Investigation of magnesia reactivity in blended cement systems under hydothermal conditions
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Addition of supplementary cementing materials (SCM) to produce blended Portland cements (PC) is a well established practice in the manufacture of construction materials. It enables an overall reduction in the consumption of PC thereby reducing CO₂ emissions. This study reports findings on the hydrothermal chemistry, physical and mechanical properties of autoclaved PC-quartz blends where PC was partially replaced by reactive magnesia (MgO), which requires less energy for calcination than calcia (CaO) from their naturally occurring carbonates, and alumina-silica rich fired clay-brick (CB) waste fines, an abundant industrial waste and a proven SCM. The dependence of the reactivity of magnesia on the calcination temperature was initially investigated through the hydration behaviour and an optimum reactivity at a temperature of 600°C was identified. In order to investigate the potential of this reactive magnesia in a cement environment, the reactivity of the magnesia with colloidal silica in model MgO-SiO₂-H₂O slurry systems was first investigated under both ‘mild’ and ‘extreme’ hydrothermal reaction conditions. Amorphous magnesium silicate hydrate (M-S-H) phases were identified in the ‘mild’ conditions at 180°C with increasingly crystalline phases being developed as the temperature and time were increased up to 350°C and 16 hours of hydrothermal treatment. Two different reaction sequences were also established. For M/S ratios of 0.6, 0.8 and 1.0, the principal reaction products were M-S-H gel and talc while for M/S ratios of 1.5 and 2.0, they were brucite, deweylite and chrysotile serpentine. Morphological studies using SEM of the specimens containing chrysotile revealed that the chrysotile was not of a fibrous nature. The addition of reactive MgO to PC in hydrothermal conditions was observed to have a negative effect on the compressive strength. The only magnesium containing phase observed in XRD was brucite, indicating that MgO did not take part in the reaction during the hydration of the cement. No M-S-H phases were observed in the MgO containing mixes. Experimental results suggested that MgO was not entirely inert as the formation of tobermorite appeared to be retarded in the presence of brucite. Examination of PC-quartz mortar mixes revealed that alumina-silica rich clay-brick waste is pozzolanic where the AI₂O₃ provided a source of Al ions for the accelerated formation of Al substituted 1.1 nm tobermorite. Mechanical properties showed improvements with the incorporation of CB waste in more silica-rich environments. Moreover, drying shrinkage and resistance to carbonation were improved due to increased crystallinity of Al-tobermorite. For blended PC with the addition of both CB waste and reactive magnesia (in a 50/50 ratio), an apparent synergy was observed as minimal (or no) reduction in strength was observed for up to 20% additions of the 50/50 blend. The synergy was explained by the contrasting physical and chemical effects as a result of attaining an optimum proportion of amorphous and crystalline material and optimum physical packing conditions. Autoclaved MgO-SiO₂ only cube specimens were shown to be capable of producing strength up to 10MPa. XRD revealed the presence of talc where the talc crystallinity was higher in the MgO-silica fume specimens which corresponded to higher strength specimens. This has the potential to be used for low strength applications such as interior walls, possibly as a replacement for gypsum plasterboards. The use of fired clay-brick waste in combination with reactive magnesia as additives for the production of hydrothermally cured cement-based building products has the potential to achieve an overall positive outcome from an environmental viewpoint.
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