Pressure-Induced Geopolymerization in Alkali-Activated Fly Ash

Sol Park,Nam Lee,Jeong Jang,Haeng Lee,Hyun Yoon,Seon Kim,Joon Seo,Hammad Khalid,Hyeong Son

doi:10.3390/su10103538

Abstract

The present study investigated geopolymerization in alkali-activated fly ash under elevated pressure conditions. The fly ash was activated using either sodium hydroxide or a combination of sodium silicate solution and sodium hydroxide, and was cured at 120 °C at a pressure of 0.22 MPa for the first 24 h. The pressure-induced evolution of the binder gel in the alkali-activated fly ash was investigated by employing synchrotron X-ray diffraction and solid-state 29Si and 27Al MAS NMR spectroscopy. The results showed that the reactivity of the raw fly ash and the growth of the zeolite crystals were significantly enhanced in the samples activated with sodium hydroxide. In contrast, the effects of the elevated pressure conditions were found to be less apparent in the samples activated with the sodium silicate solution. These results may have important implications for the binder design of geopolymers, since the crystallization of geopolymers relates highly to its long-term properties and functionality.

Highlights

Geopolymers are a cementitious material belonging to a group of alkali-activated binders, which can be synthesized by alkaline activation of aluminosilicate-rich precursors [1,2]
The present study explored pressure-induced geopolymerization in alkali-activated fly ash using synchrotron X-ray diffraction (XRD) and solid-state magic angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy
The synchrotron XRD results showed that significant growth of zeolites in the H-series samples was induced by pressure, while the S-series samples remained amorphous in identical conditions

Summary

Introduction

Geopolymers are a cementitious material belonging to a group of alkali-activated binders, which can be synthesized by alkaline activation of aluminosilicate-rich precursors [1,2]. Geopolymers are known to exhibit excellent durability performance [6,7], providing a potential application to various fields in place of ordinary Portland cement [8]. Fully-reacted geopolymeric gels are totally amorphous [9]. They may present a nanostructural analogue to zeolite, possibly due to the similarities between the chemical compositions of both materials [10,11]. A recent study conducted by Brant et al [12] revealed that Q4(4Al), Q4(3Al), Q4(2Al), and Q4(1Al) Si units are predominantly present in sodium aluminosilicate gels, which are charge-balanced by three to four coordinated Na+ ions and H2O molecules, or six coordinated Al atoms. This study identified that the presence of Al3+ in sites with lower symmetry provided charge-balancing capacity [12]

Methods

Results

Conclusion