This paper presents the results of an experimental study performed to investigate the effect of activator modulus (SiO2/Na2O) and slag addition on the fresh and hardened properties of alkali-activated fly ash/slag (AAFS) pastes. Four activator moduli (SiO2 /Na2O), i.e., 0.0, 1.0, 1.5, and 2.0, and five slag-to-binder ratios, i.e., 0, 0.3, 0.5, 0.7, 1.0, were used to prepare AAFS mixtures. The setting time, flowability, heat evolution, compressive strength, microstructure, and reaction products of AAFS pastes were studied. The results showed that the activator modulus and slag content had a combined effect on the setting behavior and workability of AAFS mixtures. Both the activator modulus and slag content affected the types of reaction products formed in AAFS. The coexistence of N–A–S–H gel and C–A–S–H gel was identified in AAFS activated with high pH but low SiO2 content (low modulus). C–A–S–H gel had a higher space-filling ability than N–A–S–H gel. Thus, AAFS with higher slag content had a finer pore structure and higher heat release (degree of reaction), corresponding to a higher compressive strength. The dissolution of slag was more pronounced when NaOH (modulus of 0.0) was applied as the activator. The use of Na2SiO3 as activator significantly refined the pores in AAFS by incorporating soluble Si in the activator, while further increasing the modulus from 1.5 to 2.0 prohibited the reaction process of AAFS, resulting in a lower heat release, coarser pore structure, and reduced compressive strength. Therefore, in view of the strength and microstructure, the optimum modulus is 1.5.@en

This study is undertaken to explore the relationship between the reaction products and the drying shrinkage of alkali-activated slag (AAS) with the varying hydroxide dosage. AAS pastes were firstly produced with either Al(OH)3 or Mg(OH)2 to investigate the effects of hydroxide on the composition of reaction products and the drying shrinkage of AAS. Secondly, the main reaction products formed in AAS pastes, viz. C4AH13, M4AH10, C-S-H, C-A-S-H and C-M-S-H, were synthesized individually. Synthetic products were then taken to cast the respective paste sample to examine the performance against the drying exposure. The results show that adding Mg(OH)2 indeed improved the shrinkage resistance of AAS. This is primarily connected to the increased crystalline phases, namely the hydrotalcite-like phases and the unreacted Mg(OH)2. Whereas adding Al(OH)3 did not relieve the drying shrinkage of AAS as the interlayer spacing of C-A-S-H was enlarged, which in turn allowed for more shrinkage. Further, the investigation of individual synthetic products illustrates that their drying shrinkage scales satisfied the following order: C4AH13 < M4AH10 < C-M-S-H < C-S-H < C-A-S-H. Moreover, the shrinkage scale of C-S-H was found to evolve as the Ca/Si ratio decreased.@en

Carbonation treatment can effectively improve the performance of recycled concrete aggregate and fines due to the reactions of CO2 with CH and C–S–H gel of cement paste. To better understand the mechanisms involved in the performance improvement, the surface properties of carbonated recycled cement paste powder (CRP) and its effect on the rheology, hydration and strength development of cement paste was studied. The results showed that during the carbonation, the surface of CRP was covered by a layer of amorphous silica gel. The generated CaCO3 was wrapt by the silica gel and seldom exposed. The silica layer led to the poor flowability of CRP-cement paste due to that the silica gel on the surface of CRP has a strong affinity for H2O. During the very early hydration, the silica gel dissolved and then CaCO3 was exposed. CaCO3 is capable of chemically absorbing Ca2+, which facilitated the nucleation of C–S–H nuclei and stabilized the C–S–H phase. As a result, the C–S–H grew densely, uniformly and perpendicularly on the surface of CRP. In addition, the chemically absorbing Ca2+ enabled the chemical bond to be formed between CaCO3 and C–S–H. Due to increased C–S–H resulted from reactions of silica gel with CH at the interface and the stronger bond formed between CaCO3 and C–S–H, the interface between CRP and hydration products was much stronger than that between recycled cement paste powder (RP) and hydration products.@en

