Characteristics, microstructures, and optimization of the geopolymer paste based on three aluminosilicate materials using a mixture design methodology

Abstract

This study focuses on the application of mixture design as a novel approach to developing a geopolymer paste with optimized mechanical properties. It also aims to utilize the abundance of natural or industrial aluminosilicate precursors through their combination for synthesizing high-performance geopolymers. The synthesis factors studied are kaolin (K), metakaolin (MK), and fly ash (FA) as aluminosilicate sources. Different mixture proportions were prepared by adopting an augmented simplex-centroid design. Twelve geopolymer pastes were synthesized by activating the mixtures with an alkaline solution of sodium hydroxide and sodium silicate. After 28 days of solidification under ambient conditions, the compressive strength (MPa) and bulk density (g/cm3) of the geopolymer samples were measured. The structural and microstructural properties of the raw and elaborated materials were analyzed using a range of physicochemical techniques, including X-ray Fluorescence (XRF) spectroscopy, X-ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) spectroscopy, Scanning Electron Microscopy combined with Energy Dispersive Spectroscopy (SEM/EDS), and Transmission Electron Microscopy (TEM). The results showed that the compressive strength and bulk density of the geopolymeric pastes ranged from 7.59 to 19.48 MPa and from 1.52 to 1.82 g/cm3, respectively. According to the validated special cubic models and optimization results, the optimum mixture of FA (75 %) and MK (25 %) was found to develop the optimized geopolymer (GPO) paste with the best performance. The optimized geopolymer matrix had a non-porous structure mainly formed by a dense amorphous sodium aluminosilicate gel, which gave excellent mechanical properties (1.82 g/cm3 and 20.80 MPa). The practical application of the results of this study includes the development of an efficient geopolymer paste with excellent mechanical properties that can be employed in construction and civil engineering applications. The significance of this work lies in its potential to reduce environmental impacts and minimize waste disposal by utilizing abundant natural resources and industrial waste in the formulation of practical and efficient products. Overall, this study provides valuable insights into the development of an improved geopolymer paste through the application of a mixture design methodology, which can have a significant impact on the construction and civil engineering industries.