Geopolymeric materials prepared using Class F fly ash and elevated temperature curing

Abstract

This paper reports the results of the study of the influence of elevated temperature curing on phase composition, microstructure and strength development in geopolymer materials prepared using Class F fly ash and sodium silicate and sodium hydroxide solutions. In particular, the effect of storage at room temperature before the application of heat on strength development and phase composition was studied. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and SEM were utilised in this study.

Long precuring at room temperature before application of heat was beneficial for strength development in all studied materials, as strength comparable to 1 month of curing at elevated temperature can develop in this case only after 24 h of heat curing. The main product of reaction in the geopolymeric materials was amorphous alkali aluminosilicate gel. However, in the case of sodium hydroxide activator in addition to it, traces of chabazite, Linde Type A, Na-P1 (gismondine) zeolites and hydroxysodalite were also present. The type of zeolite present and composition of aluminosilicate gel were dependent on the curing history.

Introduction

First scientific studies of geopolymeric materials indicated a certain potential for these materials to become cement for the future [1], [2], [3], [4], [5]. Davidovits [5], [6], [7] first used the term “geopolymers” in describing synthetic minerals similar to those that form in the Earth's crust. Geopolymers belong to the same silicoaluminates family as zeolites but have a significant difference from them: they are essentially amorphous polymers. Their properties are also different: they possess high strength, thermal stability, high surface smoothness and precision, and high surface hardness. [6], [7]. These mineral polymers with empirical formula: M_n[–(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, were called polysialates [5], [6], [7].

Geopolymer materials can be formed from silica–alumina oxides mixed with alkali hydroxides and alkali silicates. Davidovits [6], [7] used the silica–alumina oxide mixture, specially prepared highly reactive clay, for example, neokaolinite, parakaolinite, halloysite, milanite, and chamoisite, calcined and milled to increase its reactivity. The resulting silica–alumina oxide has the aluminium cation in fourfold coordination and in tetrahedral position. When this silica–alumina oxide is mixed with alkali silicates, reactions of dissolution and copolymerisation occur and polysialates, polysialate–siloxo and polysialate–disiloxo are formed [5]. Compared to the described patents, this paper presents an investigation of the properties of geopolymeric materials utilising Class F fly ash and the alkaline activators, such as sodium silicate and sodium hydroxide, and cured at elevated temperature.

Previous research reported that heat is an important factor for the activation of fly ash, because of the activation barrier, which has to be overcome for the reaction to take place [8]. It was reported that the activation energy is higher for fly ash than for slag, and thus heat treatment is more important for the activation of fly ash.

There were a number of researchers studying activation of fly ash by alkalies utilising elevated temperature curing [1], [2], [3]. The study by Swanepoel and Strydom [2] investigated utilisation of fly ash and kaolinite clay in a geopolymeric material cured at temperatures up to 70 °C. The compressive strength achieved after 28 days was 8 MPa. Palomo et al. [1] presented a study of alkali-activated fly ashes cured at 65 and 85 °C at two liquid/solid ratios: 0.25 and 0.3. The study indicated formation of an amorphous alkali aluminosilicate similar to that obtained in the alkali activation of metakaolin. The strength developed was in the range of 60 MPa. No crystalline zeolitic products were observed, but the Fourier transform infrared spectroscopy (FTIR) spectra indicated a possibility of their presence. The investigations showed the importance of elevated temperature curing particularly for samples exposed to 2 and 5 h curing, where a significant increase in strength was observed at 85 °C as compared to 65 °C. It was found that the rise of strength at 85 °C as compared to 65 °C was much smaller when curing time was 24 h. This study continues the investigation of the effect of different curing regimes and types of activator on the strength development and hydration products of Class F fly ash activated by sodium silicate and sodium hydroxide. It investigates the effect of precuring at room temperature before the application of heat on strength development and products of reaction and the effect of different heat treatment regimes and activators on long-term properties of geopolymer materials.