Characterisation of Nigerian Mining Waste Clays as Sustainable Precursors for Production of Low Carbon Geopolymer Binders

The replacement of Portland limestone cement, with geopolymer as a binder is gaining global attention, due to reduced carbon-footprint, in compliance with United Nations agenda on sustainable development goals. Geopolymer binders (GBs) are usually produced from aluminosilicate precursors. Precursor’s availability at cheap price is essential for sustainability. Therefore, the possibility of utilising mining waste clays (MWCs) as precursors in the production of GBs was investigated. The MWCs were collected from mining sites located in four different states in Nigeria [(Awo, Osun state (OS), Ibeshe, Lagos state (LA), Ijero, Ekiti state (EK) and Owode, Ogun state (OG)] and were characterised for thermal properties, oxide composition, functional groups, morphology/elemental composition, and mineralogical phases using Thermogravimetry/differential thermal analysis (TGA/DTA), X-ray fluorescence, Fourier-transform infra-red (FTIR), Scanning electron microscopy/Energy dispersive spectrometery (SEM/EDS), X-ray diffraction (XRD), spectroscopic techniques. They were calcined at different temperatures and time and their geopolymer binders were produced with different NaOH/Na ₂ SiO ₃ activator ratio. Carbon emissions for GBs production was estimated using combustion energy equation. Thermal dehydroxylation of MWCs into reactive amorphous phase occurred within the range of 420–740ºC. The sum of SiO ₂ and Al ₂ O ₃ composition were 70.5–82.2%, suggesting aluminosilicate material, confirmed by FTIR peaks at 3691, 1114, 1032 and 3620cm ^− 1 indicating O-H, Si-O, Si-O-Si and Al-OH bonds, respectively characteristic of kaolinite aluminosilicate materials. The SEM’s irregular non crystalline microstructure and XRD’s hallow peaks or humps at 2Ɵ (20º − 30º) of thermally treated clays suggested amorphisation. Optimised production conditions for GBs were 600–740ºC calcining temperature, 6hrs calcining time, and 1:1.5 NaOH/Na ₂ SiO ₃ ratio. The compressive strength values of the GBs ranged from 26.1 to 33.7 MPa which were above ASTM standards, with Carbon emission reduction of 92.9% compared to cement. The Nigerian MWCs could be applied for sustainable geopolymer binders’ production.