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In the course of an investigation on using calcium stearate (CaSt) to improve performance of the alkali-activated slag (AAS) cement, the objective of the present work is to discovery its role in the AAS system. Special interest is devoted to understand the influence of CaSt on the reaction process, reaction products and microstructural features of the AAS cement. To achieve this, isothermal calorimetry, impedance characteristics, infrared spectroscopy, X-ray diffraction, thermogravimetry, nitrogen sorption, mercury intrusion porosimetry and scanning electron microscopy were carried out. According to results obtained, the CaSt has three important effects on the AAS cement. Firstly, it inhibited slag reaction with the activator through decreasing activity of alkalis, whereas the amount of C-(A)-S-H gels in the system depended on the usage of CaSt, because the CaSt could have chemical reactions with the alkali-solution and form similar reaction products. Secondly, there is less sodium and more calcium in reaction products of the CaSt added mix, which improve their stability and uniformity. Finally, microstructure characteristics (e.g. pore size distribution, pore connectivity) are optimised and defects are reduced significantly, when CaSt is added in the AAS mix. [ABSTRACT FROM AUTHOR]

This paper describes the results of a study investigating early age changes in pore structure of alkali-activated slag cement (AASC)-based paste. Capillary porosity, pore solution electrical conductivity and electrical resistivity of hardened paste samples were examined and the tortuosity determined using Archie's law. X-ray computed micro-tomography (X-ray μCT) and Scanning electron microscope (SEM) analysis were also carried out to explain conclusions based on electrical resistivity measurements. AASC pastes with 0.35 and 0.50 water-binder ratios (w/b) were tested at 3, 7, 14 and 28 days and benchmarked against Portland cement (PC) controls. Results indicated that for a given w/b, the electrical resistivity and capillary porosity of the AASC paste were lower than that of the PC control, whilst an opposite trend was observed for the pore solution conductivity, which is due to AASC paste's significantly higher ionic concentration. Further, capillary pores in AASC paste were found to be less tortuous than that in the PC control according to estimations using Archie's law and from the results of X-ray μCT and SEM analysis. In order to achieve comparable levels of tortuosity, therefore, AASC-based materials are likely to require longer periods of curing. The work confirms that the electrical resistivity measurement offers an effective way to investigate pore structure changes in AASC-based materials, despite threshold values differing significantly from PC controls due to intrinsic differences in pore solution composition and microstructure. [ABSTRACT FROM AUTHOR]

• Proper use of LS can increase fluidity and prolong setting time without losing strength. • LS can affect reaction products of AAS and its function depends on mix proportions. • Variations of pore structure of modified AAS cement relies on the dosage of LS. • Around 30% hydration degree of LS was achieved at 28 d for AAS with 40% LS. This study aims at discussing physical and chemical influence of limestone powder (LS) on the AAS system. General properties, including fluidity, setting time, drying shrinkage, and mechanical strength of AAS pastes/mortars, were examined. Pore structure change and phase assemblages of LS-AAS composites were studied. The results showed that the addition of 50 wt% LS increased the fluidity and setting time of AAS pastes, but reduced the total drying shrinkage of AAS mortars. Meanwhile, around 30% reduction in mechanical properties were found at the same replacement ratio. The pore structure was refined in the pastes with LS replacement<25 wt%, but was coarsened in the sample with 40 wt% LS. Limited new phase was formed in the sodium silicate activated slag samples, which was believed to depend on the modulus of the activator. Around 30% hydration degree of LS was achieved in 40 wt% LS-replaced AAS pastes hydrated for 7 ~ 28 d as determined by TG-DSC analysis. The main drawbacks of AAS, fluidity, setting time, and drying shrinkage, are improved by using high-volume LS, so use of high-volume LS in AAS system can be an option for the purpose of environmental protection. [ABSTRACT FROM AUTHOR]

Municipal solid waste incineration (MSWI) fly ash can cause serious early volume expansion when stabilized and solidified by cementitious materials. Many pretreatment processes have been reported, such as alkali solution, milling and chelating etc. However, these processes may lead to second pollution or higher costs. This study proposed an original pretreatment process of MSWI fly ash, and investigated its effect on the immobilization efficiency by alkali-activated slag (AAS) cement. The possible reasons for volume instability were assumed and analyzed by gas chromatography, scanning electron microscopy, X-ray diffraction, hydrostatic testing and leaching tests. It was found that the 0.377 wt% of metallic aluminum is the main expansive source of volume expansion in early stage (within 24 h). Due to the amphoteric characteristic of Al, it is possible to eliminate it using either acid or alkaline. Considering the fact that alkali-activated system and the fly ash were both alkaline, this study chose water immersion to pre-treat the fly ash, aiming to avoid the possible neutralization process of acid treatment and the entrained extra alkali component of alkali treatment, which may greatly affect many characteristics of alkali-activated slag. After pretreatment, the solidified specimens showed lower water demand (with a water to cement ratio of merely 0.27) and higher compressive strength due to the refined and smoothed particles. The volume expansive rate was greatly reduced, giving the better volumetric stability and encapsulation rate. Up to 90% MSWI fly ash-added solidified bodies feature a volume expansive rate of less than 0.09%. The alkali-activated slag pastes can effectively immobilize up to 70% treated fly ash with the leaching rate of solidified particles lower than the limits recommended by Chinese standard, Identification for extraction toxicity (CSEPA GB 5085.1, 2007). Therefore, the pretreatment process is easy to operate and is a promising way to increase the immobilization efficiency of alkali-activated slag cement. • Taking advantage of MSWI fly ash's own alkalinity to reduce volume expansion. • The pretreatment process won't produce waste water which brings second pollution. • Solidified bodies gained early and long-term volume stability after pretreatment. • The effective immobilization amount increased up to 70% after pretreatment. • AAS shows better potential than PC in immobilizing MSWI fly ash. [ABSTRACT FROM AUTHOR]

• Chloride transport coefficient of alkali-activated slag (AAS) cement could be predicted using electrical responses. • Temporal variation of strength, pore solution and transport properties of AAS is assessed. • AAS cement is more sensitive to mix proportions and additional care is needed to achieve high quality. Relatively few studies currently exist concerning temporal variation of the overall performance of alkali-activated slag (AAS) cement; a topic essential for quality control in practice. This study uses microstructural characteristics and electrical responses to reflect performance variation of AAS cement. To achieve this, four key performance parameters were assessed, pore formation factor, chloride transport coefficient, permeability coefficient and compressive strength. Classical theories used for PC-based materials were applied to estimate the performance of AAS paste. It is found that the formation factor of AAS can be assessed by combining bulk conductivity and pore solution conductivity and the permeation properties of AAS cement cannot be reliably assessed by the Katz-Thomoson (KT) model, while the Millington-Quirk (MQ) model works better. To assess the relationship between compressive strength and permeable porosity, the Balshin and Ryshkevitch equations performed best in terms of describing the relationship between these two parameters. Meanwhile, AAS cement is more sensitive to the mix proportions and additional care is needed to ensure its quality. It is the first time to use electrical response to predict the chloride transport coefficients of AAS cement, which would provide a powerful tool for its quality control and service life prediction. [ABSTRACT FROM AUTHOR]