## 2 Materials and Methods  
The WBA sample was provided by the VECSA company, which is responsible for valorizing the IBA collected from the WtE plant located in Tarragona (Catalonia, Spain), with a capacity of 140 kt-y\({}^{-1}\). The feed stream treated in this incineration plant is mainly composed of household rubbish, with a small input from commercial sources. Around 32 kt-y\({}^{-1}\) of fresh IBA is obtained in this WtE plant. The combustion temperature is 950 \({}^{\circ}\)C [42]. After combustion, fresh IBA is further processed in a conditioning plant to recover some valuable materials (ferrous and non-ferrous metals) and to obtain a homogenized granular material. Finally, the resultant IBA is subjected to natural weathering outdoors for at least 3 months to stabilize heavy metal(loid)s and to obtain WBA, which is reused as secondary aggregate. Because of the heterogeneity of WBA, 100 kg were collected from stockpiles and then homogenized and stored in 30 L plastic containers. The alkali activator used consisted of a commercial Na\({}_{2}\)SiO\({}_{3}\) solution (Scharlab S.L. ; 26.44 wt.% of SiO\({}_{2}\) and 8.21 wt.% of Na\({}_{2}\)O; \(\rho\) = 1.37 g-cm\({}^{-3}\)) and NaOH solutions prepared by using NaOH pearls (Labbox Labware S.L. ; purity \(>\) 98%) dissolved in distilled water: 2 (\(\rho\) = 1.08 g-cm\({}^{-3}\)), 4 (\(\rho\) = 1.16 g-cm\({}^{-3}\)), 6 (\(\rho\) = 1.20 g-cm\({}^{-3}\)), and 8M (\(\rho\) = 1.24 g-cm\({}^{-3}\)). The preparation to obtain the WBA powder started by quartering the whole 100 kg sample to acquire a representative sub-sample of 10 kg. The quartered sample was dried in a stove at 105 \({}^{\circ}\)C for 24 h. Then, the dried sample was sieved (by using standard sieves: 32, 16, 8, 4, 2, 1, 0.5, 0.125, 0.063 mm) to determine the particle size distribution (PSD), as shown in Figure 1. Subsequently, the WBA was crushed and milled until a powder below 80 \(\upmu\)m in particle size was obtained. Finally, a metal magnet (Nd; 0.485 T) was passed over the ground sample to remove magnetic particles. The elemental oxide composition of WBA was assessed by X-ray fluorescence (XRF) analysis with a spectrophotometer Panalytical Philips PW 2400 sequential X-ray equipped with the software UniQuant(r) V5.0. Major oxides are given in Table 1. There was a high content of SiO\({}_{2}\), CaO, Al\({}_{2}\)O\({}_{3}\), and Na\({}_{2}\)O, which are the main compounds used to obtain AACs [43]. The high SiO\({}_{2}\) content was due to the presence of primary and secondary glass in WBA [6; 44]. The CaO and Al\({}_{2}\)O\({}_{3}\) content was because WBA contains cementitious materials based on OPC, as well as synthetic ceramics from small domestic works. It is important to highlight that the percentage shown in Table 1 represents the total content of  
Figure 1: Particle size distribution of weathered bottom ash (WBA). the oxides considering both their amorphous and crystalline phases. The SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) availability (reactive phases) of the WBA sample are reported in previous studies by the authors [37]. Figure 2 shows the WBA mineralogical results obtained by a Bragg-Brentano Siemens D-500 powder diffractometer device with CuK\(\alpha\) radiation. A halo is observed between 20 and 35\({}^{\circ}\) due to the mainly vitreous nature of the WBA sample [6]. The main crystalline phases are rich in Si, Al, and Ca, such as quartz (SiO\({}_{2}\); PDF# 01-083-0539), calcite (CaCO\({}_{3}\); PDF# 01-072-1937), akermanite (Ca\({}_{2}\)MgSi\({}_{2}\)O\({}_{7}\); PDF#01-076-0841), hydrocalumite (Ca\({}_{4}\)Al\({}_{2}\)(OH)12(Cl,CO\({}_{3}\),OH)\({}_{2}\).4H\({}_{2}\)O; PDF# 016-0333), and pseudowollastonite (CaSiO\({}_{3}\); PDF# 01-074-0874). Muscovite (KAl\({}_{2}\)(AlSi\({}_{3}\)O\({}_{10}\))(OH)\({}_{2}\); PDF# 01-075-0948), microcline (KAlSi\({}_{3}\)O\({}_{8}\); PDF# 01-076-0918), kyanite (Al\({}_{2}\)SiO\({}_{5}\); PDF# 01-074-1827), and magnetite (Fe\({}_{3}\)O\({}_{4}\); PDF# 01-077-1545) were also identified as minor crystalline phases.