Insights into the Microstructure of Hydrothermal Synthesized Nanoscale K2O-Al2O3-SiO2-H2O Particles

Liu Liu,Fang Fang,Zhuang Zhuang,Li Li,Xing Xing,Shuai Shuai,Long Long,Zhu Zhu

doi:10.3390/nano10010063

Abstract

K-A-S-H (K2O-Al2O3-SiO2-H2O) gel is a key phase that forms in most alkali-activated binders (eco-friendly binders which utilize a substantial amount of industrial by-product). An in-depth understanding of the microstructure and performance of this nano-sized key phase facilitates better application to alkali-activated binders. However, such studies remain little and undetailed. Therefore, our research aims to provide insights into the microstructure of K-A-S-H particles synthesized with accurate stoichiometric control by the hydrothermal method through thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BET surface area. The results show that for materials prepared at the curing temperature lower than 80 °C, the K-A-S-H products were completely amorphous. With increased curing temperature and time, the K-A-S-H products were transformed from the amorphous phase to the crystalline zeolite phase structure, with a reduction in the specific surface area. The TG results indicate that the crystalline phase contains more non-evaporated water or zeolite water for structural rearrangement. The degree of tetrahedral polymerization slightly decreased with an increase of the K2O/SiO2 ratio as the amount of non-bridged oxygen atoms increased, whereas it gradually increased with an increase of curing temperature and time, as suggested by the FTIR and NMR results. Various K2O/SiO2 ratios resulted in the formation of zeolite K-H and K-G zeolite, both of which exhibited highly polymerized three-dimensional network structures. However, there was no significant effect of the SiO2/Al2O3 ratio on the structure of K-A-S-H products. Overall, these results provide insight into understanding the chemical stability of K-A-S-H.

Highlights

The increasing demand for low-cost and durable construction materials has stimulated the study of alternative cementitious binders [1]
To probe the state of Al in the inner structure, conditions including K2O/SiO2 ratio, SiO2/Al2O3 ratio, curing temperature and curing time were varied and their effects on the resulting gels were investigated, and the following conclusions can be drawn: (1) X-ray diffraction (XRD) results demonstrated the presence of amorphous K-A-S-H gel and two crystalline products of zeolite K-H and K-G zeolite
The high K2O/SiO2 ratios led to the formation of K-G zeolite, while zeolite K-H in low ratios

Summary

Introduction

The increasing demand for low-cost and durable construction materials has stimulated the study of alternative cementitious binders [1]. Alkali-activated binders have been widely discussed and promoted. These binders are promising cementitious materials that can be used in place of Portland cement (PC) in many applications, including concrete. Compared with Portland cement-based production, the use of alkali-activated materials can reduce related carbon dioxide emissions by more than 80% [2,3], an important concern worldwide. A primary reaction product of alkali-activated binder systems may be an alkali aluminosilicate–type gel [4], a gel type that is poor in calcium. This gel is often represented as hydrated sodium aluminosilicate N-A-S-H (N2O-Al2O3-SiO2-H2O) or, with substitution of potassium for sodium, K-A-S-H (K2O-Al2O3-SiO2-H2O) [5]. This nomenclature describes the chemical nature of the reaction product rather than its precursor, similar to the designation of C-S-H (CaO-SiO2-H2O) [6]

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Conclusion