## 5 Discussion  
Carbonation in concrete is caused by the diffusion of CO\({}_{2}\) which reacts with portlandite and the calcium-bearing structural units in CSH to lower the pH by the creation of Ca(HCO\({}_{3}\))\({}_{2}\) in the pores. Below a pH of approximately 9, the microscopic passive layer surrounding the embedded steel is destabilised and is exposed to the oxygen and water in the pore structure leading to its rusting and expansion that causes the concrete to spall [59]. In the cement under analysis here, a pH of 9 exists when over 90 g of CO\({}_{2}\) exists in the system (Figure 14), and all of the portlandite, hydrdcalcite, monocarbonate, ettringite, and siliceous hydrogarnet is depleted with minor amounts of C-S-H, re-precipitated gypsum and M-S-H, Ferrihydrite, Quartz, and gibbsite remaining. The ingress of CO\({}_{2}\) has also led to a significant increase in calcite. This provides a useful tool for concrete suppliers and those who maintain structures as a guide to when carbonation may be occurring so appropriate repair works be planned and carried out. Both the individual calcium-bearing hydrates and the calcium structural units of the CSH gel are susceptible to carbonation in the presence of carbon dioxide and water (or water vapour). The order in which these reactions occur can be predicted by thermodynamic modelling, or as demonstrated here, the maximum extent of carbonation may be predicted for a specific quantity of carbon reacting with the cement. Thermodynamics does not, of course, predict chemical kinetics, although kinetic constraints may be imposed on a reaction scheme that is known to be rapid. This has been applied in other works to model (for example) cement hydration. Figure 14: Change in pH due to ongoing carbonation. Figure 13: Change in C-S-H Ca:Si ratio with increasing seawater exposure. Carbonation of cement and concrete is often a self-limiting process, as the molar volume of the reaction products is greater than that of the reactants. The consequence of carbonation is to close the surface porosity and to limit further gas transfer to the interior of structural concrete, restricting further carbonation to a vanishingly slow rate. This metastable state may persist for many decades (often longer) in well-made structural concrete. Carbonation becomes a serious degradation mechanism when a transport route is established, which allows atmospheric gasses (CO\({}_{2}\) and O\({}_{2}\)) to react with the cement and the steel reinforcement in structural concrete. The carbonation results in a local fall of pore solution pH to the point where corrosion may be initiated. The expansive corrosion products push the concrete apart, increasing the rate of gas transport and hence corrosion. The critical factor in carbonation-induced corrosion is the ease with which carbon dioxide can reach steel reinforcement. This may be along a crack, joint, or poorly compacted region of locally high porosity, or through a section where the depth of cover over the reinforcement was too low to provide adequate protection from carbonation. Temperature influences the behaviour of mortars undergoing sulfate attack both in terms of the change in solid hydrate and the time at which expansion occurs. Delayed ettrinigite formation (DEF) occurs when concrete is cured at higher temperatures (>70 \({}^{\circ}\)C) during precast concrete production or steam curing, and re-equilibrates at lower temperatures. [PERSON] et al. [37] discussed how DEF can lead to greater ettrinigite expansion and gypsum re-formation at later stages as sulfates continue to react with hydrates [59]. In addition, monosulfate, now the dominant Afm phase replacing monocarbonate at the higher temperature, converts to ettrinigite adding to its increased volume. This is shown in Figure 15 where the cement here is modelled at a curing temperature of 80 \({}^{\circ}\)C. As may be seen, ettrinigite is not stable above approximately 35 \({}^{\circ}\)C where it dissolves and monosulfate re-precipitates [60].