Development of reactive magnesia cement-based formulations with improved performance & sustainability
Date of Issue2019-02-07
School of Civil and Environmental Engineering
Portland cement (PC) production is accountable for 5-7% of anthropogenic carbon dioxide (CO2) emissions. This has led to a growing interest in sustainable practices in line with the increased pressure on the cement and construction industries to develop alternative technologies with reduced CO2 emissions. An example of new binders developed with this goal in mind is reactive magnesia (MgO), which is produced at lower temperatures than PC (700-1000 vs. 1450 oC), and has the ability to absorb CO2 in the form of stable carbonates during accelerated CO2 curing and gain strength accordingly. The presented research incorporates two main areas involving the use of reactive magnesia within the construction industry: (i) Enhancement of carbon sequestration, mechanical performance, strain-hardening and self-healing behaviors of reactive magnesia-based and PC formulations under 10% CO2, water, air and bacterial curing, and (ii) Life cycle analysis of reactive magnesia production in comparison to PC. The first area focused on the enhancement of the carbonation process, mechanical performance, microstructural development, and strain hardening and self-healing potentials of reactive magnesia-based formulations under 10% CO2, water, air and bacterial curing, with PC formulations included as a comparison in this area. This investigation involved the evaluation of the influence of various parameters including mix design (i.e. cement, water and aggregate contents), introduction of additives (i.e. air-entrained agent (AEA), PFA and GGBS), use of alternative sources for reactive magnesia (i.e. calcined dolomite), and incorporation of fibers under 10% CO2, water, air and bacterial curing on the performance of reactive magnesia-based and PC mixes. The outcomes of this research were presented in terms of fresh and hardened properties involving the measurement of porosity, density, workability, thermal conductivity, water sorptivity, rheology, resonance frequency, crack healing efficiency, hydration and carbonation reaction mechanisms, mechanical properties and microstructural characterization of various reactive magnesia-based concrete mixes. Overall, the results indicated that introduction of AEA reduced early strength of reactive magnesia formulations due to increased air contents under 10% CO2 curing. This was compensated by increased CO2 penetration, carbonation and strength gain in longer periods. Samples in which 50% of the binder component was replaced by PFA subjected to 10% CO2 curing indicated the highest strength development, which was associated with a reduction in sample porosity due to the filler effect of PFA as well as the formation of strength providing phases through the hydration and carbonation reactions. The use of calcined dolomite was also investigated as a new source of reactive magnesia within concrete mixes. The presence of undecomposed carbonate phases in dolomite facilitated the continuation of the hydration and carbonation reactions and both reactive magnesia and calcined dolomite samples benefited from the use of high humidity (90%), whereas elevated temperatures (60 ºC) presented an advantage only in reactive magnesia samples under 10% CO2 curing. The final section of this study led to the development of reactive magnesia based strain-hardening composites (SHC) via the use of 10% CO2 curing, whose self-healing capacity was also demonstrated. The adequate binder content and w/b ratio necessary for desirable fiber dispersion was determined. The autogenous self-healing capacity of the developed binders under the presence of water and elevated CO2 concentrations was highlighted, during which the formation of hydrated magnesium carbonates (HMCs) that were capable of sealing the cracks was observed. A subsequent investigation involved the use of microbial induced carbonate precipitation (MICP), which was more effective than the use of water and CO2 in terms of crack healing efficiency in reactive magnesia-based samples, resulting in a high extent of healing products in a short time period. The second area focused on the assessment of the environmental impacts of reactive magnesia and PC production. In terms of production, reactive magnesia had a lower impact on the overall ecosystem quality and resources than PC, but posed a larger damage to human health due to the high coal usage by most plants. The research presented in this study contributed to the literature via the information generated on the relationship between the fresh and hardened properties of reactive magnesia formulations. This involved the identification of the influence of phase formations on the mechanical and microstructural development of reactive magnesia samples under 10% CO2 curing. This study is novel as it is the first in the literature to investigate the environmental impacts of the production of reactive magnesia as a binder in comparison with PC, introduce several additives (AEA, PFA and GGBS) and alternative sources for magnesium (calcined dolomite) into reactive magnesia mixes and develop reactive magnesia-based strain hardening composites via the use of accelerated CO2 curing with the ability to undergo self-healing. The reported findings have indicated the high potential of reactive magnesia-based formulations to be used in various building applications based on their ability to be produced from alternative sources and gain strength via the sequestration of CO2.