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This study investigates the effects of high temperatures and exposure duration on the flexural behavior and microstructure of hybrid basalt textile and steel fiber reinforced alkali-activated slag panels. Three-point bending tests were conducted after heating specimens to 400 °C, 600 °C, and 800 °C for durations of 1 and 2 h. The effects of thermal exposure on the matrix and the basalt fibers from the panels were investigated with scanning electron microscopy. Element and phase analyses of the matrix were performed after exposure to high temperatures via energy dispersive spectroscopy and X-ray diffraction, respectively. The first crack and peak flexural strength of the specimens not exposed to heat reached 8.9 and 20.5 MPa, respectively. Obvious decreases in flexural performance occurred as temperature and duration increased as a result of the decomposition of the alkali-activated slag mortar (AASM) matrix and the deterioration of the bonding performance between the basalt textile and the matrix. Changing of the Ca/Si ratio, the Al/Si ratio, and the crystalline phase of the AASM matrix indicated that phase transformation occurred after heat exposure at 800 °C. [ABSTRACT FROM AUTHOR]

Low field nuclear magnetic resonance (LF-NMR) was used to monitor the evolution characteristics of transverse relaxation time (T 2) of the water in sodium hydroxide (SH) and sodium silicate (SS) activated slag pastes. According to the T 2 weighted average value evolution curve, the early-age reaction process of SH-activated system could be divided into three typical stages: initial setting stage (I), acceleration stage (II) and deceleration stage (III). However, another two distinguishable stages including the second acceleration stage (Ⅳ) and the second deceleration stage (Ⅴ) could be identified in SS-activated system. The essential reactions in different reaction stages of the two types of alkali-activated pastes were discussed. The obtained relaxation signals were employed to determine the chemically bound water content in different alkali-activated slag (AAS) pastes at specific reaction stages, as well as the dynamic change of the relative contents of gel water, capillary water and free water with reaction time. The influence mechanism of the initial active SiO 4 4− oligomer in activator solution on the early-age reaction of AAS was revealed. A distinct water release behavior was observed in the geopolymerization process at the acceleration stages of each AAS paste. • The T 2 evolution characteristic of water in AAS pastes is monitored by LF-NMR. • The chemically bound water content at specific reaction stages is calculated. • The evolution of relative content of gel water, capillary water and free water is revealed. • A water release behavior is observed during the reaction process of AAS pastes. [ABSTRACT FROM AUTHOR]

Generally, high-water-content of dredged sediment (DS) tends to suffer from inferior mechanical properties and obvious shrinkage after solidification, so finding solutions to this issue is helpful for promoting the recovery and recycling of DS. In this paper, in reference to natural gypsum (NG), phosphogypsum (PG) was incorporated into DS solidified with alkali-activated slag (AAS) system. The effect of PG (0 %-20 %) on the hydration process (0–168 h), mechanical properties (3 d, 7 d and 28 d) and autogenous shrinkage (0–7 d) of DS solidified with AAS was investigated. It is found that the addition of PG not only induces the generation of ettringite to compensate for shrinkage, but also accelerates the formation of C-A-S-H by providing active calcium to promote stiffness to resist shrinkage. This results in a reduction of autogenous shrinkage by 74.3 % and an increase of compressive strength by 28.5 % when PG dosage is 15 %. Compared with NG, the difference in 28d-compressive strength of PG group is not more than 7.34 % under equivalent dosages. The dissolved SO 4 2– from PG could be adsorbed on C-A-S-H and preserved in pore solution in the form of Na 2 SO 4. The decrease in S/Si from 0.31 to 0.09 indicates stored SO 4 2– could be released back into system to promote the further generation of ettringite. To obtain superior mechanical properties and volume stability, appropriate PG dosage is 10 %-15 %. Compared with the control group, it increases the content of ettringite and amorphous phase by 2.4 %-4.6 % and 3.3 %-3.7 %, respectively. This research not only provides theoretical support for DS solidified with AAS to realize efficient utilization of solid waste resources (i.e., DS, PG and slag), but also gives a new insight into solidification of other high-water-content system, such as backfill mining, grouting materials and treatment of soft soil foundations. • Different solid wastes were utilized to treat high-water-content system. • PG induces the formation of ettringite in DS solidified with AAS system. • PG accelerates C-A-S-H formation and increases stiffness to resist shrinkage. • Ettringite consolidates massive water and compensates for shrinkage of ASD system. [ABSTRACT FROM AUTHOR]

• The micromechanical properties of different phases in FNS particles are revealed. • The micromechanical property of N-A-S-H gel in AAFNS is studied. • The effects of FNS and activator type on the microstructural characteristics of AASF are investigated. In recent years, low-calcium and high-magnesium ferronickel slag (FNS) has been gradually used in the preparation of alkali-activated cements (AACs). An in-depth exploration of the microstructural characteristics is crucial for a thorough understanding and enhancement of the macroscopic behaviors of AACs. In this study, the microstructural composition and micromechanical properties of the alkali-activated ground granulated blast furnace slag (GGBS)-FNS cements (AASF) prepared with sodium silicate and sodium hydroxide as alkali activators were quantitatively studied. Backscattered electron image analysis (BSE-IA) and nanoindentation were selected to reveal the micromechanical properties of different phases in FNS particles and AASF pastes. The results show that the elastic modulus of sodium aluminosilicate hydrate (N-A-S-H) in alkali-activated FNS cement is about 17 GPa, which is similar to that of the gel phase in alkali-activated fly ash cement (AAFA) and alkali-activated metakaolin cement (AAMK), confirming that the micromechanical property of N-A-S-H gel is independent of the kind of precursor materials. The Ca/Si and Al/Si of the hydration products in AASF gradually decline as FNS is incorporated, whereas the Mg/Al steadily rises, increasing the elastic modulus of the inner hydration product (IP) phase in AASF. The reaction degree of GGBS is higher in the sodium silicate-activated AASF system, while more N-A-S-H gels are formed in the sodium hydroxide-activated AASF system. The incorporation of FNS is conducive to improving the reaction degree of GGBS, resulting in the increase of calcium silicoaluminate hydrate (C-A-S-H) content and a significant decrease in IP content in AASF. [ABSTRACT FROM AUTHOR]

