## 1 Introduction  
Spent pot lining (SPL) is a hazardous waste originating from the production of aluminum [1, 2]. In the Hall-Heroult process, aluminum is reduced electrochemically from alumina using graphite electrodes and a cryolite electrolyte. This process happens in a pot, which has to be replaced every 5 years [1, 3]. This results in 2 tonnes of spent pot lining for each 100 tonnes of Al produced adding up to a total of >80,000 tonnes/year in Europe. The lining of the pot is in contact with the cryolite electrolyte. The inner part of the lining is made of graphite and is surrounded by chamotte (aluminosilicate) refractory bricks, the outer part of the lining. During operation the pot lining is slowly impregnated with cryolite and other reaction products forming during aluminum electrolysis. At the end of the lifetime of the pot, the graphite and refractory lining is therefore contaminated with up to 40 wt% of fluorides, around 1 wt% of cyanides and reactive metals and metal oxides that can form explosive gases and toxic leachates in contact with water [1, 2, 4, 5, 6]. The spent pot lining is therefore classified as a hazardous waste (waste classification code 16 11 01*) [7]. Recent developments show that a detoxification process involving a treatment with NaOH can decrease the concentration of contaminants drastically [8, 9]. The spent graphite lining can be used in the production of new graphite or in the pyromental-lurgical industry. For the application of the chamotte aluminosilicate refractories, several options exist, however, because of the residual sodium level after the detoxification process, direct application in new refractories is not possible. Efforts have been made to introduce these spent pot lining aluminosilicates in cement production, where it can represent up to 12 wt% of the raw meal [10]. An alternative--not known in literature--is the use of milled material in composite cement as a supplementary cementitious material (SCM). The aluminosilicate composition is desirable for use as SCM. However, further studies are needed to assess whether the refractory part of spent pot lining can contribute to the cement hydration reactions without negative effects on the performance, durability or environmental compatibility. The embodied greenhouse gas emissions of cement can be lowered by replacing part of the Portland cement clinker with SCMs. The production of Portland cement clinker is associated with 0.8 tonne CO\({}_{2}\)/tonne clinker [11]. The embodied CO\({}_{2}\) emissions of a waste, like spent pot lining, are conventionally seen as zero through the economic allocation set as best practice for life-cycle assessments in ISO 14044. Each wt% of clinker replacement with a waste material thus delivers a reduction of 0.008 ton CO\({}_{2}\)/ton cement. A replacement level of 30 wt% roughly decreases the CO\({}_{2}\) emissions by 30% to 0.56 tonne/tonne cement. On top of this environmental benefit, the durability is often positively impacted by the use of SCMs [12]. In particular aluminosilicate SCMs are known to react with Ca(OH)\({}_{2}\) and provide additional hydration products. These additional hydrates fill up space and contribute to the strength development of a mortar or concrete. This space filling is also associated with pore refinement and therefore leads to a lower permeability of the microstructure. A lower permeability is inherently associated with a higher durability in diffusion controlled degradation mechanisms, such as chloride ingress [12,13]. Additionally, the higher Al content of the pore solution when using these aluminosilicate SCMs hinders the dissolution of other silicates, thereby mitigating the alkali-silica reaction. Recent developments show that a synergetic interaction between limestone and aluminosilicate SCMs can lead to an additional decrease in the clinker content of cements [13]. An important characteristic of an SCM to assess whether it can have a beneficial effect on cement hydration is its reactivity [14].