Abstract
The increased awareness of global warming, CO2 emissions, and natural resource depletion has urged the construction industry to research greener eco-friendly alternative binders to Ordinary Portland Cement (OPC) in line with the circular economy and sustainable development goals (SDGs) 9, 11, 12, and 13. The construction industry is a major energy consumer, accounting for 42% of global energy use and 35% of CO2 emissions. OPC is particularly energy-intensive, consuming 7% of global energy and contributing 8% of CO2 emissions. Geopolymer has evolved as a greener sustainable third-generation binder alternative to OPC as it has comparable strength, low CO2 emission, and reduced embodied energy. With increasing population and urbanization, the generation of industrial waste, such as fly ash (FA) and phosphogypsum (PG), has surged, necessitating improved waste disposal and recycling, which currently stands at below 50%.
Limited research on the physico-chemo-mechanical performance and mix design modelling of ambient-cured FA-PG blended geopolymer paste and mortar threatens broader adoption by the construction industry fuelling environmental pollution and reliance on OPC. FA-PG geopolymer systems contain complex gel structures that influence setting time, workability, strength, mix design, and durability. There is a need to study the role of PG addition on the performance mechanisms of ambient-cured FA geopolymer paste and mortar to broaden mix design options and promote durable standardized preparation, thus lowering environmental pollution and reliance on OPC. Therefore, this research aims to evaluate the physico-chemo-mechanical performance of ambient-cured fly ash-phosphogypsum blended geopolymer paste (FPGP) and mortar (FPGM) and establish a performance-based mix design model for structural applications. The study addressed four aligned objectives. First, the study characterized the FA and PG waste materials to assess their behavior and reactivity as potential geopolymer precursors. Second, the study investigated the effects of adding PG on the physico-chemo-mechanical properties of FA-based geopolymers. Third, the study developed strength-based mix design models for FPGP and FPGM based on the experimental datasets collected in the preceding section. Finally, the study investigated the durability of the developed FPGM when exposed to a water and sulfate environment at different durations.
Experimental testing and analysis were done based on ASTM standards, X-ray fluorescence (XRF), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The geopolymer was formed by reacting binary powder precursor (FA+PG) and alkaline activator (NaOH+Na2SiO3) and cured at ambient temperature for 3, 7, and 28 days. The FA was substituted with varying weight proportions of PG (i.e., 10%, 20%, 30%, 40%, 50%). The optimum mixture conditions for developing FPGP and FPGM were 30 wt% PG, alkaline liquid/precursor of 0.4, 10M NaOH, Na2SiO3/NaOH of 1.5, and binder/aggregate of 1.0. A specific output value of compressive strength was determined for specific mix design input parameters. The multivariate regression approach was used to model the relationship between compressive strength and alkaline liquid/precursor. A practical performance-based mix design model was proposed and validated using a trial experiment, statistical metrics, and existing literature data. The Statistical Package for Social Sciences (SPSS) v29 was used to show model significance and the analysis of variance (ANOVA). A correlation heatmap developed in Python was used to explore the key features influencing the properties of FPGP and FPGM. Thereafter, water absorption, porosity, and sulfate resistance tests were conducted on the FPGM prepared at 0 wt%, 30 wt%, and 40 wt% PG replacement of FA content in the mixture. The mix proportions were determined experimentally at 10M NaOH, alkaline liquid/precursor of 0.4, Na2SiO3/NaOH of 1.5, and binder/aggregate of 1.0. The samples were
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immersed in potable water and magnesium sulfate solution. The changes in weight, length, and compressive strength were monitored.
The material characterization results showed that FA is rich in amorphous silica and alumina, and PG is rich in calcium oxide and sulfate implying that both waste materials have the reactive compounds necessary for developing geopolymeric binders. Due to the favourable material characteristics, the FA and PG were suitable precursors for developing FPGP and FPGM. The workability of the developed FPGP ranged from 112 mm – 185 mm while that of FPGM ranged from 100 mm – 145 mm. The FPGP and FPGM had considerably lower initial and final setting times of (18 – 37 min, 81 – 155 min) and (14 – 29 min, 67 – 142 min), respectively. The compressive strength of FPGP ranged from 7.3 MPa to 27.24 MPa while that of FPGM ranged from 9.5 MPa to 43.27 MPa. The flexural strength of FPGP ranged from 1.46 MPa to 3.98 MPa while that of FPGM ranged from 1.61 MPa to 5.03 MPa. There was a strong connection between compressive and flexural strength giving R2 values > 0.8. The mineral phases of FPGP and FPGM were quartz (SiO2), mullite (3Al2O3.2SiO2), bassanite (CaSO4.0.5H2O), ettringite (Ca6Al2(SO4)3(OH)12·26H2O), C-A-S-H and N-A-S-H. The higher strength and dense morphology of the FPGP30 and FPGM30 were attributed to the formation of C-A-S-H, N-A-S-H, and AFt. Based on the adjusted R2 values and ANOVA, the proposed mix design models were statistically significant since the p-value (0.000) was less than the 0.05 significance level and the value of F (197.953) was greater than the critical value of 1. At high curing duration, the NaOH concentration and alkaline liquid/precursor ratio influenced the dissolution of Ca2+ in PG and Si4+ and Al3+ in FA forming a C-(N)-A-S-H matrix crucial to the strength development. The durability results showed that FPGM with 30 wt% PG had lower water absorption, porosity, and sulfate attack than FPGM with 0 wt% PG attributed to hydrated gels forming a dense microstructure, improving strength.
In summary, the research showed that adding PG to FA-based geopolymers improves their fresh and hardened engineering properties, microstructure, and durability. Advanced knowledge of the performance mechanisms, mix design models, microstructure, and durability of FA-PG geopolymer systems enhances our understanding of the material behaviour broadening the mix design options, promoting standardized preparation, and offering a durable alternative binder to OPC. The statistical metrics implied a good performance accuracy and reliability of the proposed model equations making them applicable in construction materials design. The FPGM performed comparably to OPC mortar and other cementitious materials reported in the literature promoting its usage in sulfate-rich environments. Using FA-PG geopolymers for bricks, mortars, concrete repair, and ground improvement can foster medium-to-large-scale businesses, improving global socioeconomic welfare. The research enhances industry competitiveness through sustainable innovative practices, data-driven material design, skills development, and standardization. In the long term, the development of FA-PG geopolymer paste and mortar offers a sustainable construction material that reduces reliance on OPC, promotes the circular economy, and supports SDGs 9, 11, 12, and 13. For optimal performance, it is recommended to use the strength-based mix design models within the specified parameters employed in this study.