The effects of salt-loss soda residue and oxalate acid on property and structure of fly ash-based geopolymer

• Published in: Construction & building materials 2023-02, Vol.366, p.130214, Article 130214
• Main Authors: Wang, Haoyu, Zhao, Xianhui, Gao, Han, Yuan, Tiebiao, Zhang, Xijin
• Format: Article
• Language: English
• Subjects: Fly ash; Geopolymer; Mechanical strength; Microstructure; Stability
• ISSN: 0950-0618
• DOI: 10.1016/j.conbuildmat.2022.130214

•A low-calcium fly ash (FA)-based geopolymer is prepared by incorporating salt-loss soda residue (SLSR).•Oxalic acid (OA) was used to pretreat SLSR in fly ash-based geopolymer.•The addition of 20% SLSR improves the 90-d flexural and compressive strengths of samples incorporating OA.•The product gels of alkali-activated paste with SLSR, FA, and OA is mainly composed of CSH and (N, C)-ASH. When industrial solid waste soda residue (SR) is subjected to water erosion, certain soluble salts are lost, which influences the reuse of SR in construction materials. This research studies the possibility of using salt-loss soda residue (SLSR) in fly ash (FA)-based geopolymers activated by sodium hydroxide solution (NS). To achieve a wider application, the SR and SLSR were proposed with an oxalic acid (OA) treatment. Fluidity, electrical conductivity, and densitywere assessed to examine the effects of SLSR and OA on fresh and hardened characteristics. In addition, flexural and compressive strengths, as well as drying shrinkages, were tested to investigate the role of SLSR and OA. Furthermore, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and Fourier Transform Infrared Spectroscopy (FTIR) tests were carried out to evaluate the mineralogy, morphology, and products. Thermal stability was further evaluated using Thermo-Gravimetric and Differential Thermal Synchronous Thermal Analysis Spectrometer (TG-DSC) tests. The results show that SLSR and OA can be used to synthesize FA-based geopolymers at room temperature. When proper OA is introduced, SLSR contributes to higher compressive strength, improved drying shrinkage resistance, and increased thermal stability of FA-based geopolymers than SR due to the loss of soluble salts and the formation of calcium oxalate in the matrix. Furthermore, it is revealed that the structure of hardened paste (V4) prepared with SLSR, FA, and OA is composed of chemical CSH and (N, C)-ASH gels together with physically filled particles (such as calcite, calcium oxalate, sodium oxalate, and unreacted glass microsphere). The research findings give guidance for promoting the preparation of SLSR products and the utilization of organic calcium sources in geopolymer.