Optimization and mechanism analysis of multi-solid wastes-based geopolymer using response surface methodology

Abstract

The escalating production of industrial solid waste, combined with the dwindling availability of natural resources, has intensified the focus on waste recycling. However, the heterogeneity and complexity of waste pose significant challenges to determining process parameters. In this study, burnt coal cinder (BCC), granite powder (GP), and high-calcium fly ash (Class-C FA) were used as raw materials, and the response surface methodology (RSM) and single-factor experiments were applied to optimize the process parameters for geopolymer preparation. The optimized precursor powder composition was determined to be a mass ratio of 1.6:0.9:7.3 for BCC, GP, and Class-C FA. The NaOH-precursor powder ratio and liquid–solid ratio were adjusted to 0.084 and 0.222, respectively. The curing condition was set at 80°C for 24 h. The resulting 28 d-aged multi-solid wastes-based geopolymer exhibited a high compressive strength of 61.34 MPa. The microstructure, mineral phase, and atomic bonding of geopolymers were investigated using X-ray diffraction (XRD), thermal analysis (TA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Findings indicate that the compressive strength of geopolymer is most significantly influenced by the Class-C FA, followed by BCC. Furthermore, a minor addition of GP can optimize the structural density of the geopolymer. The Ca present in the Class-C FA participates in the geopolymerization, forming a hybrid N–(C)–A–S–H gel. RSM optimization facilitates the synergistic utilization of multi-solid wastes, ensuring an even distribution of gel and filler. This research establishes a theoretical framework for optimizing the preparation parameters of multi-solid wastes-based geopolymer and its subsequent applications; it holds significant scientific implications for the circular economy, resource transformation, and environmental conservation.

1. Introduction

Facilitating technological advancements and material innovations geared toward efficient recycling and environmentally responsible disposal of solid waste is pivotal in tackling environmental pollution and alleviating resource scarcity [1–4]. As solid waste accumulates rapidly, research has increasingly focused on using it to produce building materials, with geopolymer standing out as a sustainable and eco-friendly option [5–7]. Geopolymer is typically prepared from high-activity aluminosilicate precursors such as metakaolin and low-calcium fly ash (Class-F FA) using alkali activators [8–9]. However, given the limited availability of metakaolin and the significant added value of high-activity solid waste, interest is increasing in exploring cost-effective solid waste substitutes in the geopolymer field [10–13].

Current research on single solid waste utilization often follows a linear approach, focusing on individual materials without considering their potential synergistic effects. This limitation leads to various adverse effects, such as low utilization efficiency of solid wastes, limited technological innovation, and poor economic benefits [14]. In contrast, multi-component co-utilization offers a promising solution by integrating different solid wastes to enhance material properties and reduce dependence on single sources [15–16]. Sarıdemir and Çelikten [17] studied the synergistic effect of slag and Class-F FA. The concrete with the two solid wastes has a superior frame and better acid/salt resistance. Utilizing Bayer red mud, steel slag, carbide slag, aluminum ash, and flue gas desulfurization gypsum as raw materials, Li et al. [18] developed high-performance red mud-based cementitious materials through the synergistic multi-solid waste system under high temperature treatment conditions.