Abstract
Abstract. The cement industry involves high-energy consumption that generates high CO2 emissions into the atmosphere. Environmental concerns can be addressed by replacing parts of Portland cement clinkers with pozzolanic materials in mortars and concrete. Slag, fly ash and silica fume are materials considered for the planned replacement. Research studies on clay minerals, such as kaolinite, are being followed with special attention by the scientific community and the cement industry. It is well known that these minerals require an activation process to transform kaolinite (K) into metakaolinite (MK). MK is an amorphous material from the transformation of K with high pozzolanic activity, which is its capacity to react with the portlandite released during the hydration of Portland cement, generating compounds such as C–S–H gels and some aluminum-phase hydrates. One of the MK production methods is heat treatment controlled by kaolinite at temperatures in the range of 600–900 ∘C. Different residues have been used (coal mining, paper sludge and waste from a drinking water treatment plant) activated at 600 ∘C for 2 h to elaborate blended cements. Due to their good behaviour as future eco-efficient additions, this research is a study by x-ray fluorescence (XRF), x-ray diffraction (XRD) and scanning electron microscopy (SEM) of their influence on the performances of blended cement mixtures (binary and ternary one), with substitutions of pozzolan ratio at 28 days of hydration. The porosity of pozzolanic cements decreases because of the formation of hydrated phases during pozzolanic reaction.
Highlights
The cement industry traditionally uses in its production process a huge quantity of waste and by-products in different stages: alternative fuel, activated drinking water treatment waste, raw materials and/or pozzolanic additions in order to reduce the environmental impact
The coal mining waste activated at 600 ◦C during 2 h (ACW) is mainly by low quartz, micas and calcite (Table 1)
In pastes containing APS + FA, the compressive strength is lower at this age for 10 % addition, probably due to the reactivity of FA being slower than MK-based pozzolans; the value for 20 % replacement is very close to that obtained for Ordinary Portland cement (OPC) paste
Summary
The cement industry traditionally uses in its production process a huge quantity of waste and by-products in different stages: alternative fuel, activated drinking water treatment waste, raw materials and/or pozzolanic additions in order to reduce the environmental impact. Vigil et al (2007) and Frías et al (2008) reported that MK obtained from paper sludge, calcinated at temperatures between 650 and 700 ◦C for 2 h, has similar pozzolanic activity as other highly pozzolanic additions, such as commercial MK or silica fume, improving the performances of blended cements (Bai et al, 2003; Rodríguez et al, 2009). Another interesting research line to obtain recycled MK is the recycling of coal mining waste or activated drinking waste (Frías et al, 2014).
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