## 1 Introduction  
The chemical environment of cement maintains a high pH pore solution (>12.5) which, in turn, keeps the protective oxide film on steel reinforcement in the passive state and protects it from corrosion. Once the passive layer is broken down, aggressive ions in the concrete including air, water, salt, etc., are free to attack the reinforcement which can, over time, lead to structural instability and failure unless preventive measures are taken. Atmospheric carbon dioxide (CO\({}_{2}\)) readily dissolves in cement and the carbonic acid this produces partially neutralises the alkalis, reducing the overall pH, with carbonate and bicarbonate ions remaining in solution [1]. In terms of solid hydrates, the dissolution of CO\({}_{2}\) during hydration causes the destabilisation of portlandite and ettrinigite while calcite, gypsum, and aluminum hydroxide precipitates [1]. These changes in the solid hydrates are also accompanied by changes in the pore solutions which affect the sorptivity of reducing alkalis by the C-S-H as calcium is diminished in the system due to ongoing carbonation [2; 3; 4; 5; 6]. The changes in the hydrated cement due to carbonation as reported by [7] includes the preference of monosulfate and Friedel salt to monocarbonate which, in turn, forms stratlingite, high Ca\({}_{2}\)Si C-S-H gels to low Ca:Si C-S-H gels, calcite and water as ettrinigite dissolves and gibbsite precipitates, leaving sulphate ions in solution. Sulfate ingress into concrete and mortar can lead to expansion, softening, cracking, and spalling which allow further aggressive agents to enter and cause severe durability issues [8; 9; 10; 11; 12]. Sulfates such as Na\({}_{2}\)SO\({}_{4}\) and MgSO\({}_{4}\) can be found in groundwater and wastewater that, unless properly accounted for in the design, will lead to costly shutdowns while the concrete is being repaired or replaced. The influence of external sulfates like Na\({}_{2}\)SO\({}_{4}\) lead to additional or'secondary' internal sulfate phases like ettrinigite and gypsum precipitation along with thaumasite formation that leads to the creation of cracks. Previous work in this area has led to mixed opinions on the influence of cement solid phases on the true cause of crack formation and expansion in cementitious materials subject to sulfate attack [13;14]. The ingress of seawater into cement leads to the conversion or replacement of mono-carbonate with Friedel's salt, along with the removal of typical solid hydrate phases, such as C-S-H, portlandite, ettringite, etc., with a corresponding decrease in solid volume [15; 16; 17; 18; 19]. These phases are replaced with Friedel salt, Magnesium Silicate Hydrate (M-S-H), and the re-precipitation of brucite. These changes come with an initial increase in the total volume of ettringite primarily due to the presence of sulfate, carbonate, and magnesium in the seawater. Thermodynamic modelling of cement hydration offers a reliable means of simulating changes in an anhydrous clinker during its hydration and providing predictions of phase assemblages and pore solution chemistries over time for many cement systems. Accurate predictions are possible once suitable input parameters (clinker oxide proportions, water/cement (w/c) ratio, curing temperature, relative humidity, and the Blaine fineness), an appropriately formatted thermodynamic database, and suitable geochemical software are used with a degree of expertise and a level of understanding to include other practical factors such as oxide components dissolved in the OPC clinker phases, oversaturation of specific phases during the first 12 h of hydration, and the release and uptake of alkali elements (K and Na) by the C-S-H. Most hydration simulations use these factors over time using empirical rate equations to describe the dissolution of OPC clinker phases. The authors have successfully used [20;21] the freely available geochemical software PHREEQC [22] to predict the hydration behaviour of Portland cement over time.