## 3 Literature Review
### DK in Cements for Mortars and Concrete
#### 3.4.2 DK as SCM in Portland Cement-Based Concrete
The general chemical reactions representing each stage are as follows:  
\[\text{Al}_{2}\text{Si}_{2}\text{O}_{7}\ (\text{s})+4\text{OH}^{-}+7\text{H}_{2} \text{O}\to 2\text{Al}(\text{OH})_{4}\text{${}^{-}$}+2\text{Si}(\text{OH})_{4} +2\text{H}_{2}\text{O}\ (\text{Dissolution}) \tag{5}\]  
\[\text{Si}-\text{OH}+\text{HO}-\text{Si}\rightarrow\text{Si}-\text{O}-\text{ Si}+\text{H}_{2}\text{O}\ (\text{Polycondensation}) \tag{6}\]  
\[\text{Si}-\text{OH}+\text{HO}-\text{Al}\rightarrow\text{Si}-\text{O}-\text{ Al}+\text{H}_{2}\text{O}\ (\text{Hydrate gels}) \tag{7}\]  
\[[-\text{AlO}_{4}\text{${}^{-}$}]+\text{M}^{+}\rightarrow[-\text{AlO}_{4} \text{${}^{-}$}\cdot\text{M}^{+}]\ (\text{Hardening}) \tag{8}\]  
where M\({}^{+}\) represents a monovalent alkali metal cation such as Na\({}^{+}\) or K\({}^{+}\). The chemical composition of the aluminosilicate precursor, alkali combination used in Equation (5) and the gel concentration and hard composite geopolymer formed in Equation (8) depends on several variables such as particle size, dissolution and alkaline concentration, among others. The following section reviews research publications in DK used as precursor for geopolymers through the process described in Equations (5)-(8). DK has also been explored in the development of AAC/geopolymer cements. As major constituents of DK are silica and alumina, DK is likely to be a good candidate as AAC/geopolymer precursor. Nine studies were identified in the literature, three of which investigate DK and metakaolin (MK) precursor system. Namely, in [16], mortars consisting of various blends of DK and MK were examined to evaluate their compressive strength performance. A total of 13 mortars with different DK/MK ratios (ranging from 20% DK to 70% at 5% intervals) were tested, with sodium hydroxide (NaOH) used as the alkali activator. Water content and NaOH were used to maintain a fixed water-to-sodium oxide (H\({}_{2}\)O/Na\({}_{2}\)O) molar ratio of 10 in all 13 mixes. Results showed that the mortars containing 35% and 40% DK exhibited the highest compressive strength values at both 7 and 28 days of curing. The values of compressive strength for the 40% DK mortar at 7 days and 28 days were 41.4 MPa and 51.3 MPa, respectively. SEM/EDX showed a more homogenous matrix and a smooth surface with lower cracks in mixes with highest strengths. Another study with a wider scope tested 26 mortars with different DK/MK ratios (ranging from 0% DKto 60% at 5% intervals) [15]. Water and NaOH were used to form two amounts of Na\({}_{2}\)O (1.94 M and 2.32 M) and a unified H\({}_{2}\)O/Na\({}_{2}\)O molar ratio of 13.7. The mix with 25% DK (with the lower Na\({}_{2}\)O = 1.94 M) showed the best results in terms of highest compressive strength, lowest water absorption, and lowest porosity. Compressive strength values of this mix at 7 days and 28 days of curing were 55 MPa and 64 MPa, respectively, compared to the 0% DK mix (11 MPa and 18 MPa, respectively). Porosity and water absorption of the 25% DK mix were 12.5% and 6.5%, respectively, compared to the 0% DK mix (16.5% and 8.5%, respectively). Similar results were also obtained by [PERSON] et al. [18] who tested nine DK/MK mortar mixes using three different activator concentrations. The 50% DK mix had the highest compressive strength at 7 days of 63 MPa. Flowability and setting time were also tested and it was shown that their values reduced with increasing DK contents.