Resistance of geopolymer materials to acid attack

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Abstract

This article presents an investigation into durability of geopolymer materials manufactured using a class F fly ash (FA) and alkaline activators when exposed to 5% solutions of acetic and sulfuric acids. The main parameters studied were the evolution of weight, compressive strength, products of degradation and microstructural changes. The degradation was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The performance of geopolymer materials when exposed to acid solutions was superior to ordinary Portland cement (OPC) paste. However, significant degradation of strength was observed in some geopolymer materials prepared with sodium silicate and with a mixture of sodium hydroxide and potassium hydroxide as activators. The deterioration observed was connected to depolymerisation of the aluminosilicate polymers in acidic media and formation of zeolites, which in some cases lead to a significant loss of strength. The best performance was observed in the geopolymer material prepared with sodium hydroxide and cured at elevated temperature, which was attributed to a more stable cross-linked aluminosilicate polymer structure formed in this material.

Introduction

Acid resistance is a desirable property for structural materials used in the aggressive environment of chemical, mining, mineral processing and other industries. Basic in nature, concretes made with Portland cement and alkali-activated slag deteriorate in the acid environment [1], [2]. In the case of acid attack on ordinary Portland cement (OPC) concrete, calcium salts of the attacking acid rapidly form and the concrete loses its strength and deteriorates quickly. When tested in acetic acid solution of pH 4 for 12 months, AAS concrete had about 33% strength reduction as compared to 47% reduction in OPC. Thus, AAS was found superior in durability to OPC. Its good performance was attributed to low Ca content (∼40% CaO) compared to Portland cement (∼65% CaO) and the glassy state of the slag, which is poorly soluble in the acid solution [2]. Geopolymer materials prepared with the class F fly ash (FA) contain very low calcium (3–4% CaO) and it could be expected that they possess high durability in the acid environment.

Geopolymers are synthetic minerals belonging to the same family of aluminosilicates as zeolites, but unlike zeolites, they are essentially amorphous polymers. The properties of these geopolymer materials prepared using FA are not well studied but there are reports of superior durability and heat-resistant properties of geopolymer materials prepared using metakaolin [3], [4], [5]. Geopolymer materials are produced by a sol–gel process utilising alumina and silica oxides activated by alkali hydroxides and/or alkali silicates. The starting materials dissolve in high pH alkaline solution and the geopolymers are precipitated; this process being facilitated by heat. In the process of the polymerisation reactions, polysialates, polysialate siloxo, and polysialate disiloxo are formed [3]. The mineral polymers have empirical formula: Mn[–(SiO2)z–AlO2]n·wH2O, where z is 1, 2 or 3; M is an alkali cation, such as potassium or sodium, and n is the degree of polymerisation [4], [5]. In a previous article, the processing and characterisation of geopolymer materials made using class F FA and formed at elevated temperature were discussed [6]. This article presents a study of durability in the acid environment of three geopolymer materials utilising class F FA activated by sodium silicate, sodium hydroxide and a mixture of sodium and potassium hydroxides.

Section snippets

Materials

The chemical and mineral compositions of FA are shown in Table 1 and Fig. 1, respectively. FA used was sourced from Gladstone in Queensland, Australia. It is mainly glassy with some crystalline inclusions of mullite, hematite and quartz. Laboratory grade sodium silicate solution type D with Ms (ratio of silica to sodium oxide) equal to 2, and 14.7% Na2O and 29.4% SiO2 was supplied by PQ Australia, while 60% w/v sodium hydroxide solution was supplied by Sigma. Potassium hydroxide pellets were

Results

Geopolymer specimens had very small change in appearance after 5 months of immersion in the acidic solutions. Some softening of the surface cover and insignificant lightening of the colour could be noticed in the 8FA, 8FASS and 8FAK geopolymer specimens after exposure to the solution of sulfuric acid. In the acetic acid, there was no change in appearance of 8FAK specimens and very small change in appearance of 8FASS and 8FA specimens.

Visual examination of specimens exposed to the sulfuric acid

Discussion

To understand the process that takes place in the geopolymer materials immersed in the acidic solution, let us start with the reactions that occur in the materials before immersion. The possible chemical processes of dissolution of the starting materials and condensation of silicon and aluminium monomers can be represented as the following reactions [11]:(Al-Sisolid)(flyashparticle)+OH(Aq)⇔Al(OH)4monomer+OSi(OH)3monomerOSi(OH)3+OHOSi(OH)2O+H2OOSi(OH)3+M+⇔M+OSi(OH)3monomerOSi(OH)2O+2

Conclusions

The deterioration of geopolymer materials in acidic media is connected to depolymerisation of aluminosilicate polymers and liberation of silicic acid, replacement of Na and K cations by hydrogen or hydronium ion and dealumination of the geopolymer structure. It is also connected to condensation of siliceous polymers and zeolites, which in some cases lead to a significant loss of strength.

In acidic environment, high-performance geopolymer materials deteriorate with the formation of fissures in

Acknowledgements

The author is grateful to the Australian Research Council for financial support under Grant DP0209501 and to Civil Engineering Department and School of Physics and Materials Engineering, Monash University, for providing access to equipment used in this investigation.

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