Synergistic effect o f hydration and carbonation on the development of MgO-based cementitious materials

Abstract

Portland cement (PC) production is accountable for 5-8% of anthropogenic carbon dioxide (CO2) emissions. The concrete and construction industries have been under increasing pressure to develop alternative materials and technologies that produce fewer CO2 emissions and potentially absorb CO2, resulting in an increased interest in sustainable practices. Compared to cement clinker, reactive MgO (RM) from calcining MgCO3 has a lower calcination temperature (700–900 °C vs. 1450 °C needed for calcining CaCO3). It can gain strength by permanently sequestering CO2, and it can be largely recycled after completing its lifecycle. This thesis investigates four major aspects related to using reactive magnesia in the construction industry: (1) Enhancement of carbon sequestration and mechanical properties of reactive magnesia-based cements under 15% of CO2 and ambient curing; (2) The effectiveness of amino acids on controlling performance of magnesiumbased carbonate; (3) The feasibility of using magnesium-based carbonate in Portland metakaolin cement blends; and (4) The role of magnesia in alumina/silicate hydrate binder systems. The first and second investigations (i.e. Chapter 4) involved the evaluation of the influence of amino acids (i.e. L-arginine (L-Arg) and L-aspartic (L-Asp)) on the performance of reactive magnesia-based mixes. The results of this research were presented in terms of hardened properties such as bulk density and compressive strength. The mechanisms of hydration and carbonation and microstructural characterization of the mixes were also studied. The results revealed that the use of amino acids as additives resulted in the formation of unstable nesquehonite (MgCO3·3H2O) and maintained its polymorph during the carbonation of MgO. Additionally, the carbonated composites produced with amino acids had a higher carbonation degree and higher stability, compared to the base batch without amino acids. The presence of amino acid resulted in the formation of denser carbonation products with a different morphology than those in the control mix, leading to significantly enhanced carbonation and compressive strength. Chapter 5 (third aspect) investigated the influence of carbonates (limestone, magnesite, various hydrated magnesium carbonates (HMCs), and labproduced HMCs) and non-carbonate (quartz) on the performance of Portland metakaolin cement binder. The mixtures exhibited a distinct difference in strength at early ages (i.e. from 3 to 7 days), while at the late ages (after 28 days) compressive strengths were comparable call the binders. Also, the microstructure of the mixtures was densified by the lab-produced HMCs. The synergistic reaction between metakaolin and high-reactivity carbonate produced carboaluminates, which may explain the observed increase in strength. Moreover, the observed increase in strength cannot solely be attributed to the carboaluminates; other hydration products may also play a role. strätlingite and C-A-S-H gel) also play an important role. Based on the findings from the last two chapters, which approved the feasibility of magnesia-based materials in the alumina/silicate hydrate binders, Chapter 6 investigated the practical applications for magnesia in alumina/silicate hydrate binder systems. The study introduced reactive magnesia (i.e. MgO) and sodium carbonate, together with an early-age oven curing regime, as an approach to clay solidification. The results indicated that incorporating MgO effectively solidified the clay by providing additional Mg2+ and OH- ions. The morphology of hydration phases plays a more important role than their contents and porosity in strength development.Another investigation conducted in this thesis is to replace microsilica with waste glass for preparing MgO-SiO2 formulations to form magnesium silicate hydrate (MS- H). The results suggested that waste glass can partially replace microsilica (i.e. up to 50 wt.% replacement level) in MgO-SiO2 formulations without sacrificing the compressive strength, whereas the complete replacement with waste glass did not reveal favourable outcomes in terms of performance. The reactivity and solubility of the silica source played a key role in the formation of M-S-H, which contributed to the strength development in samples containing microsilica. This study is original as it was the first in the literature to introduce amino acids into reactive magnesia mixes to control the polymorphs of HMCs in carbonated magnesia composites to develop low carbon MgO-clay binder systems via the use of the polymorphs controlled HMCs. The findings have suggested the high potential of reactive magnesia-based formulations to be used in various building applications based on their ability to gain strength via the sequestration of CO2 and their lower sensitivity to impurities, enabling the utilization of large quantities of industrial waste.