Use of phosphogypsum in alkali-activated binders: radiological and leaching assessment

Abstract

Introduction In the phosphate industry vast amounts of phosphogypsum (PG) are produced during fertilizer production when the phosphate ore is treated with sulphuric acid. The PG contains impurities such as P2O5, F-, organic substances, alkali metals and enhanced concentrations of naturally occurring radionuclides (NORs), predominately from the 238U series, restricting its use. Worldwide, 170 million tons of PG are disposed annually and the recycling rate is still very low, i.e. less than 5% while the worldwide production is increasing. (IAEA, 2013; Hilton, 2007; Tayibi et al., 2009; Wang et al., 2018)

Alkali-activated materials (AAMs), produced on the basis of by-products or residues, can replace Ordinary Portland Cement (OPC) reducing the CO2-emissions by up to 80% compared OPC while offering comparable technical performance (Van Deventer, 2012). This work focusses on the incorporation of phosphogypsum in AAMs based on ground granulated blast furnace slag (GGBFS).

Methodology A combination of GGBFS (Belgian steel company) and PG (IAEA reference material 434) was used as precursor for the production of AAMs. Non-volatile inorganic NORs and non-radiological elements leaching was assessed by means of an up-flow percolation test as described in CEN/TS 16637-3. Thermal and epithermal neutron activation analysis (NAA) was used for 238U and 232Th while other NORs were analysed via gamma spectrometry. ICP-OES and ion chromatography were applied for quantitative analysis of non-radiological elements. A SARAD Radon Scout PMT radon monitor was used to perform a massic radon exhalation study.

Results & conclusion The matrix properties from the AAM matrix can be adapted to reduce radon release and to develop an optimal immobilisation matrix for the incorporated NORs and and impurities form PG (e.g. F-). For hardened AAMS, the decline in radon exhalation was assigned to the mechanism of physical encapsulation, which is enhanced by lowering both the pore volume and the specific surface area. From the leaching study it was found that both the porosity and formation of multiple leachable and nonleachable complexes are determining factors for the release of elements from the AAMs.

Acknowledgement This work was supported by the Fund for Scientific Research Flanders (FWO), and the authors would like to acknowledge the networking support of the COST Action TU1301, www.norm4building.org.

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