article · Construction and Building Materials
Geopolymer concrete made from fly ash offers an alternative to conventional materials, particularly because traditional Portland cement cannot be recycled once hydrated. This research explores creating a recycled binder from hardened fly ash-based geopolymer concrete. Old parent concrete was crushed and sieved to detach the paste from aggregates, then pulverised in a ball mill to generate recycled geopolymer cement. This powder was blended with virgin fly ash at proportions between zero and one hundred per cent, mixed with an alkaline activator of sodium hydroxide and sodium silicate, and cured at either 23 or 80 degrees Celsius. Introducing the recycled material generally reduced workability, setting time, and tensile and compressive strengths, while increasing drying shrinkage. However, mixtures containing up to 60 per cent recycled geopolymer cement still preserved performance levels adequate for structural applications.
Portland cement contributes significantly to industrial waste and emissions because it cannot be chemically recycled into new cement after hardening. Demonstrating that geopolymer concrete can be reclaimed and pulverised into a reusable binder supports circular construction practices, potentially reducing the need for virgin raw materials and lowering the volume of demolition waste destined for landfills.
This applied laboratory study demonstrates a method for concrete recyclers and building material manufacturers to convert demolition waste from geopolymer concrete into structural-grade binder. The process relies on standard mechanical processing equipment like crushers and ball mills. Because the findings are currently limited to laboratory-scale testing of fresh and hardened properties, further development and pilot-scale trials would be needed before commercial use in construction products.
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This paper presents an investigation into the potential of recycling fly ash - based geopolymer concrete, to produce a recycled binder for circular re – use as starting material. The aforementioned would be a significant advantage over Portland cement which is non – recyclable, owing to its irreversible hydration. The experiment conducted involved crushing and sieving old parent fly ash – based geopolymer concrete, to separate out the paste from aggregate. The paste was then pulverized using a ball – mill to produce the recycled aluminosilicate starting material, herein referred to as recycled geopolymer cement (RGPC). Fly ash was blended with 0 to 100% RGPC, then used to prepare fresh geopolymer paste and mortar mixtures activated using a binary alkali activator consisting of sodium hydroxide and sodium silicate. Samples were cast then cured at 23 or 80 °C. The various tests done were workability, setting time, compressive strength, splitting tensile strength, drying shrinkage, water absorption, and volume of permeable pores. Analytical studies were done using X – ray diffraction, scanning electron microscopy and Fourier – transform infrared spectroscopy. Results showed that incorporation of RGPC into geopolymer mixtures generally led to reduction of workability and setting time, reduction of compressive and tensile strength, along with increase in drying shrinkage. It was, however, found that up to 60% RGPC can be incorporated into geopolymer mixtures while maintaining adequate performance suitable for structural applications. Unlike Portland cement, hardened geopolymer concrete is evidently recyclable to produce a circularly re – usable binder.
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DOI: 10.1016/j.conbuildmat.2024.135098
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