## 1 Introduction  
Portland cement (PC) is an important material at several low- and intermediate-level radioactive waste disposal facilities in most countries, because of its high-quality performance in various environments and its ability to act as a chemical barrier to prevent or retard the transport of radionuclides [1; 2]. In the past, a considerable amount of research has been carried out to assess the suitability of cementitious materials for immobilisation of various radionuclides, to ensure the reliable long-term performance of the disposal system [1; 3; 4; 5; 6]. Seawater has been used as a cooling water in many nuclear power plants, and the resultant precipitates in the pipes, including NaCl, have been removed by sulfuric acid. This process produces highly concentrated sodium sulphate, up to 25% by weight, as a low-level radioactive waste needing disposal [7; 8]. These liquid wastes are solidified with cement systems at elevated temperature before they proceed to waste disposal. It is important to study the performance of solidified waste because of the chemical interaction between liquid wastes and the cement matrix, and the crystallisation of soluble solids. It has long been recognised that the chemical interaction of sulphate with cement hydrates forms expansive products such as ettringite, gypsum, and thaumasite, resulting in spalling, swelling, cracking, and eventual failure of concrete structures [9]. Over the past couple of decades, intensive work has been carried out on internal and external sulphate attack in cementitious materials [8; 9; 10; 11; 12; 13]. The formation of sulphate-bearing phases depends on the concentration of sulphate solution, the chemical composition of the cement, cement additives, and the porosity of hydrated cement [9; 12]. In external sulphate attack, ingress of sulphate ions induces chemical degradation of cement matrix. Furthermore, leaching of calcium and hydroxide ions from the pore solution of the cement matrix to external environments causes decalification of cementitious material, and thus increases the porosity and permeability of the material [10; 12]. The formation of sodium sulphate crystals or their phase change can also induce degradation [14]. Crystallisation of mirabilite (Na\({}_{2}\)SO\({}_{4}\):10H\({}_{2}\)O) and thenardite (Na\({}_{2}\)SO\({}_{4}\)), or their phase changes (mirabilite to thennardite or vice versa), due to temperature or sodium sulphate concentration do not involve any chemical interaction with cement hydrates. Although many studies have reported chemical and physical sulphate attack, the associated mechanisms and influencing parameters of sulphate attack are incomplete. A critical concern of highly concentrated sodium sulphate nuclear waste with the co-hydration of PC is the degradation of the cementitious materials, by chemical interaction of sulphate with cement hydrates and crystallisation of sodium sulphate. Further, the repository environment and its changes significantly affect the long-term performance of waste storage systems [1]. The potential variation of physicochemical properties of cementitious materials, as a result of PC hydration in aqueous solid waste, and the transport of ions are important factors to consider in maintaining performance over the time scale required. Hydration and reactive-transport models are essential tools to evaluate the performance of waste storage systems, because the degradation of the materials needs to be predicted precisely as a function of time and depth. Several studies have published reactive transport simulations for performance evaluation of cementitious materials in nuclear waste repositories using various geochemical codes, and the models have been improved by incorporating various features and mechanisms and coupling between them [15; 16; 17; 18; 19; 20]. However, research on the long-term performance of sodium sulphate waste solidified with cementitious materials is very limited.