Development of High Performing, Commercially Relevant Concretes with High Levels of Clinker Replacement by Manipulating Microstructure and Controlling Crystal Growth

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Copyright: Fu, Jiayi
The increasing focus on sustainability has driven manufacturers to develop greener cementitious products in order to reduce the associated carbon dioxide emissions. Supplementary cementitious materials (SCM) such as ground-granulated blast furnace slag (GGBFS) are commonly used in the development of low carbon, blended cement formulations. The reduced strength development due to the addition of slag can be improved by the addition of chemical activators such as sodium sulfate (Na2SO4). The reactivity and early strength of cement:slag binders is significantly enhanced by the addition of Na2SO4 though the mechanisms underlying the relationship between the enhanced hydration reactions and the strength behaviour remain unclear. This thesis explores the impact of Na2SO4 on the early age chemical reaction mechanism in blended systems containing a high percentage of blast furnace slag. Early hydration reactions and resultant compressive strength in a 50:50 slag:cement binder in the presence of Na2SO4 were investigated. Early strength increases in the presence of Na2SO4 were shown to be due to a combination of increased alite hydration and increased slag dissolution. Increased alite hydration was due to neither reduced dissolved Al concentration nor increased alite under-saturation, but related to increased ionic strength due to the presence of Na2SO4. Increased slag dissolution was associated with both increased pH and decreased Ca activity with the two being connected through the portlandite solubility limit. Na2SO4 was shown to substantially enhance slag dissolution at fixed pH 13 with this action attributed to greater under-saturation of slag as a result of ettringite formation. Na2SO4 was shown to be superior to alternate activators in a slag:cement binder. Microstructural development in the presence of Na2SO4 was investigated utilizing mercury intrusion porosimetry (MIP), NMR relaxometry, and XRD. Increased rates of early strength development and decreased rates of late strength development due to the presence of added Na2SO4 were linked to effects on capillary porosity refinement. While the degree of hydration at later age was shown to have been lower in the presence of Na2SO4, and may have been responsible for the higher capillary porosity, a clear alteration in the pathway of microstructural development had occurred with inhibition to hydration of the slag component due to earlier microstructural development. Subsequently, different cement types with varying clinker chemistry were used to investigate which clinker phase was primarily responsible for the acceleration reaction with Na2SO4. Early hydration reactions were examined and monitored utilizing mercury intrusion porosimetry (MIP), calorimetry, and XRD while the resultant compressive strength in a 50:50 slag:cement binder in the presence of Na2SO4 was investigated for up to90days. Forallcementsexamined,astronginversecorrelationbetweenearlystrength development and late age strength was apparent with results suggesting that the retardation of the early hydration reactions due to the presence of sodium sulfate resulted in a higher late age strength development. Previous studies indicate that Na2SO4 activation improves the early strength of blended cements due to enhanced ettringite formation. In this study, we examine whether triethanolamine (TEA) provides further early strength increase through additional ettringite formation by investigating the effect of TEA at 0 to 0.5% w/w of cement in a 2.5% w/w sodium sulfate activated 47.5:50 GGBFS:Portland cement blend. Results of XRD analyses indicate that TEA encouraged ettringite formation through enhanced calcium aluminoferrite (ferrite) and gypsum consumption, and, to a lesser extent, enhanced tricalcium aluminosilicate (C3A) consumption. Increased porewater Fe(III) concentrations provided further evidence of TEA-facilitated ferrite dissolution. Compressive strength results correlated with the degree of calcium silicate hydration and ettringite transformation, being highest in the 0.2% TEA blend but lowest in the 0.5% TEA blend. Finally, the effect of higher gypsum content in blended cement:slag can led to C3A dissolution retardation. Associated early age strength development was examined using calorimetry, pore solution chemistry, XRD, and MIP. Gypsum was found in these cements to exert no effect on alite hydration as it is consumed during C3A hydration and ettringite formation. Compressive strength measurements in the “gypsum studies” reinforced the inverse relationship between early and late strength development and the critical role of alite during early hydration towards late age strength development. This work hopes to contribute a more comprehensive understanding of the effect of Na2SO4 on the early hydration stages of GGBFS incorporated Portland cement blend systems from a geochemical approach in effort to achieve greater sustainability through higher cement replacement.
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PhD Doctorate
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