# Alkali-Activated Materials with Pre-Treated Municipal Solid Waste Incinerator Bottom Ash
## 3 Results and Discussion
### Hardened-State Properties  
#### 3.5.1 Compressive Strength  
Figure 8 shows the results of compressive strength at 7, 28, 91, and 182 days for the OAA and CAA mixes. The maximum 28-day compressive strength was that of the 100% FA mix: 61 and 39 MPa for the OAA and CAA designs, respectively. The minimum values were those of mixes with 100% MBIA; 5.36 and 5.72 MPa for the OAA and CAA designs, respectively. The 28-day and 182-day compressive strength values of the mixes with 50/50 of MBIA/FA were 19.8 and 25.2 MPa, respectively. Marginal strength development was observed in OAA mixes containing 25% FA in comparison with those without it (i.e., ~100% increase after 28 days). For longer curing periods, the trend maintained regarding the mixes with the highest and lowest compressive strengths. This indicates that there is an enhanced strength development with increasing FA content. The crosslinking of the reaction product in the activation process of type F FA (a precursor characterized for its high content of Al and Si and a low content of Ca) is analyzed as a monolithic alkali-activated gel, which is not yet completely understood [62]. By comparison, MBIA is more similar to a precursor with  
Figure 8: Compressive strength of: (**a**) OAA mortars and (**b**) CAA mortars. Figure 7: Fresh and hardened-state densities of: (**a**) OAA mixes and (**b**) CAA mixes. a high Ca content, where the main reaction products correspond to C-A-S-H gel and its structure is strongly associated with the activator used [63]. The type of gel that is formed when mixing FA and MIBA-like precursors will depend on the reaction mechanisms, which will be analyzed in subsequent studies. By mixing the two materials to obtain an AAM, the FA structure may function as a bound network that envelops the MIBA particles, which present low pozzolanic activity. This means that MIBA does not completely deteriorate the mechanical properties of the new material but reduces them proportionally to the percentage of addition. Other hypotheses about the reaction mechanism in which the MIBA particles are fixed in the structure formed by the alkaline activation of the FA consist of the previous formation of zeolites before gelation [64], where the smaller MIBA particles may be adsorbed in the pores generated by the zeolites produced from the hydrothermal conditions of the amorphous aluminosilicates present in the FA in highly basic solution (pH greater than 13) [15; 65; 66]. [PERSON] et al. [67] recently identified and characterized the zeolitic phases in the alkaline activation of FA type F, identifying the presence of amorphous metastable aluminosilicates, which are the intermediate phases in the formation of zeolites. Both hypotheses about the reaction mechanics between FA and MIBA will be clarified in upcoming stages of research. [PERSON] et al. [28] studied mixes where type C FA was replaced with MIBA at 0%, 20%, 40%, and 100%. These authors concluded that the 20% and 40% replacement levels of FA with MIBA led to the highest compressive strengths (53 and 45.6 MPa, respectively, when compared with 42 MPa of the mix without MIBA; 100% MIBA exhibited a compressive strength of 10.6 MPa). The improved performance when adding MIBA may have been partly due to the greater presence of waste glass particles, which are known to actively contribute to the strength development of alkali-activated mixes. In another study, [PERSON] et al. [57] studied binary mixes, where type F FA was replaced at 0%, 25%, 50%, 75%, and 100% with ferroinckel magnesium slag. The alkaline activator used was a mix of NaOH and Na2 SiO3, and two curing conditions were used: thermal curing at 60 \({}^{\circ}\)C for 24 h; and ambient curing. With thermal curing, when the content of ferroinckel increased from 0% to 75%, the compressive strength increased from 33 to 45 MPa at 28 days of curing. However, for the AAM with 100% ferroinckel slag, the compressive strength was substantially reduced to 16 MPa.