Mechanical performance and microstructural development of MgO-SiO2-H2O systems
Date of Issue2018-12-31
School of Civil and Environmental Engineering
Reactive magnesium oxide (MgO)-based cement mixes present several advantages due to their lower production temperatures than Portland cement (PC), similar rate of strength development when compared to PC mixes, and ability to be produced from various alternative routes involving waste materials such as reject brine or seawater. The combination of reactive MgO and SiO2 leads to formation of magnesium-silicate-hydrate (M-S-H), which is known to contribute to significant strength gain in cement-based formulations. The high performance achieved by MgO-SiO2 mixes under ambient conditions has led to scientific interest in the exploration of their potential to be utilized in structural and non-structural construction applications. In line with this interest, the main goal of this study was to analyse the mechanical performance and microstructural development of MgO-SiO2-H2O systems and evaluate the stability of the formed phases under various conditions. Initial investigations focused on the effect of mix design as well as different curing conditions on the strength development of MgO-SiO2 mixes. Furthermore, the use of waste materials such as rice husk ash (RHA), which replaced microsilica as a SiO2 source in MgO-SiO2 formulations, was investigated to improve the environmental and economical aspects of the developed mixes. These efforts were followed by the development of a multi-component binder, which involved the formation of a dense network composed of both hydrate and carbonate phases via the introduction of carbonation in the MgO-SiO2-H2O system. Finally, the developed formulations were tested under different durability conditions for an assessment of their performance and stability under aggressive environments. These investigations involved the use of isothermal calorimetry and pH measurements to analyse the hydration process. Mechanical performance was assessed via compressive strength measurements at different durations. Microstructural analysis was performed via various techniques such as x-ray diffraction (XRD), thermogravimetric/derivative thermogravimetric analysis (TG/DTG), Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). In the initial studies, where the influence of mix design and different curing conditions were discussed, mix compositions with a MgO/SiO2 weight ratio of 1-1.5 resulted in the best mechanical performance, depending on the water/binder ratio. Although elevated temperatures (60°C) led to rapid strength development and M-S-H formation at early ages, high humidity sealed curing resulted in the best long-term mechanical performance within MgO-SiO2-H2O systems. From these results, a correlation between the compressive strength and the formed M-S-H content was established. Studies on the use of alternative silica sources such as RHA demonstrated that amorphous RHA can effectively be used as a silica source in MgO-SiO2 formulations to partially or fully replace microsilica. Evident formation of M-S-H led to a satisfactory mechanical performance within samples incorporating amorphous RHA. Further studies investigated the incorporation of carbonation curing in MgO-SiO2 systems, which enhanced strength development. This increase in strength was associated with the densification of microstructure via the formation of Mg-carbonates, which were assessed via microstructural analysis. Finally, subjecting MgO-SiO2 samples under different durability conditions for up to 180 days revealed that MgO-SiO2 based concrete maintained its strength development under various aggressive environments and performed better than PC-based concrete samples under magnesium chloride (MgCl2) and magnesium sulfate (MgSO4) environments. This advantageous performance of MgO-SiO2 samples was associated with the stability of hydrate phases, namely brucite (Mg(OH)2) and M-S-H, under several durability environments including carbonation, sodium chloride (NaCl), MgCl2 and MgSO4 environments. The findings of this research shed light on the enhancement of the performance of MgO-SiO2 based formulations through a detailed investigation of key parameters such as mix design, curing conditions, use of alternative materials and durability. The obtained results contribute to the literature via the information generated on the formation and stability of hydrate phases under various conditions and their effect on the strength and microstructural development of MgO-SiO2 mixes. When compared to similar PC-based mixes, the MgO-SiO2 formulations developed in this study demonstrated the potential to be used in various building applications due their satisfactory mechanical performance and durability under various environmental conditions.