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Flue gas desulphurization (FGD) gypsum is a kind of industrial solid waste containing impurities (i.e., heavy metal ions Zn2+ and Ni2+), which may lead to threats to the ecological environment and public health. To safely use the FGD gypsum, this study investigated the mechanical strength, releasing property of SO42- and leachability of heavy metal ions of a series of FGD gypsum (40%) based mortar with various cement to slag ratios. The hydration process, pore structure, and morphology of the sample were then characterized and analyzed by isothermal calorimetry, H1 low-field nuclear magnetic resonance (NMR), and scanning electron microscope with energy dispersive X-Ray analysis (SEM-EDx). It was found that the highest mechanical strength could be obtained when the C/S ratio was 1:5, and the decrease of C/S ratios from 4:2 to 1:5 leads to less SO42- releasing from the mortar. The heavy metal leachability of the mortars is lower than that of the limitation of solid waste stipulated by the specification of GB5085.3-2007, and less of the heavy metal are leached from the sample as the C/S ratios decrease. The heat flow for the paste sample with 40% of FGD gypsum was delayed as the cement to slag ratio decreased from 5:1 to 2:4. In addition, the released amount of Zn2+ ions linearly decreases with the pore size of gel pores and capillary pores but increases with the pore size of macropores, and the ettringite in the paste exhibits the transformation from the needle to the clubbed shape when smaller cement to slag ratio was used. This work helps to understand how to reduce the heavy metals leachability of FGD gypsum from cementitious material and provides baseline for safely using the FGD gypsum for the preparation of building materials. [ABSTRACT FROM AUTHOR]

In this paper, a brief overview of the current literature on the dispersion mechanism of polycarboxylate-type superplasticisers (PCEs) is first presented. Then, the available free water amount and fluidity of cement paste with various water-to-cement ratios ( w/ c) and PCE dosages are measured. The phase and particle size distribution of early cement hydrates are analysed by X-ray diffraction and laser particle size analysis, respectively. As a result, a surprising tendency is observed: the increase rate of fluidity increases with decreasing w/ c ratio when the dosage of PCE gradually increases. This phenomenon cannot be explained by the steric effect. The authors also conclude that not less than 2·0% of solids volume of 'AFt-PCE' structures (early hydrates of ettringite (AFt) coated with PCE, < 200 nm) are dispersed well in the interstitial pore space along with non-adsorbed PCE. Based on these results, the authors present a possible dispersion mechanism in the PCE-cement system studied here based on the repulsion originating from the exclusion of free polymers (non-adsorbed PCE, 'AFt-PCE' structure and some other fine matters coated with PCE) from the gap between cement particles. [ABSTRACT FROM AUTHOR]

• NMR can measure the microstructure, bleeding extent and PCEs concentration. • PCEs synthesized with high acid to ether ratio can reduce bleeding. • PCEs molecules in bleeding water were diluted during bleeding process. • More bleeding water results in larger microstructure change of fresh cement paste. This study investigated the bleeding properties of fresh cement paste mixed with various polycarboxylate superplasticizers (PCEs) using low-field nuclear magnetic resonance (NMR). The PCE superplasticizers were synthesized with various acid to ether ratio, molecule weight of methyl allyl alcohol polyoxyethylene ether (HPEG) and functional groups. The bleeding process of fresh cement paste mixed with the PCE superplasticizers was monitored by low-field NMR and the bleeding rate was obtained with the calculation of transverse relaxation time (T 2) peaks. Results showed that the proposed NMR method can not only evaluate the bleeding extent, probe the microstructure of fresh cement paste but also provide information about the PCEs concentration of bleeding water. It was found that PCE superplasticizers synthesized with high acid to ether ratio would reduce the bleeding, and PCE superplasticizers synthesized with the HPEG of molecule weight 4400 and functional group of methyl methacrylate (MMA) would lead to less bleeding when the fresh cement paste reached the same fluidity. The T 2 relaxation time of bleeding water increased with bleeding rate, which implies the PCEs molecules in bleeding water was diluted during the bleeding process. Furthermore, the results suggest more bleeding water coming out from the pore water results in larger microstructure changes due to sedimentation. [ABSTRACT FROM AUTHOR]

Based on detection technology of helium pycnometry, this study developed a novel approach for characterizing the absolute volume change of cement pastes in early-age hydration process. This method gives an accurate and continuous surveillance on the absolute volume change of cement pastes excluding all the empty pore space even below nm-scale. Traditional methods, such as chemical shrinkage test (based on ASTM C 1608), isothermal calorimetry test and X-ray diffraction (XRD) analysis, are also adopted for comparison and analyzation. It was found that the data obtained by chemical shrinkage test is discontinuous and inaccurate, the data gotten by isothermal calorimetry test is vague and unspecific, and the data gained by XRD analysis is discontinuous and unspecific. Compared with the traditional methods, the results captured by helium pycnometry are continuous, accurate and specific for describing the hydration process, especially the changing details of the absolute volume change. But at the same time, helium pycnometry has a good correspondence with the traditional methods, whose trends are almost the same. Based on the trends of the absolute volume change and heat flow, the absolute volume change of cement pastes in early-age hydration process was then divided into four periods (dissolution period, induction period, fast reaction period and stable period). [ABSTRACT FROM AUTHOR]