DKt initially slightly reduced setting time at 5% replacement but retarded it above 5% replacement. The authors explained that DKut contains a higher amount of alumina (13%) versus only 5.6% in cement, which can promote the formation of crystalline hydration products when used with cement, thereby accelerating both hydration and setting. This is consistent with [29], which confirms the presence of significant reactive alumina content in DK, available in a soluble form, and observing that DK rapidly consumes lime and forms aluminous hydration products, including ettrinigite at high Ca(OH)\({}_{2}\) availability and allophone-type phases at low Ca(OH)\({}_{2}\) availability.