RESUMO
To better understand the pozzolanic activity in fly ash used as a supplementary cementitious material in cement or concrete, calcium silicate hydrate (C-S-H) has been synthesized by adding silica fume to a supersaturated calcium hydroxide solution prepared by mixing calcium oxide and ultrapure water. Thermogravimetric analysis results have revealed the variation in the weight loss due to C-S-H in the samples and the conversion ratio of calcium oxide (the µCaO value), which represents the proportion of calcium oxide in the initial reaction mixture used to produce C-S-H, with curing time. The weight loss due to C-S-H and the µCaO value were both maximized (13.5% and 90.4%, respectively) when the initial C/S molar ratio was 1.0 and the curing time was 90 d. X-ray diffraction (XRD) analysis has indicated that C-S-H in the samples after curing for 7 d had the composition Ca1.5SiO3.5·xH2O. 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis has revealed that the degree of polymerization of C-S-H increased with an increase in curing time for samples with an initial C/S molar ratio of 1.0. The ratio of internal to terminal tetrahedra (Q2/Q1) increased from 2.29 to 4.28 with the increase in curing time from 7 d to 90 d. At curing times ≥ 28 d, a leaf-like C-S-H structure was observed by scanning electron microscopy (SEM). An ectopic nucleation-polymerization reaction process is proposed for the formation mechanism of C-S-H.
RESUMO
Steel slag is a calcium-containing alkaline industrial solid waste that can replace limestone for flue gas desulfurization. It can remove SO2 and coproduce silica gel while avoiding CO2 emission from limestone in the desulfurization process. In this study, steel slag with a D 50 of 3.15 µm was used to remove SO2. At room temperature, with a solid-liquid ratio of 1:10, a stirring speed of 800 rpm, and the mixed gas introduced at a flow rate of 0.8 mL/min, 1 ton of steel slag could remove 406.7 kg of SO2, a SO2 removal efficiency typical of existing calcium-rich desulfurizers. As limestone desulfurization can release CO2, when limestone desulfurization was replaced with steel slag of equal desulfurization ratio, CO2 emissions could be reduced by 279.6 kg and limestone could be reduced by 635.5 kg. The yield of silica gel was 5.1%. Silica gel pore structure parameters were close to those of commercially available B silica gel. Products after desulfurization were mainly CaSO4 ·2H2O, CaSO4 ·0.5H2O, CaSO3 ·0.5H2O, and silica gel. With a silica gel dosage of 30 mg, a temperature of 20 °C, a pH value of 6.00, a stirring time of 0.5 h, and a methylene blue concentration of 0.020 mg/mL, the removal ratio of methylene blue adsorbed by silica gel was 98.4%.
