## 1 Introduction
Recently, an efficient test method has been developed in the framework of RILEM TC 267-TRM: the R3 test [15,16]. The SCM is mixed in a solution of Ca(OH)\({}_{2}\) with small amounts of KOH, K\({}_{2}\)SO\({}_{4}\) and CaCO\({}_{3}\) and the reaction is monitored using isothermal calorimetry or thermogravimetric analysis. The heat release or bound water content is used to assess the reactivity. A good correlation with 28 days strengths has been observed for conventional SCMs. SCMs rich in silica may be used in the production of autoclaved aerated concrete (AAC) blocks. Autoclaved aerated concrete blocks are porous blocks, which are used as structural insulation material. Commercial products are characterized by a density of 300-800 kg/m\({}^{3}\) and compressive strength of 2-12 MPa [17, 18, 19]. The density and strength are inversely related to each other. The production process involves mixing cement and/or lime, quartz powder, anhydrite and aluminum powder with water [17,19]. The main components of autoclaved aerated concrete are lime, cement and quartz. The amount of quartz powder in the solid part of the mixture ranges from 50 to 75 wt% [18, 19, 20]. Lime and cement can both represent 0-35 wt%. Anhydrite is usually added at 5 wt%, although research exists where blocks are produced without or with partial replacement of the anhydrite [19,20]. The amount of aluminum powder varies between 0.1 and 0.5 wt%, depending on the desired porosity. The water to powder ratio is high: usually water/solid mass ratios of \(\geq\)1 are used (although also in this aspect not all literature follows the values for commercial products [19]). These high ratios exceed the amount of liquid needed to obtain a workable mixture, but are needed to enhance the porosity of the autoclaved block. Aluminosilicate residues and byproducts have been investigated to replace the quartz in the mixture by other authors. The addition of aluminates in the system accelerates the reaction kinetics and changes the morphology of the obtained tobermorite phase, while also forming additional hydration products such as katoite [21]. The use of blast furnace slag, coal fly ash and copper tailings have been explored to this end [18,22], suggesting that any aluminosilicate source may be usable for this application. During precuring the pH rises due to the dissolution of cement and/or lime. This increase in pH accelerates the oxidation of the aluminum powder and reduction of water, resulting in the formation of hydrogen gas bubbles. The mixture expands and a foam is produced. This precured foam is autoclaved to further develop the binder and strength of the block. During curing in the autoclave, usually at 180 \({}^{\circ}\)C, the fine quartz reacts with Ca(OH)\({}_{2}\) and water and tobermorite is formed, which is the main binding phase that provides strength to the resulting blocks [19; 23]. The anhydrite in the starting mixture is needed to promote tobermorite formation [20]. There is no consensus in literature on the mechanism of tobermorite promotion by the addition of anhydrite, but most sources indicate that the influence of sulfates on the morphology and microstructure of the C-S-H intermediate is important [24; 25]. The presence of sulfates significantly adapts the pore structure of the binder [26] as it adsorbs to the surface of the C-S-H [27]. On the other hand, the presence of sulfates complicates the recycling of autoclaved aerated concrete blocks, because of environmental leaching of sulfates [28]. This paper focuses on applying the aluminosilicate refractory part of spent pot lining in construction materials. The applications as SCM in composite cement and the use as silica source in AAC blocks are investigated. The reactivity of the aluminosilicate and its influence on cement hydration is determined. The compressive strength evolution of mortar bars is investigated, including the interaction with limestone filler.