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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]

• Using X-CT combined with MIP is a reliable method for investigating the pore feature in printed concrete interlayer interface. • The bond strength in the middle interface of the printed element is higher than that of the upper and bottom interfaces. • The pores in the bottom interface tend to be interconnected or continuous due to the pressure of upper printed layers. 3D printed concrete is an emerging intelligent construction method, and it has been attracting an exponential attention in this sector. However, one of the major issues in printed concrete is the cold joint between the neighbor layers. In this study, an experimental study on the relation between the number of stacked layers vertically and the interlayer adhesion of 3D printed element was conducted. Splitting method was carried out to test the interlayer bond strength, mercury intrusion porosimetry (MIP) and X-ray computed tomography (X-CT) scanner were used to investigate the pore structure of the interlayer zone. The experimental results indicated that a markable influence of the stacked layers vertically on the interlayer adhesion is found. The interlayer bond strength in the middle interface of the printed element is higher than that of the upper and bottom interfaces, especially the bond strength in the bottom interface is the least. Such result is also reflected in the results of the MIP test: the lower total pore volume and total pore surface area existed in the middle interfaces. The X-CT results showed that a large number of elongated and flat pores present in the interlayer zone, the pores in the bottom interface tend to be interconnected or continuous due to the pressure of upper printed layers. An empirical model was proposed to account for the formation of the irregular pore and the change of bond strengths between the vertical neighbored layers of 3D printed element. [ABSTRACT FROM AUTHOR]

To investigate the relationship between morphology and specific ion transport properties in low-density (LD) and high-density (HD) calcium-silicate-hydrate (C-S-H), a numerical method of directly achieving diffusivities is developed. Firstly, a multiscale structural model is proposed to reconstruct C-S-Hs in which self-similarity and quantitative physical properties are compared to experimental results. Subsequently, electrical double layer (EDL) theory is introduced into analysis of ion transport in pore solution. The quantitative analysis of various influence factors of EDL on ion diffusivities in pore solution is carried out and average diffusivity in pore solution is also obtained. Moreover, a numerical method for solving Fick’s first law equation is performed on the simulated C-S-H structures to obtain numerous diffusivities of common ions in C-S-H. Finally, the significant parameter for durability in reinforcement concrete, chloride diffusivity in hardened cement paste, is predicted using simulated diffusivities in C-S-Hs, based on a multiscale method. It is found that compared to the atomic force microscope image and surface area of C-S-H, this self-similar model can well reconstruct real C-S-H structures. In addition, the influence factors of EDL including overlapping diffuse layer, relative potential, cation/anion, valence, zeta potential and bulk solution have great effects on ion diffusivities, which demonstrates EDL effects cannot be ignored when investigating ion transport behavior in C-S-H. Furthermore, using an appropriate case of pore solution with diffusivities of 8.2 × 10 −12 m 2 /s in LD C-S-H and 2.0 × 10 −13 m 2 /s in HD C-S-H, estimated chloride diffusivities in cement pastes agree well with measured values, which can indirectly indicate this modelling work is reasonable. [ABSTRACT FROM AUTHOR]

• An innovation apparatus is developed based on the analyzation of a theoretical model. • The GIP method has been developed based on the proposed apparatus. • The porosity of cementitious materials tested by gas is comparable with that by water (WIP). • Compared to GIP, mercury intrusion porosimetry (MIP) has underestimated the porosity. • Gas is a more proper medium for intrusion porosimetry compared to water and mercury. Pore-structure in cementitious materials determines their chemical and physical properties. However, the quantitative characterization of pore-parameters has yet been investigated in depth. In this work, gas intrusion porosimetry (GIP) is accomplished by a proposed apparatus based on the theoretical model. The porosities of cementitious materials measured by GIP are compared to those obtained by conventional methods including water intrusion porosimetry (WIP) and mercury intrusion porosimetry (MIP). The three methods are assessed by an established empirical-model for the porosity-strength relationship of cementitious materials. The results show that the porosities obtained by GIP are near to those obtained by WIP and the difference is below 2.88%. The porosities determined by the MIP technique were 7.15%~16.71% smaller than those determined by GIP. The results deduced from the empirical model agree well with the measured data obtained by GIP (R2 > 0.95). Both the WIP and MIP decreased the empirical-model effectiveness to predict a trend. [ABSTRACT FROM AUTHOR]