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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]

Water repellent (WR) has been widely used to improve the durability of cementitious materials (CMs) by reducing the absorption of water and aggressive ions. In this study, cement pastes were prepared with three types of WR, namely, waterborne pure nanosilicone emulsion (NS), isobutyltriethoxysilane (IS), and octyltriethoxysilane (OCT). The effects of WR on setting time, compressive strength, and water absorption were investigated. The influence of WR on the transverse relaxation time spectrum from 0 min to 28 days during the hydration process was monitored with a low-field nuclear magnetic resonance (LF-NMR). In addition, X-ray diffraction (XRD) and heat flow were employed to study the effect of WR on hydration characteristics. The results showed that the WR specimens exhibited a limited reduction of compressive strength compared with the control sample at 28 days. The use of WR reduced the water absorption of cement paste when compared to that of the control specimens. The results showed that the WR exhibited a retarding effect on hydration reactions, and the capillary and gel pores of the cement paste increased compared with that of the reference sample. XRD analysis showed that the formation of hydration products had been restricted with the addition of WR. The heat flow results showed that the retardation effect was enhanced as the WR dosages increased. [ABSTRACT FROM AUTHOR]

Previous studies focused on the determination and simulation of the bleeding extent of fresh cement paste and provided objective conclusions. However, few studies exist on the influence of bleeding on the final structure of cement paste. This study focused on the microstructure development during the hydration of cement paste with various bleeding caused by the water-cement ratio, the slag replacement, and the superplasticizer. To obtain information on the porosity profile and pore size distribution, the sample was measured using a low-field nuclear magnetic resonance (NMR) with Hahn spin sequence and Carr-Purcell-Meiboom-Gill (CPMG) sequence from top to bottom. The results showed that increasing bleeding thickness leads to a heterogeneous structure from top to bottom in terms of porosity profile and pore size distribution. Moreover, hydration can reduce the difference in the pore structure, especially when the bleeding depth is small. It was also found that the influence of bleeding depth on the pore size distribution of gels pores during hydration is minor compared with that of the capillary pores. Furthermore, detailed discussions on the connections between the bleeding and the final structure are presented in this paper. [ABSTRACT FROM AUTHOR]

• NMR can measure the Na+ ions and Cl- ions during hydration. • The consumption of the Cl- and Na+ ions is closely linked to the hydration. • Fly ash in the cement will result in less Cl- ions binding. • Cl- ions will be bound earlier compared to the Na+ ions. Lacking effective and in particular direct methods, the study on Cl- and Na+ ion binding during hydration has rarely been reported. In this study, a specially developed Nuclear Magnetic Resonance setup was employed to study the Cl- and Na+ ion binding of cement pastes during hydration. Influence factors, such as slag and fly ash replacement, on the 1H, 35Cl and 23Na relaxation of the cement paste made with 1 mol/L NaCl solutions were investigated. It was found that the increase of slag in the cement gives rise to bigger pores during hydration, and the fly ash resulted in less water being consumed. The Cl- signals would first be stable then gradually decrease as a function of time. The slag in cement has minor impacts on the Cl- ions binding, and the fly ash replacement in the cement would result in less Cl- ions binding during the hydration. Results also suggest that the slag could result in later Na+ binding, and the fly ash in the cement will significantly reduce the Na+ binding. Besides, the Cl- ions will bind earlier compared to the Na+ ions, which is probably related to the characteristics of the hydrates. The normalized 35Cl and 23Na signal intensity, in general, decrease with water consumption reflecting the hydration. Furthermore, the early formed hydrates would consume relatively less water but bind more Cl- and Na- ions. [ABSTRACT FROM AUTHOR]

Based on the analytical 1H low-field nuclear magnetic resonance (NMR) technique, we developed a novel method and corresponding model to analyze the hydration degree (α t) of cement in cement paste under different curing conditions. The hydration degree can be calculated quantitatively based on the transverse relaxation time (T 2) distributions using NMR measurements. The T 2 distributions of the cement paste samples with the water/cement (W/C) ratios of 0.25, 0.35 and 0.45 under standard curing, steam curing at 40 °C and steam curing at 60 °C were obtained. The α t of cement was studied by the hydration heat method and portlandite (CH) quantitative measurements. The results show that the intensity-weighted T 2 value of non-surface water peaks (T 2 ¯) has a significant correlation with α t that can be expressed by an exponential equation. In this paper, the derivation of this exponential equation is given, and the effects of W/C ratio and curing condition on its parameters are analyzed. [ABSTRACT FROM AUTHOR]