Characterisation and assessment of recycled concrete aggregates used in a permeable reactive barrier for the treatment of acidic groundwater
The acidification of coastal waterways because of acid sulphate soil is an environmental, economic and social problem within Australia. A pilot-scale permeable reactive barrier (PRB), using recycled concrete aggregates as reactive material, was installed in low-lying acid sulphate soil terrain for acidic groundwater remediation. Column experiments were previously undertaken with synthetic groundwater to ascertain the dominant reactions occurring within the PRB. Results showed that armouring of the reactive material surface by precipitated Al- and Fe-bearing minerals significantly reduced its acid neutralisation capacity (ANC). The purpose of this current study was to validate this decline in ANC through characterisation of the virgin and armoured concrete aggregates, and precipitates that formed on the concrete. Samples of concrete aggregates and precipitates were analysed using X-ray diffraction (XRD), X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS) and X-ray micro-computed tomography (μCT). The conclusions drawn from these analyses are that Al-bearing (gibbsite 14.3%, boehmite 10.9%) and Fe-bearing (goethite 38.2%) mineral precipitates of diverse morphology form as a thin layer coating the aggregate surfaces. A reduction of CaO in the armoured concrete aggregates by 47% correlates with the reduction in ANC of the virgin concrete by 50% due to armouring.