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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. References *International Atomic Energy Agency (IAEA), Radiation Protection and Management of NORM Residues in the Phosphate Industry, Safety Reports Series No. 78, IAEA, Vienna, 2013. *Hilton, J. 2007. Towards a management and regulatory strategy for phosphoric acid and phosphogypsum as co-products, in: W.C. Gerken (Ed.), Proc. Fifth Int. Symp. Nat. Occur. Radioact. Mater. (NORM V), IAEA, Seville, Spain, pp 281–295. *Tayibi, H., Choura, M., López, F.A., Alguacil, F.J., López-Delgado, A., 2009. Environmental impact and management of phosphogypsum. J. Environ. Manag. 90, pp 2377–2386. * Van Deventer J.S.J., Provis, J.L., Duxson, P., 2012. Minerals Engineering 29, pp 89–104. * Wang, L., Yu, K., Li, J., Tsang, D.C.W., Poon, C.S., Yoo, J., Baek, K., Ding, S., Hou, D., Dai, J., 2018. Low-carbon and low-alkalinity stabilization/solidification of high-Pb contaminated soil. Chem. Eng. J. 351, pp 418–427.

Alkali-activated materials (AAMs) are currently under research for providing opportunities for re-use of industrial residue streams in the construction sector. The valorisation of industrial residues into sustainable building materials has become an important feature for reducing the need of disposal and avoid the scarcity of primary resources. The use of ordinary Portland cement (OPC) as primary construction material has also been questioned extensively due to its environmental impact, i.e. CO2 emission during production. The aforementioned issues have prompted various researches in an attempt to develop AAMs as sustainable alternative binders. The most popular industrial solid residues used in making AAMs are: ground granulated blast furnace slag (GGBFS), steel slag, phosphorus slag, fly ash, bauxite residue, among others. In some cases (phospho-)gypsum and flue gas desulfurisation gypsum are also used. This wide range of solid residues provides a wealth of potential precursors for AAMs. GGBFS, produced during iron production, is an excellent raw material for making high-strength building materials for civil engineering applications. The slag’s main components are CaO, SiO2, and Al2O3, available in a glassy state. GGBFS is used in a wide range of applications, in which some of these contain a significant amount of sulphate (e.g. flue gas desulfurisation gypsum). For this reason, GGBFS-based AAMs serve here as a test matrix in order to allow the incorporation of sulphate-rich residues. Alkali silicate based activator is the most used activating solution for GGBFS based AAMs. The modulus (MS) of the activator solution (molar ratio SiO2/Na2O) is an important parameter for mechanical strength and is usually optimal between a ratio of 1 to 2. It is the aim of this paper to investigate the influence of curing regimes on the mechanical properties of AAMs, focussing solely on pastes based on GGBFS and calcium sulphate dihydrate. FWO

The objective of this work is to identify the crucial variables for rendering the production of alkali-activated materials (AAMs) based on Flemish phosphogypsum and blast furnace slag mineral wastes profitable. Therefore, a preliminary techno-economic analysis has been carried out for a process design especially developed for their valorisation. After developing a process diagram for the production of AAMs, the net present value of the cash flows generated has been determined. The minimum selling price of the produced building materials has been determined, taking into account uncertainties by performing Monte Carlo sensitivity analysis. Finally, this preliminary techno-economic analysis is used to identify the key variables for the profitability of the production of AAMs from phosphogypsum and blast furnace slag. The environmental aspects of the process are expressed in global warming potential (GWP). The evaluation of GWP is based on a life cycle assessment.

Alkali-activated materials (AAMs) based on ground granulated blast furnace slag (GGBFS) and phosphogypsum (PG) were investigated in order to quantify leaching of naturally occurring radionuclides (NOR) and inorganic non-radiological elements according to an up-flow percolation column test as described in CEN/TS 16637-3. Gamma spectroscopy and neutron activation analysis (NAA) were applied for radiological characterization, inductively coupled plasma optical emission spectrometry (ICP-OES) and ion-chromatography (IC) for chemical characterization. Upon leaching, 238U, 226Ra, 210Pb, and 228Ra were retained very well. Both for 232Th and 40K, a decrease in activity concentration was observed due to leaching and their release was influenced by the use of different alkali activators, which was also the case for the leaching of non-radiological elements. Only a small amount of Al (0.5-0.8%), Ca (0.1-0.2%) and Si (0.1-0.3%) was mobilized, while highest release was observed for K (56-94%), Na (49-88%) and S (71-87%). At first glance, drinking water is not endangered by leaching of NOR following the requirements of the European Drinking Water Directive. From the results for porosity, obtained with mercury intrusion porosimetry (MIP), it was concluded that both the porosity and formation of multiple leachable and non-leachable complexes are determining factors for the release of elements from AAMs.

The current presentation deals with scenarios for the development of novel and more effective immobilisation matrices based on NORM (Naturally occurring Radioactive Materials) containing construction materials for the safe encapsulation of various types of wastes. NORM containing by-products such as slags, fly-ashes from several industrial sectors and phosphogypsum form the basis of the newly constructed construction materials that are considered for encapsulation. In particular, results regarding the immobilization potential of construction materials, such as alkali activated materials, for naturally occurring and artificial radionuclides will be discussed. The immobilisation properties and the investigation of the leaching behaviour of radionuclides from the construction materials are discussed considering the Euratom water directive. The authors would like to acknowledge networking support by the COST Action TU1301 (www.norm4building.org).