## 3 Results and Discussion
The main crystalline compounds of APS are mica (2%), quartz (1%), illite (1%), dolomite (1%) and calcite (95%) (Figure 2). The absence of the characteristic peaks of K is notable in the two thermally activated wastes, which demonstrates their total transformation in the MK amorphous phase. The elemental analysis of Ordinary Portland cement (OPC) by means of XRF determines the concentration of the principal elements in the cement (Ca, Si, Fe, Al), which in accordance with the Bogue formula [34] can be used to determine its mineralogical composition: C\({}_{3}\)S (41.7%), C\({}_{2}\)S (28.0%), Al\({}_{2}\)O\({}_{3}\) (8.33%) and C\({}_{4}\)AF (8.89%). Figure 1: Presence/absence of crystalline materials in the coal mining residues before CW/after ACW thermal activation and after the pozzolanic reaction at 2 and 28 days. CW: coal waste; ACW: activated coal waste; LDH: Layered Double Hydrooxides. The pozzolanic properties of the samples were determined by the quantity of lime that they fixed between 1 and 90 days. In Figure 3, the results of the lime that is fixed in the samples over various reaction times are shown. Through these chemical values, the presence or absence of pozzolanic activity may be valued, an essential condition for material to be used as an active addition to cement. In general, both wastes presented a high pozzolanic activity, as between 70-80% of the lime was available 6 days into the reaction of the fixed lime, progressively increasing up to 80-90% at 90 days. These values are comparable to those obtained for a 100% natural MK. Different pozzolanic behaviors were observed between the clayey wastes under analysis. The most important differences were those observed during the first 24 h of the reaction, where factors such as fineness, reactive silica content and morphology of the MK particles play a principal role in the short-term rate of reaction. Thus the ACW consumed half (50%) of the available lime; while the APS only reacted with a quarter (25%) of the lime, despite the latter waste presenting a higher quantity of kaolinite (21%) in its initial chemical composition. This fact could be related to the loss of specific surface of the MK with the increase of the calcination temperature [35]. Consequently, although both  
Figure 3: Evolution of the pozzolanic activity for activated products. Figure 2: Presence/absence of crystalline materials in paper sludge residues before PS/after APS thermal activation and after the pozzolanic reaction at 2 and 28 days. wastes were activated under the same conditions (600 \({}^{\circ}\)C and 2 h), the presence of organic matter (cellulose) in the APS changed those conditions. The analysis of ACW in how it reacts with calcium hydroxide at 2 and at 28 days into the pozzolanic reaction (ACW/Ca(OH)\({}_{2}\) system) presents the following crystalline phases:  
Tetracalccium aluminate hydrate was observed in trace concentrations by XRD two days into the reaction, while after 28 days it increased by 5% (Figure 1). Stratlingite, inexistent at 2 days, quickly formed until it had a concentration of 20% after 28 days. In turn, monosulfoaluminate hydrate reached 5% after two days of reaction time, and was almost imperceptible after 28 days. Layered Double Hydroxides (LDH) compounds (phyllosilicate/carbonate), which at an early age had a concentration of 1%, reached 13% after 28 days. Both the micas (2%) (Figure 4F) and the spinel (1%) as well as the calcite (30%) remained constant at both ages, while the quartz evolved from 61% at two days to 29% at 28 days of reaction time. SEM/EDX as an amorphous majority phase (Figure 4A) identified C-S-H gels, rich in calcium and presenting a spongy appearance. The nanostructure of the C-S-H gels consisted of interlaminar Ca with chains of silica tetrahedra linked on the sides with some aluminum substituting the silica in the tetrahedra. The CaO/SiO\({}_{2}\) ratio was 1.26 (Table 1), which, according to the [PERSON] classification [36], corresponds to gel type I.