No abstract

No abstract

Alkali-activated aluminosilicate cements, as a sustainable and high-performance class of cementitious materials, can benefit from the development of admixtures that can enhance various aspects of their performance. Alkali activation generally occurs at a faster pace than the hydration of Portland cement (PC). In this investigation, citric acid was evaluated as an admixture for controlling the hydration kinetics, enhancing the fresh-mix rheology and delaying the setting of alkali aluminosilicate cement (AAC) without compromising its long-term strength. The obtained experimental results showed that the use of 3 wt% citric acid reduced the heat of hydration, improved the fresh-mix rheology and increased the hardened material compressive strength of the AAC. The results demonstrate the merits of using citric acid as an effective admixture in alkali-activated cement to make it more compatible with the construction practices established for PC.

In order to mitigate problems associated with environmental pollution, alkali-activated nickel slags (AANSs) may be used as an alternative to cementitious material. However, the understanding of their mechanical properties and microstructure is currently very limited. This paper therefore explores the influence of different types and contents of three solid alkali activators, Na2SiO3, NaOH, and Na2SiO3/Na2CO3, on the compressive strength of one-part AANS. Their microstructure, hydration components, and pore structure are analyzed by means of a scanning electron microscope, an energy-dispersive spectrometer, an X-ray diffractometer, an infrared spectrometer, and a mercury analyzer. The results show that the AANS with an Na2SiO3/Na2CO3 activator has a denser microstructure, lower porosity, and a smaller pore size when compared with the AANS with the Na2SiO3 or NaOH activators. Consequently, the compressive strength of the Na2SiO3/Na2CO3 sample reached a higher compressive strength (96 MPa) than that activated by Na2SiO3 or NaOH. This strength is optimal as well as more economical as Na2O, which increased from 0.107 mol to 0.123 mol, contributes little to compressive strength. The final part of the article discusses an optimal design for the engineering application of one-part AANS.

The article describes an experimental study on the bond–slip performance between the pultruded glass fiber-reinforced polymer (GFRP) tube and the nano-CaCO3 concrete. Taking the nano-CaCO3 concrete strength and GFRP tube thickness as primary parameters, nine specimens were designed and tested to study the influence of these parameters on the bond strength of the specimens. Besides, three specimens filled with the ordinary concrete were also tested by using the push-out tests to make comparisons with the bond performance of the specimens filled with nano-CaCO3 concrete. A total of four push-out tests were conducted on each specimen. The experimental results indicate that there are two types of axial load–slip curves for each specimen in four push-out tests. Moreover, comparison of the results of the push-out tests in the same direction shows that the bond failure load of the specimen decreases with the increase in the number of push-out tests. Based on the analysis of the test results, it is shown that the bond performance between the GFRP tube and the nano-CaCO3 concrete is better than that between the GFRP tube and the ordinary concrete. Furthermore, as the nano-CaCO3 concrete strength increases, the bond strength of the specimens decreases, indicating that the concrete strength has a negative effect on the bond strength. When the nano-CaCO3 concrete strength is relatively smaller (C20), the bond strength of the specimens decreases with the increase in the thickness of the GFRP tube. However, when the nano-CaCO3 concrete strength is relatively larger (C30 and C40), the bond strength of the specimens increases as the thickness of the GFRP tube increases.