Effects of Si/Al Ratios on the Bulk-Type Zeolite Formation Using Synthetic Metakaolin-Based Geopolymer with Designated Composition

Guoru Ma,Meirong Wang,Chengying Bai,Peigang He

doi:10.3390/cryst11111310

Abstract

In this paper, synthetic metakaolin with fixed composition (Al2O3·2SiO2) was produced by a simple chemosynthetic route. The chemosynthetic metakaolin can eliminate the influence of impurities in metakaolin from natural kaolin minerals. The synthetic metakaolin together with NaOH and SiO2-sol were used to prepare Na-based geopolymer precursors with various molar ratios of Si/Al. The molar ratios of Si/Al from 1 to 2 were tailored by adding different contents of SiO2-sol. Zeolite/geopolymer composites or monolith-type zeolite were successfully fabricated from synthetic metakaolin-based geopolymer through a hydrothermal process. The effects of Si/Al ratios on the phase composition and microstructure of the produced zeolite/geopolymer composites or zeolites were studied. The results proved that the composition of synthetic metakaolin and geopolymer precursors can be facilely tuned, and the monolithic geopolymer precursors can be mostly, or even totally, transformed into zeolite after hydrothermal treatment.

Highlights

Porous geopolymers have attracted a lot of attention, due to their low price, extensive sources of raw materials, excellent mechanical properties, high thermal and chemical stability, easy fabrication, consolidation/setting at room or low temperature as well as offering a wider range of applications than dense geopolymers [1,2]
The obtained synthetic metakaolin (S-MK) powders (Al2O3 · 2SiO2) were used to produce a geopolymer precursor for zeolite formation; it indicated that when the molar ratio of Si/Al ≤ 1.25, zeolite/geopolymer composites can be obtained as the existence of geopolymer gels, which was confirmed by XRD, FTIR, and SEM data
Almost all of the geopolymer gels can be transformed into zeolite at a higher molar ratio of Si/Al

Summary

Introduction

Porous geopolymers have attracted a lot of attention, due to their low price, extensive sources of raw materials, excellent mechanical properties, high thermal and chemical stability, easy fabrication, consolidation/setting at room or low temperature as well as offering a wider range of applications than dense geopolymers [1,2]. Various reinforcing and/or functional agents (e.g., zeolite, active carbon, SiC, hollow spheres, graphene, chitosan, and resin) were added into the geopolymer matrix to further enhance or optimize the performance, or to endow new functions [17]. In these composite materials, there are more and more research studies about zeolite/geopolymer composites, due to the advantageous combination of zeolite (functional agent with high surface area, porosity, and good adsorption performance) and geopolymers (matrix with high mechanical strength and good chemical stability) [17,18]. Zeolite/geopolymer composites can be formed mainly by three routes [17,18]: zeolite as a second phase directly added into the geopolymer slurry [19,20]; zeolite phase can be formed together with the geopolymerization process [21,22]; geopolymer is partially or completely transformed into zeolite after hydrothermal treatment [23,24]

Methods

Results

Conclusion