Abstract
This article demonstrates the possibility of producing alkali-activated hybrid cements based on fly ash (FA), and construction and demolition wastes (concrete waste, COW; ceramic waste, CEW; and masonry waste, MAW) using sodium sulfate (Na2SO4) (2–6%) and sodium carbonate (Na2CO3) (5–10%) as activators. From a mixture of COW, CEW, and MAW in equal proportions (33.33%), a new precursor called CDW was generated. The precursors were mixed with ordinary Portland cement (OPC) (10–30%). Curing of the materials was performed at room temperature (25 °C). The hybrid cements activated with Na2SO4 reached compressive strengths of up to 31 MPa at 28 days of curing, and the hybrid cements activated with Na2CO3 yielded compressive strengths of up to 22 MPa. Based on their mechanical performance, the optimal mixtures were selected: FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, and CDW/30OPC-10%Na2CO3. At prolonged ages (180 days), these mixtures reached compressive strength values similar to those reported for pastes based on 100% OPC. A notable advantage is the reduction of the heat of the reaction, which can be reduced by up to 10 times relative to that reported for the hydration of Portland cement. These results show the feasibility of manufacturing alkaline-activated hybrid cements using alternative activators with a lower environmental impact.
Highlights
Alkali-activated cements and geopolymers are materials that, thanks to their good durability and mechanical, physical, and thermal properties, compete with materials such as Portland cement; red or kaolinitic clays; and many raw materials used to manufacture common or special ceramic materials, such as refractory ceramics, photoluminescent ceramics, and antibacterial ceramics
Cristelo et al [16] propose a hybrid cement based on fly ash (FA), iron and steel slag, and ordinary Portland cement (OPC) activated with Na2SO4, which presented compressive strength at 28 days of up to 40 MPa
The effects of the Na2SO4/Na2CO3 and OPC contents on the compressive strength Molecules 2021, 26, x FOR PEER REVIEWof the alkaline-activated hybrid systems can be observed in Figures 3 and 4
Summary
Alkali-activated cements and geopolymers are materials that, thanks to their good durability and mechanical, physical, and thermal properties, compete with materials such as Portland cement; red or kaolinitic clays; and many raw materials used to manufacture common or special ceramic materials, such as refractory ceramics, photoluminescent ceramics, and antibacterial ceramics. Cristelo et al [16] propose a hybrid cement based on FA, iron and steel slag, and OPC activated with Na2SO4, which presented compressive strength at 28 days of up to 40 MPa. The authors attribute the mechanical resistance to the greater dissolution of the precursor, as a consequence of the formation “in situ” of NaOH, and to the densification of the matrix by the formation and precipitation of ettringite. This article aims to investigate the compressive strength and microstructure of hybrid cements (10–30% OPC) based on different precursors such as FA, concrete waste (COW), and red ceramic waste (CEW) using sulfate (Na2SO4) and sodium carbonate (Na2CO3) as activators. It is expected that the results of this study will be the basis for the future use of FA and CDW as precursors for the manufacture of hybrid cements activated with sodium sulfate and carbonate, which will generate a lower environmental impact
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