This study aims to investigate the effects of ferronickel slag (FNS) on the hydration heat, mechanical strength and microstructure of alkali-activated slag (AAS). Sodium hydroxide (SH) and sodium silicate (SS) were used as alkali activators to prepare various alkali-activated slag-FNS cements (AASF). The incorporation of FNS has no significant effect on the reaction process of the SH-activated AASF system while delaying the reaction process of the SS-activated AASF system. The influence mechanism of FNS on the reaction process of AASF is unaffected by variations in alkalinity and modulus of the alkali activators. The delay effect of FNS on the SS-activated AASF system is diminished at 60 °C. The reaction degree of FNS at a later age is significantly higher than that at an early age. Mg in FNS participates in the formation of Mg 4 Al 2 (OH) 14 ·3H 2 O and gel products. The reaction degree of FNS increases with the rise of FNS content in the SH-activated AASF system but has no relationship with the FNS content in the SS-activated AASF system. • The incorporation of FNS delays the reaction process of the sodium silicate-activated AAS system. • The influence of FNS on the reaction process of AAS is unaffected by variations in alkalinity and modulus of activators. • The reaction degree of FNS in the sodium hydroxide-activated AASF system increases with the rise in FNS content. [ABSTRACT FROM AUTHOR]

This paper presents comprehensive investigation on the microstructure and micromechanical properties of the hydration products in alkali-activated slag by adopting the coupled techniques of nanoindentation, optical microscopy, backscattering electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS), aiming to provide insight into the formation mechanism of the dark rim in AAS. It is found that the dark rim is primarily comprised of a mixture of Mg-Al layered double hydroxide (LDH) and C-A-S-H gel, and the existence of this LDH phase contributes to the relatively high hardness and elastic modulus of the dark rim. The dark rim is in-situ formed in the outer layer of GGBFS. The dissolution and diffusion of Ca and Si in the outer layer of GGBFS results in a lower average atomic number in the dark rim region, contributing to its relatively lower grey level. Based on the experimental results, the formation mechanism of the dark rim is discussed in this investigation. • The formation mechanism of the dark rim is proposed. • The dark rim is in-situ formed in the outer layer of slag. • The hardness and elastic modulus of the dark rim is higher than that of C-A-S-H gel in the matrix. • It is the composition rather than the density which should be responsible for the low grey value of the dark rim. [ABSTRACT FROM AUTHOR]

In the present study, the early-age reaction process of alkali-activated slag (AAS) pastes was in-situ monitored by using embedded piezoelectric transducer based ultrasonic monitoring system. The effects of different types of activator (sodium hydroxide and sodium silicate) on reaction process, products and their microstructure evolution were investigated. The experimental results reveal that the early-age reaction process of AAS can be effectively in-situ observed by use of the ultrasonic method. It is found that the early-age reaction process of AAS pastes made with sodium silicate can be divided into five stages: (Ⅰ) dormant period, (Ⅱ) acceleration period, (Ⅲ) deceleration period, (Ⅳ) second acceleration period and (Ⅴ) second deceleration period. However, only three featured stages could be identified in those activated with sodium hydroxide. The soluble SiO 4 4− ions in activator solution are critical to the formation process of reaction products and the microstructure evolution, especially for the former four stages. In specific, when activation is with sodium silicate, the increase of its modulus (Ms, molar ration of SiO 2 to Na 2 O) leads to the improvement of reaction extent in stage Ⅰ, prolongs the reaction time of stage Ⅱ; meanwhile, the deceleration period of stage Ⅲ is shortened due to the advancement of stage Ⅳ. • Early-age reaction process of AAS is in-situ monitored by ultrasonic measurement. • Five featured stages are distinguished in AAS activated with sodium silicate. • Three typical stages could be identified in AAS activated with sodium hydroxide. • Soluble SiO 4 4− ions affect each specific stage of the early-age reaction process. [ABSTRACT FROM AUTHOR]

This paper presents the leaching behavior of hydration products and unhydrated particles in Portland cement (PC) and alkali-activated slag (AAS) systems with a new in-situ observation method. After leaching in NH 4 Cl solution, obvious decalcification can be found in unhydrated C 3 S and C 2 S in PC, while the unhydrated slag keeps stable in AAS. The Ca in the gel of AAS is more vulnerable to leaching than that in PC due to the lack of phases with buffering capability like portlandite. In addition, the Mg–Al layered double hydroxide, which mainly exists in the dark rim of AAS, is not stable in NH 4 Cl solution. The in-situ observation method proposed in this study provides a new methodology to investigate the leaching behavior of different phases directly and helps to understand the effect of leaching on cementitious materials from a new perspective, it can therefore be used to instruct the design of durable cementitious systems. • A new method is proposed to in-situ observe the leaching behavior of phases in PC and AAS pastes. • The decalcification of gels in AAS is more serious than that in PC due to the lack of phases with buffering capability. • After leaching, the C 2 S and C 4 AF particles can still maintain integrity, while the C 3 S particles suffer from obvious erosion. • The Mg–Al layered double hydroxide in the dark rim of AAS is vulnerable to leaching. [ABSTRACT FROM AUTHOR]