## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
In PC systems, the reactive silica in DK reacts with portlandite (Ca(OH)\({}_{2}\)) to form additional calcium silicate hydrate (C-S-H), the principal phase contributing to long-term strength. This explains the strength enhancements observed in SCM mixes incorporating DK, particularly at moderate replacement levels (20-25%). Additionally, the relatively low alumina (Al\({}_{2}\)O\({}_{3}\)) content in DK may help limit the formation of delayed ettringe (C\({}_{6}\)A(SO\({}_{4}\))\({}_{3}\cdot\)H\({}_{32}\)), which typically occurs due to the reaction of calcium aluminates with sulphates in sulphate-rich environments. In systems with sufficient sulphate, ettringe can form early via the hydration of C\({}_{3}\)A, as illustrated below:  
\[\text{C}_{3}\text{A}+3\text{CaSO}_{4}\cdot 2\text{H}_{2}\text{O}+26\text{H}_{2} \text{O}\to C_{6}\text{A(SO}_{4})_{3}\cdot\text{H}_{32} \tag{3}\]  
\[\text{SiO}_{2}\text{ (amorphous)}+\text{Ca(OH)}_{2}+\text{H}_{2}\text{O} \rightarrow\text{C-S-H} \tag{4}\]  
where C = CaO, S = SiO\({}_{2}\), A = Al\({}_{2}\)O\({}_{3}\), and H = H\({}_{2}\)O. The high surface area and amorphous structure of DK enhance its pozzolanic reactivity, contributing to improved long-term strength and reduced permeability. In some DK-containing systems, rapid setting has been observed, which may result from soluble sulphate species and residual reactive alumina promoting early hydration reactions and localised ettringe and C-S-H formation. However, the formation of ettringe is highly dependent on system chemistry and lime availability, as shown by the absence of ettringe in low-line DK systems studied in [29]. Figure 5: Compiled compressive strength results of DK used in SMS systems—based on [PERSON] et al (2010) [34], [PERSON] (2023) [35], [PERSON] et al. (2023) [42], [PERSON] (2010) [36], and [PERSON] et al (2023) [43]. 3.4.3. DK in Alkali-Activated or Geopolymer Cement Systems  
Alkali-activated cements (AACs) are PC-free binders originating from the reaction of an alkali metal source with an (alumino-) silicate. They are commonly under two main categories and respective activation models, namely (1) high-calcium (Ca+Si) model, such as the activation of blast furnace slag (with a CaO-SiO\({}_{2}\) > 70%) to form C-(A)-S-H (calcium (alumino-) silicate hydrate) gel as a main reaction product, which is like the gel obtained during PC hydration (except here Al is also present), and (2) low-calcium (Si+Al) activation model, comprising activation of materials rich mainly in aluminium and silicon, e.g., type F fly ash (low in calcium), metakaolin or silica fume. Although DK has not been discussed in this context in the reviewed literature, it would belong to the latter category. In this category, the main reaction product is N-A-S-H (alkaline aluminosilicate) gel, a three-dimensional inorganic alkaline polymer that can be regarded as a zeolite precursor, known as a geopolymer [6]. The main process of geopolymer formation involves three main steps: (1) dissolution of aluminosilicate precursors, releasing silicate and aluminate monomers; (2) polycondensation and reorganisation of these monomers into semi-crystalline three-dimensional structures; (3) gel hardening, where the aluminosilicate gel condenses further into a rigid, cross-linked network. Monovalent alkali metal cations (e.g., Na\({}^{+}\), K\({}^{+}\)), generally denoted as M\({}^{+}\), are required to balance the negative charge of AlO\({}_{4}\)\({}^{-}\) units, and water is progressively released during the condensation process.