# Modelling Leaching Using C-S-H Solid Solutions
## 1 Introduction  
Leaching from cement-based materials is the gradual release of various chemical components, such as calcium (Ca) and silica (Si), into the surrounding environment and the breaking down of solid phases, such as calcium-silica-hydrate (C-S-H) and portlandite. It can affect performance and durability, as well as have a negative environmental impact, if these components enter groundwater or sensitive soils. An increase in calcium in groundwater can affect its alkalinity, with the resulting change in the pH affecting the drinking water quality and aquatic life. Any new cement or concrete should, therefore, consider leaching, especially if they are in environmentally sensitive areas. Not understanding leaching not only affects the durability and longevity of concrete structures but also poses potential environmental risks. Understanding cement leaching is a necessary consideration as new sustainable cement and concrete are developed, so their behaviour in this environment is crucial for determining their viability. Leaching is affected by the environment and temperature and can take several weeks, months, or even years to observe. Therefore, calibrated prediction models that account for changing temperature and accurately represent the incongruent dissolution of calcium and silica within appropriate solid-solution models of C-S-H gels are vital to understand and mitigate the effects of cement leaching. [PERSON] et al. [1] and [PERSON] et al. [2] reported on the dissolution of cement into pore water and the measurement of the changing pH and ion concentrations. Cement paste dissolution models were developed by [PERSON] et al. [3] and [PERSON] [4] to model the long-term change in the composition of the leachate. [PERSON] et al. [5] concluded in their work that hardened cement paste was altered by ongoing leaching and dissolution, and changes in the solid hydrate phases occurred as a result. Others [6; 7; 8] summarised in their findings on the leaching of cement in deionised water that diffusion was the main transport mechanism of ions into the leachate. [PERSON] et al. [1] found that research into the structural changes of hardened cement pastes due to dissolution had hardly been reported, and [PERSON] et al. [9; 10] reported on the dissolution kinetics of aluminosilicate precursors to provide a comparative framework for Si behaviour in leaching. [PERSON] et al. [2] provided a summary of the influence of C-S-H and portlandite on the chemical properties of the liquid phase in a hydrating cement. The high initial pH (>13) created from the dissolution of sodium and potassium hydroxides from the cement was lowered to approximately 12.5 because of the dissolution of other constituent minerals, including portlandite and C-S-H. They reported that previous research [11; 12] measured the ongoing decalcification of C-S-H pastes due to leaching until a constant value of the Ca/Si ratio was reached. C-S-H is the main hydrate phase of Portland cement. The hyphens are used to symbolise the variable composition and poorly defined crystalline structure [13]. The C-S-H Ca/Si ratio and calcium concentration ultimately control the type of gel formed [2; 14], which can change due to due to the addition of supplementary cementitious materials (SCMs) or leaching and chemical attack by external sulphates and/or carbonation [15]. There are many descriptions of C-S-H. [PERSON] [13] provided C-S-H (I) and C-S-H (II), which are similar to tobermorite and jennite, respectively [2]. C-S-H's solubility can be modelled using solid-solution models [16; 17; 18] or, to a limited extent, using a surface complexation approach [15; 19; 20]. Others have employed discrete solid phases (DSPs) to model C-S-H, which provide reasonable comparisons with measured data of changing pH values and Ca and Si concentrations for molar Ca/Si ratios ranging from 2.7 to 0. Deriving DSPs for any ideal or non-ideal solid solution has been proposed by [PERSON] et al. [18] and [PERSON] et al. [21]. This was further developed by [PERSON] et al.