Abstract
In this paper, slag is used as a calcium source to make alkali-activated coal gangue–slag (AACGS) based material. The reaction mechanism of AACGS materials was discussed in depth by means of XRD, FT-IR, 29Si MAS-NMR (nuclear magnetic resonance) and SEM-EDS (energy dispersive spectrometer). The experimental results show that coal gangue can be used as a raw material for preparing alkali-activated materials. The liquid–solid ratio is the most influential factor on AACGS paste fluidity and strength, followed by slag content. As the modulus of sodium hydroxide increases, the depolymerization process of the reactant precursor is accelerated, but the high sodium hydroxide concentration inhibits the occurrence of the early coal gangue–slag polycondensation reaction, and exerts little effect on the 28 d compressive strength. Ca2+ in the slag promotes exchange with Na+, and the product is converted from N-A-S-H gel to C-(A)-S-H gel, and C-(A)-S-H is formed with higher Ca/Si ratio with the increase of slag content. The slight replacement of coal gangue by slag can greatly improve the reaction process and the strength of AACGS materials.
Highlights
Cement is one of the world’s most widely used construction cementitious materials.About 1.6 billion tons of cement is used each year, releasing 1.5 billion tons of CO2, and about5% to 8% of global CO2 emissions are caused by cement [1,2,3,4]
It is obvious that calcined coal gangue belongs to the silica–alumina material, while slag belongs to the calcium–silica–alumina material
Paste fluidity rose as the slag content increases
Summary
Cement is one of the world’s most widely used construction cementitious materials. About 1.6 billion tons of cement is used each year, releasing 1.5 billion tons of CO2 , and about. Alkali-activated materials are products of the reactions between alumino-silicate materials (fly ash, slag, metakaolin, coal gangue, ceramic, etc.) and alkaline solutions at ambient temperature [11,12]. All materials rich in active silicon-aluminum oxides (slag, fly ash, metakaolin, coal gangue, etc.) can undergo polymerization reactions with high-concentration alkalis [13]. The reaction products of alkali-activated slag (rich in CaO) are different from those of alkali-activated metakaolin, coal gangue or fly ash (rich in Si and Al) the former are C-(A)-S-H gels while the latter are N-A-S-H gels [14,15], Materials 2019, 12, 2250; doi:10.3390/ma12142250 www.mdpi.com/journal/materials. With silica–alumina coal gangue as the main material, slag as the calcium source, NaOH and Na2 SiO3 to be the alkali activator, the AACGS materials is prepared.
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