## 3 Simulation Results and Discussions
### Influence of Na\({}_{2}\)SO\({}_{4}\) Concentration and Temperature  
Figure 7 shows the amount of Na\({}_{2}\)SO\({}_{4}\) addition on the hydration of PC. The simulation conditions, input parameters, and temperature profiles (Figure 4), except Na\({}_{2}\)SO\({}_{4}\) addition, are the same as used in Section 3.2. A higher addition of Na\({}_{2}\)SO\({}_{4}\) resulted in a higher formation of sulphate-bearing products, which may induce cracking due to high volume of hydrates. Mirabilite could only be formed after  
Figure 5: (**A**) Hydrate assemblage of PC without Na\({}_{2}\)SO\({}_{4}\), (**B**) hydrate assemblage of PC with 25% of Na\({}_{2}\)SO\({}_{4}\), and (**C**) pore solution concentration of PC with 25% of Na\({}_{2}\)SO\({}_{4}\), for varying temperature.  
Figure 6: CASH end member of hydrating PC, (**A**) without Na\({}_{2}\)SO\({}_{4}\), and (**B**) with 25% of Na\({}_{2}\)SO\({}_{4}\).  
17% of Na\({}_{2}\)SO\({}_{4}\) addition and there was no formation of themadite, because the hydration temperature at 28 days of curing was 20 \({}^{\circ}\)C (Figure 4). As shown in Figure 2, the mirabilite was in equilibrium at around 1.3 mol/L of Na\({}_{2}\)SO\({}_{4}\) at 20 \({}^{\circ}\)C; the concentration of sulphate ions in the pore solution with the addition of Na\({}_{2}\)SO\({}_{4}\) up to 17% was less than 1.3 mol/L, hence the mirabilite could not form. Sodium sulphate addition did not influence the stability of main hydration products such as CASH and portlandite, and other hydration products of hydrotalcite and C\({}_{3}\)FH\({}_{6}\), but hemicapoaluminate was formed at lower additions of Na\({}_{2}\)SO\({}_{4}\) and was changed to monocarbonoaluminate with increasing Na\({}_{2}\)SO\({}_{4}\). The formation of monosulfoaluminate was restricted to in very low (<2.5% of Na\({}_{2}\)SO\({}_{4}\)) concentrations or in the complete absence of Na\({}_{2}\)SO\({}_{4}\), and it changed to ettringe with availability of sulphate. Thus, the simulation results provide useful insights into the degradation of cementitious materials relating to mineralogical distribution with Na\({}_{2}\)SO\({}_{4}\) addition.  
In order to examine the influence of Na\({}_{2}\)SO\({}_{4}\) concentration on the hydrated PC, a thermodynamic calculation between hydration products and sodium sulphate solution was carried out. Solid products and pore solution concentration at 28 days of hydrated PC (as shown in Figure 3C,D) were used as initial input parameters. Figure 8 shows changes of equilibrated hydrates as a function of Na\({}_{2}\)SO\({}_{4}\) concentration, which had a significant influence at equilibrium concentrations higher than almost 2 mol/L at 25 \({}^{\circ}\)C (Figure 8A). Mirabilite and monosulfoaluminate were produced and ettringe was destabilised at high concentrations of Na\({}_{2}\)SO\({}_{4}\) equilibrated with hydrates at 25 \({}^{\circ}\)C. From the solubility of mirabilite (Figure 2), mirabilite could form at around 2 mol/L of Na\({}_{2}\)SO\({}_{4}\) at 25 \({}^{\circ}\)C and destabilised the ettringe. The remaining sulphate and aluminium ions from the destabilisation of ettringe were in equilibrium with monosulfoaluminate to produce monosulfoaluminate. However, thenadite could only be formed at high concentration of Na\({}_{2}\)SO\({}_{4}\) and elevated temperature, which changed ettringe to monosulfoaluminate independent of sodium sulphate concentration. The environment in the pore solution of cementitious materials induced formation of mirabilite or thenadite depending on sodium sulphate concentration and temperature, which affected stability of the other sulphate-bearing phases, mainly ettringe and monosulfoaluminate. The stability of CASH and portlandite did not depend on Na\({}_{2}\)SO\({}_{4}\) concentration or temperature, as shown in Figure 8.  
Figure 7: Distribution of hydrates at 28 days of hydration of PC as a function of Na\({}_{2}\)SO\({}_{4}\) addition.