Development of magnesium silicate hydrate binder systems.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Studies on magnesium-based cement have been of interest to mitigate the increasing impact of climate change due to the high CO2 emissions associated with cement production industry. Although magnesium silicate hydrate (M-S-H) phases in concrete were discovered in the 1950s, the strengths of M-S-H binders as construction materials have been known only since 2006. Research on M-S-H binders is very limited, compared to Portland cement (PC), and mainly focused on properties of cement paste and mortar. The publications focused on mechanical properties and initially studied the reaction mechanisms of the formation of hydration products and the influence of different material sources. There was a controversy on the strength capacity of M-S-H binders while very little research characterized long-term properties, chemical compositions of pore solutions, and properties of concrete with the effect of aggregates. The poor workability was also reported as a significant limitation of the development of M-S-H cement for environmental benefits.
In this research, a comprehensive experimental programme was conducted to improve properties of M-S-H binders, derived from the characterization of the microstructures and chemical reactions of the binders, then followed by the optimization of the binders for strength, workability, and durability.
Firstly, microstructures and hydration processes have been analyzed by various methods such as XRD, SEM/EDS, and pore solution analysis (ICP-MS). Binders included MgO and silica within the optimal contents from 40-60% and were cured for a long-term duration up to 365 days age. The XRD and SEM/EDS results confirmed the formation of the hydration products including brucite and M-S-H phases. The brucite formed in the first stage and mainly completed after 7 days whereas M-S-H phases were formed slowly and still developed significantly from 28 to 90 days, which is very different from the hydration of PC. SEM/EDS images provided the microstructures of raw materials and hydration products, showing highly porous structures of M-S-H binder compared to PC. Pore solution analysis by ICP-MS showed important characteristics such as pHs and ion concentrations, which reveal the trend of forming hydration products and directly relate to the durability of M-S-H cementitious materials.
Secondly, M-S-H binders were found to have adequate strength for construction materials. Optimized M-S-H mortar using ternary binder systems (MgO - silica fume- quartz filler) obtained high strength of approximately 90 MPa. The optimal binder composition for strength and working was the balance between maximizing hydration products and improving the microstructure homogeneity and packing density. In addition, among high range water reducers, the use of polymer-based super plasticizer Sika Viscocrete-5-555 was a key factor to reduce w/c ratio to below 0.40 to improve strength and workability. Quartz filler also played an important role in improving microstructure and packing density, resulting in superior strength and improved workability of M-S-H binders.
Thirdly, M-S-H binder concrete was produced and tested based on the optimization of M-S-H cement pastes and mortars in previous phases. The binder using 60% MgO-40% silica fume with 10% quartz filler yielded strength of over 40 MPa which could be sufficient for structural concrete materials. However, some unexpected results were also observed such as low tensile strength and elastic modulus. In addition, further studies are required regarding the long-term strength and the permeability of the M-S-H binder concrete.
Another finding showed the strong influence of silica sources on the microstructure and mechanical properties of resulting M-S-H cementitious materials. The high-performance M-S-H binders require high reactivity binder constituents. The M-S-H phases readily formed with highly reactive silica such as silica fume, natural pozzolan, and rice husk ash. However, poor performance was observed with FA type F.
The extensive experimental programmes in this research have yielded valuable results for a variety of related topics which are of fundamental importance for the application of M-S-H binders. The research findings have closed a number of current research gaps, and also give directions for future studies of the M-S-H binders as new cementitious materials focused on sustainable development.