Variable Factors Controlling Amorphous-Zeolite Phase Transformation in Metakaolin Based Geopolymer

Abstract

Metakaolin based geopolymer binder was synthesized using variable factors such as Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O molar ratios as well as curing time. Metakaolin was used as an alumino-silicate source incorporating with strong alkali activators; sodium hydroxide solution to undergo polycondenation and hence a formation of hardened geopolymeric binder. This work was aimed to investigate the effect of Na2O/SiO2, Na2O/Al2O3 and H2O/Na2O ratios, as well as curing time on phase transformation. The chemical composition, phase development and microstructure of reaction products were examined by X-ray fluorescence, X-ray diffraction analysis and scanning electron microscopy with respect to their final compressive strength. Increasing the molar ratios of Na2O/Al2O3 and H2O/Na2O tended to favour the transformation of amorphous gel to various types of crystalline zeolite. Sodalite (pseudo-phase zeolite) and zeolite belonging to the Faujasite group, as well as zeolite X and A were identified in these mixtures. Based on this study, systems that favoured the formation of zeolitic products tended to possess lower strengths.Keywords: Geopolymer binder, Microstructure, Zeolite, Amorphous gel, Aluminosilicates IntroductionGeopolymer or alkali polysialate is an entirely known material that could be used in a wide range of potential applications includes: fire resistant materials, thermal insulation, ceramic tiles, thermal shock refractories, cements and concretes, high-tech composites aircraft interior and automobile, radioactive and toxic waste containment, arts and decoration. These can be formed by mixing aluminate and-silicate materials such as silica fume, fly ash, rice husk ash, slag, waste glass, industrial wastes, kaolinite and metakaolin with strong alkali activators or solution [1][2][3]. The basic of geopolymerization process involve dissolution of solid alumino-silicate oxides in alkali hydroxide or alkali silicate solution, diffusion or transportation of the dissolved Si and Al complexes from the initial raw material, formation of a gel phase and finally hardening of the gel phase. The hydration products forming in the thee-dimensional rigid network geopolymer are composed of variety structures, such as amorphous phase, semi-crystalline phase, as well as fully crystalline zeolite phase. A high content of amorphous geopolymeric gel resulted to the high compressive strength, while a formation of zeolite phase in geopolymer matrix produced lower strength [4][5][6][7].

Correlation between initial SiO2/Al2O3, Na2O/Al2O3, Na2O/SiO2 and H2O/Na2O ratios on phase and microstructure of reaction products of metakaolin-rice husk ash geopolymer

“…The particulate zeolite types Y and faujasite were observed in these samples at all curing times (1 and 90 days). These results agreed with our previous works (using only NaOH as activation), which using molar ratio of SiO 2 /Al 2 O 3 = 2.0 in range of Na 2 O/Al 2 O 3 ratio = 0.6-2.4, Na 2 O/SiO 2 = 0.31-1.22 and H 2 O/Na 2 O = 7.5-27.5[6,7] as demonstrated in Fig.1(m), (n) and (o). We repeated the experiments for comparison in this work.…”

“…Sodium (or potassium) hydroxide and sodium (or potassium) silicate or their combinations are normally used as alkaline activators [4,5]. In many cases, alkaline hydroxide can be used without alkali silicates depending on the reactivity of starting materials [6][7][8][9]. Metakaolin (MK) is often used as the sources of reactive SiO 2 and Al 2 O 3 .…”