Geopolymers are three-dimensional amorphous Si-O-Al networks that generally can be synthesized from low-Ca aluminosilicate mineral sources. Such materials were first introduced as a sustainable construction material but today their application goes beyond the building industry. So far, a broad range of aluminosilicate minerals including fly ash, natural pozzolans, kaolin and metakaolin have been used to produce geopolymers; however, there are limited studies on the geopolymerization of porous and crystalline aluminosilicate minerals such as mined zeolites. Use of zeolite as it is for commercial applications depends on the shape and architecture of these materials. Therefore, the hypothesis was that geopolymerization provides the possibility of using mined zeolite in different shapes. Moreover, zeolite as a nontoxic mineral material with an inherent 3D structure may result in the formation of the cleaner geopolymeric product with different physical properties compared to when waste materials such as fly ash are employed. In this study, the viability of creating geopolymers from mined zeolite has been demonstrated. The aim of this study was to evaluate the influence of different parameters such as zeolite particle size, curing temperature, reagents ratio and time on amorphous content and mechanical strength. The conversion of the crystalline phase of mined zeolite to amorphous gel and/or synthetic zeolite phases was comprehensively studied using X-ray diffraction. It was found that finer zeolite particles resulted in the formation of a material with higher amorphous content (max ~60%) and higher mechanical strength (max ~33 MPa). It was also shown that the higher amorphous content did not necessarily translate to higher mechanical strength due to the formation of intermediate species that cannot transfer into the polycondensation stage. It was revealed that the formation of analcime and chabazite may occur through the geopolymerization process. Microstructure studies using infrared spectroscopy confirmed the geopolymer formation and development over time.

Introduction

Geopolymers are three-dimensional amorphous Si-O-Al networks that generally can be synthesized from low-Ca aluminosilicate mineral sources using a strong alkaline activator (Hajimohammadi and van Deventer, 2016; Provis and Bernal, 2014). It is believed that the use of geopolymers goes back to the ancient Egyptians, but geopolymerization as we know it today, was first defined by Davidovits in 1978 (Rao and Liu, 2015). These materials are considered an environmentally friendly alternative to cement in the construction industry (Reddy et al., 2016). This is because the geopolymer production releases about six times less CO2 in comparison to production of Portland cement. These materials have great potential for various applications due to their sustainable inherent features such as being lightweight, fire resistant, of high workability and good chemical stability (Aguirre-Guerrero et al., 2017; Cheng and Chiu, 2003; Majidi, 2009). Some of the applications are production of light weight construction material (Pimraksa et al., 2011), heavy metal stabilization (El-Eswed et al., 2015), manufacturing high quality, non-burning and fire resistant ceramic breaks (Liew et al., 2016a), and thermal insulation (Ferone et al., 2019). Geopolymerization is a low-cost method to convert different types of inorganic supplies from waste materials to natural resources into a more valuable, environmentally friendly product.