Abstract
The use of supplementary cementitious materials as a partial replacement for Portland cement is the most effective way to reduce the carbon footprint of the concrete industry. Raw clays containing kaolinite (kaolin) are promising substitute materials. In the field, raw clays are often mixed with calcite and this is thought to affect their behaviour after calcination. This study explores the influence of calcite impurities on the mineralogy and reactivity a kaolinitic clay. A kaolin sample was blended with different quantities of calcite. The results show that during calcination calcite is decomposed, but no significant amount of free lime or amorphous calcium carbonate are formed. A granular deposit was observed that partially covers the kaolinite particles. The decomposition of calcite and formation of the deposit is associated with a reduction in specific surface area, which increases with the amount of calcite that is intermixed in the raw clay. TEM-EDS analysis showed that the deposit corresponds to a new phase formed from the interaction of kaolinite and calcite, with an Al/Si ratio ranging from 0.74 to 0.88 and Ca/Si ratio between 0.86 and 1.65. Reduction of the calcination temperature to 700 °C reduces the calcite decomposition and the negative impact on reactivity.
Highlights
Mineral additions, commonly referred as supplementary cementitious materials (SCM), are widely used either in blended cements or added to concrete separately in the mixer [1, 2]
This study explored the effect of calcite impurities in kaolinitic clays on reactivity after calcination
Based on the results presented in this study, the following conclusions can be drawn: 1. Calcite impurities reduce the specific surface area of calcined clay as compared to clays without calcite, which explains most of the measured reduction in reactivity in the samples with calcite impurities
Summary
Commonly referred as supplementary cementitious materials (SCM), are widely used either in blended cements or added to concrete separately in the mixer [1, 2]. Clay minerals are unique among supplementary cementitious materials because of their abundance worldwide [6,7,8]. The three most abundant clay mineral types are kaolinite, illite and montmorillonite [9]. Kaolinite is dehydroxylated in the temperature range 400–600 °C. The dehydroxylation of kaolinite leads to a more disordered material known as metakaolinite [10,11,12]. Raw clay is calcined in the range 700 and 800 °C to maximize the amount and degree of disorder of metakaolinite in the calcined clay. Higher temperatures lead to reduced reactivity due to sintering and eventually recrystallization [11]
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