The Long-Term Mechanical and Durability Properties of Indonesian Fly Ash-Based Geopolymer Concrete

The sustainability of the construction industry has to be considered an essential focus in today's society. The building industry sector is responsible for almost a quarter of global mineral extraction. It consumes a high quantity of raw materials and, through the use of environmentally unfriendly materials. Ordinary Portland Cement (OPC) concrete use triggers environmental problems globally due to carbon emissions during the manufacture of OPC, producing about 3.0 billion tons of CO2 per year. In the last few decades, fly ash has been utilized as a supplementary cementitious material in concrete production to reduce dependence on OPC. Many researchers have studied the potential of fly ash as the main binder in geopolymer materials. It has been reported that fly ash-based geopolymer concrete (FAGC) has superior mechanical properties and durability compared to conventional PC concrete. However, only a limited number of studies have addressed the long-term behaviour of FAGC. Durability is fundamental to concrete due to deterioration and damage relating to the resistance in harsh environments, such as from acid and sulphate attacks. Conflicting reports in previous studies also persist in the case of sulphate and peat acid exposure. In the Indonesian environmental context, these types of chemical attacks are of primary focus due to Indonesia having a large area of peat and acid sulphate soils. This study is important for the implications of FAGC in the native Indonesian environment and will be valuable information for future construction and concrete industry applications. Besides, FAGC has significant potential in Indonesia due to the considerable availability of fly ash (the utilization rate: 45%). Coal consumption in Indonesia is relatively high, with around 111.9 million tonnes in 2021. Fly ash in Indonesia is generally divided into types C and F; This study focuses on the examination of the long-term exposure of high Fe type F geopolymer concrete in sulphate, acid, and peat environments This work provides comprehensive data regarding the deterioration of FAGC when exposed to 5% sodium sulphate, 5% magnesium sulphate, 1% and 3% sulphuric acid, and a simulated peat solution. The durability tests include compressive strength, change in mass and length, and pH profile; and employ various analytical and chemical methods, including XRD, SEM/EDS, TGA, FTIR, and Micro-CT on specimens exposed for up to 18 months in acid, sulphate and peat environments. Portland cement concrete is also investigated in a similar environment as control concrete. In addition, the carbonation and chloride resistance (RCPT and salt ponding) of FAGC are also assessed. Mechanical properties, such as compressive strength, tensile strength, flexural strength, and modulus of elasticity, coupled with durability properties, including density, water absorption and volume of permeable voids, air and water permeability, ultra pulse velocity, and resistivity of FAGC, are also conducted. Furthermore, this work studies the microstructural development of geopolymer mortar and paste. This study also undertakes a life cycle and cost-benefit analysis of FAGC compared to PC concrete in the Indonesian context. The study on geopolymer mortar and paste confirmed the potential of Indonesian fly ash based on strength properties. An average strength of 44.9 MPa and 76.0 MPa are noted at 28 days for geopolymer mortar and paste, respectively. Alkali modulus (AM) 1.375 is found to be the optimum concentration. Both materials observe the incorporation of Al in the geopolymer matrix increases the compressive strength, while increases Fe in the geopolymer matrix result in a decrease in compressive strength. A Na2O dosage of 7.5% generates a relatively slow increase in compressive strength but provides a consistent increase in strength with time (up to 365 days) in geopolymer mortar. The study also reports the relation between AM and Na2O dosage with respect to pore size distribution and their contribution to strength development. The FAGC highlights crack propagation, which negatively impacts the long-term performance of FAGC. This results in decreased compressive strength of FAGC when subjected to sulphate and acid solution at 18 months. The crack development is triggered by drying shrinkage, decreases compressive strength and reduces the strength of bonding at the ITZ, resulting in low tensile strength in long-term FAGC specimens. Chloride resistance demonstrated a low RCPT of 994.1± 32.8 coulombs, a chloride diffusion value of 2.90x10-11 m2/s, and a surface concentration of 0.104 %. Accelerated carbonation gave an accelerated coefficient of FAGC of 4.26 mm/√month. Furthermore, in a sulphate environment, FAGC concrete performed better when exposed to sodium sulphate, with higher strength observed at 12 months compared to specimens immersed in water, while in magnesium sulphate, the compressive strength is reduced. There are no expansion products found in the sulphate FAGC. Meanwhile, 1% and 3% sulfuric acid resulted in a decline in the compressive strength of the FAGC, coupled with a slight mass reduction. Peat solution had minimal impact on the performance of the FAGC. Compared to PC concrete, this study demonstrates that the FAGC generally had better resistance in sulphuric acid, sulphate and peat solutions. The cradle-to-gate environmental performance and economic benefits of FAGC compared with comparable strength PC concrete in the Indonesia region concludes the environmental impacts of FAGC concrete were relatively lower than PC in only two categories, GWP and MAETP. Sodium silicate and transportation were observed as the two dominant factors that influenced the LCA analysis. This study also emphasized the allocation method's role and mode of transport type in the LCA. Furthermore, the cost analysis noted FAGC could be potentially nearly 30% lower than PC-based concrete, with the cost of the alkaline activator being the most significant contributor. Finally, this study provides recommendations for future studies, including a nanoscale microstructure analysis, due to a strong indication that nanoparticles affect the properties of geopolymer, particularly in porosity due to nano zeolite formation. Future studies should focus on the investigation of shrinkage behaviour, and its relation to crack development in the short and long term due to the potential long-term deterioration of FAGC. In addition, field investigation on peat acid attack is suggested to confirm the current finding before construction application.