The interface between filler and hydration products can have a significant effect on the mechanical properties of the cement paste system. With different adhesion properties between filler and hydration products, the effect of microstructural features (size, shape, surface roughness), particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system is supposed to be different, as well. In order to understand the effect of the microstructural features, particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system with either strong or weak filler-matrix interface, microscale simulations with a lattice model are carried out. The results show that the strength of the filler-matrix interface plays a more important role than the microstructural features, particle distribution and area fraction of filler in the crack propagation and the strength of blended cement paste. The knowledge acquired here provides a clue, or direction, for improving the performance of existing fillers. To improve the performance of fillers in cement paste in terms of strength, priority should be given to improving the bond strength between filler particles and matrix, not to modifying the microstructural features (i.e., shape and surface roughness) of the filler.@en

The high autogenous shrinkage of alkali-activated fly ash/slag (AASF) poses a significant concern for the widespread application of AASF in structural engineering. The present study compares the efficacy of activator and mineral admixtures in mitigating the autogenous shrinkage of AASF, and discusses the underlying mechanism. The results show that the use of activators with a lower silicate modulus and a lower sodium content, as well as incorporating metakaolin (MK) or silica fume, can reduce the autogenous shrinkage of AAMs. These approaches delay the appearance of the second exothermic peak, which corresponds to the later formation of C-A-S-H gels and slower development of capillary pressure. The inclusion of MK not only retards the reaction but also facilitates the formation of N-A-S-H gels, resulting in a coarse pore structure and reduced water consumption. The use of the activator with a lower silicate modulus (reduced from 1.5 to 1.0) leads to a higher internal relative humidity and the reduced pore volume of silt-shaped and ink-bottle pores (2–50 nm) in AASF, thereby reducing the autogenous shrinkage without significant strength reduction.@en

It is of great significance for the optimization and performance improvement of aggregates to investigate the influence of aggregate chemical properties on the nucleation and growth of calcium silicate hydrate (C-S-H)and its mechanism. In this work, limestone, quartz and magnesite were used as aggregates. The interactions between different aggregates and ions in a simulated solution were determined via zeta potential measurement. Also, the nucleation and growth of C-S-H on the surface of these aggregates were analyzed by scanning electron microscopy. The hydration heat of cement pastes with the micro-aggregates was examined to investigate the effect of aggregate chemical properties on the early hydration of cement. The results show that limestone surface has a strong adsorption of Ca2+ (i.e., chemical adsorption), leading to a high density of C-S-H nuclei and directional growth of C-S-H, and thus greatly promoting the hydration of cement. The adsorption of Ca2+ on quartz surface is due to electrostatic force, which is weaker than that due to the chemical adsorption. The density of C-S-H nuclei on quartz surface is thus much lower than that on limestone surface. In the case of magnesite, its surface has a high affinity for SO42- rather than Ca2+, reducing the nucleation density of C-S-H on the surface of magnesite, and thus slowing down the early hydration of cement.@en

Investigating the mechanism of multi-walled carbon nanotube (MWCNT) in the early cement hydration enables better utilization of the potential of MWCNT as reinforcing agents in cement composites. The interaction of MWCNT with ions and its effect on cement hydration were investigated. Zeta potential measurement was intended to discuss the interaction of the MWCNT surface with Ca2+. Then, the formation processes of C–S–H were observed with a scanning electron microscope (SEM). The influence of MWCNT on the early cement hydration was investigated using isothermal calorimetry. The results showed that MWCNT significantly accelerates early cement hydration. It is likely attributed to its high surface area and strong adsorption for Ca2+, which greatly promotes the migration of ions, especially Ca2+, and thus the precipitation of ions on the cement surface. This facilitates the C–S–H nucleation and growth process, thus the cement hydration. These results also indicated that the MWCNT would not provide a stable nucleation site for C–S–H, since the size of MWCNT is less than that of C–S–H.@en