## 4 Conclusions  
The kaolinite-based industrial wastes selected in the present study were of different natures and presented different kaolinite contents. The CW was of a silico-aluminous nature with 14% kaolinite; while the PS was of a silico-aluminum-calcite nature with 21% kaolinite.  
Thermal activation at 600 \({}^{\circ}\)C and over 2 h in an oven were sufficient for a total transformation of the K into MK, a highly reactive pozzolan. The rest of the minerals present in the wastes remained unaltered under these activation conditions.  
The MK-based activated products, ACW and APS, presented a high pozzolanic activity, taking into account the high values of fixed lime after 28 days of reaction time. However, a turning point was observed on day 7 of the reaction. Clearly, it may be seen that the reaction rate of the APS pozzolan in the short term was 50% lower than the one presented by the ACW pozzolan, but after day 28 its pozzolanic activity slightly improved with regard to the CW.  
The results showed that the pozzolanic activity of the waste (in terms of fixed lime) was similar up to 90 days but that its reaction rate was different.  
Due to these pozzolanic differences between the studied wastes, both the formation and the evolution of the mineralogical crystalline phases formed during the pozzolan reaction, in APS, were stratlingite and LDH, while in the ACW, C\({}_{4}\)AH\({}_{13}\) appeared in addition to the latter two (stratlingite and LDH compounds).  
The C-S-H gel is the principal non-crystalline mineralogical phase from hydration in the two types of wastes. In the ACW waste, type I gels were identified, while type II gels were identified in the APS. Type II gels present different Ca/Si ratios, due to the APS waste that generates gels that are rich in calcium and poor in Al, unlike those obtained with the ACW waste. Likewise, the chains of the gel obtained with the ACW are shorter than those obtained with APS waste. From the point of view of the Ca/Si ratio of the final product, the initial concentration of the sample (Al, Si, etc.) is not as important as the mineralogical phases present and their reactivity.  
The C-S-H gel generated with coal waste had very short chains, was poorer in Ca and rich in Al. The addition of both wastes to cement modified its mineralogical composition in comparison with a conventional cement, favoring the formation of C\({}_{4}\)AH\({}_{13}\) over the formation of ettringite.  
With both additions to the cement, the formation of C\({}_{4}\)AH\({}_{13}\) was favoured, but with some differences. In the case of the APS pozzolan no ettringite was formed and LDH appeared at 28 days, while ettringite was initially formed with ACW, although it decreased at 28 days to the detriment of the formation of C\({}_{4}\)AH\({}_{13}\). This reaction produced gels with a low incorporation of Al even at 28 days in the ACW samples, but with high Ca/Si ratios. Besides, the gels obtained from the cement plus APS mixture incorporated more Al in its structure and therefore had lower Ca/Si ratios.  
The evolution of the compressive strengths in both pozzolans was different: APS showed better mechanical behavior than ACW and even exceeded the mechanical performance of the reference mortar in relation to both percentages of substitution and also in relation to the same curing times. However, ACW, at any age, shows a worse mechanical strength than OPC used as the reference.  
The authors are grateful to the Spanish Ministry of Economy and Competiveness (MINECO/ FEDER) by the financial support (BIA2015-65558-C3-1/2/3-R) y a la Sociedad Anonima Hullera Vasco-Leonesa (Leon, Spain) and the Spanish Cement Institute (IECA, Madrid, Spain) for their support in this research.  
[PERSON] has designed the paper and performed the XRD analysis. [PERSON] has carried out the mechanical and pozzolanic tests. [PERSON] has performed and interpreted the SEM/EDX analysis and [PERSON], the XFR and Fourier-transform infrared spectroscopy (FTIR) analysis.  
The authors declare no conflict of interest.