## 2 Materials and Methods  
Two metakaolin-based wastes were used: (a) coal waste (CW) from the open-cast coal mines was supplied by the Sociedad Anonima Hullera Vasco-Leonesa, located in Santa Lucia, Leon, Spain; and (b) paper sludge (PS) was supplied by the firm Holmen Paper Madrid, which uses 100% recycled paper as a raw material for recycling the cellulose. The organic material contained in the sludge was at almost 32.35%.  
Both wastes, once characterized, were subjected to a process of thermal activation at 600 \({}^{\circ}\)C over 2 h in an oven, which are the optimal conditions from the energetic and environmental points of view. Under these conditions, the organic material is eliminated, the total transformation of K into MK takes place, and the decarbonation of the carbonates takes place, avoiding the formation of free lime (CaO), a compound known for its expansive properties during the hydration phase of the cement. Subsequently, the products were ground and sieved through a 63-micron mesh.  
An extra pure calcium hydroxide (PhEur, USP, BP chemical reagent) was used in the pozzolan/ lime system, with a minimum content of 99% CaO.  
The cement employed in this study is a Portland cement classified as CEM 1 52.5R, supplied by Financiera y Minera (Italcementi Group, Bergamo, Italy).  
The elemental composition of the materials was analyzed via X-ray Fluorescence (XRF) equipment using an energy dispersive Philips Pw-1404 spectrometer, anode Sc/Mo, under working conditions of 80 kV and 35 mA. The mineralogy of the crystalline materials was determined by X-ray Diffraction (XRD), using a SIEMENS D-5000 (Madrid, Spain) X-ray powder diffractometer with an automatic divergence slit, and graphite Cu K\(\alpha\) radiation with a graphite monochromator. The XRD data were collected at an angular interval of 3\({}^{\circ}\)\(\leq\) 20 \(\leq\) 65\({}^{\circ}\) with steps of 0.03\({}^{\circ}\) and with 3 s count times. The current intensity and voltage of the X-ray generator tube was 30 mA and 40 kW, and the divergence and reception slits were set at 1\({}^{\circ}\) and 0.18\({}^{\circ}\), respectively. The characterization of the bulk samples was performed via the Rietveld method [28; 29]. Ratile was used as an internal reference patron at a concentration of around 5% in all cases.  
The sample morphology was determined with scanning electron microscopy (SEM) with a PHILIPS XL-30 ((Hillsboro, OR, USA) equipped with a Wolfram cathode. The powder samples were fixed on the metallic runner by means of a graphite plate, on the BIO-RED SC-502 (General Electric, New York, NY, USA) equipment. The energy-dispersive X-ray spectroscopy (EDX) test was done by chemical analysis with a silicon/lithium X-ray detector and a DX41 analyzer.  
The combined CaO (mmol/L) was obtained as the difference between the concentration in the lime-saturated control solution (17.68 mmol/L) and the CaO content in the solution in contact with the sample. At the end of the reaction time, the hydrated solid sample was filtered, washed with ethanol and heated at 60 \({}^{\circ}\)C for 24 h, to stop the hydration reaction. The calculations are based on the European standard of the pozzolanicity test for pozzolanic cements [30].  
The 1 \(\times\) 1 \(\times\) 16 cm specimens of cement paste were prepared following the procedure described by [PERSON] [31], with partial substitutions of 10 and 20% of each of the activated paper sludge (APS) and the activated coal waste (ACW) pozzolans, with a water/cement ratio of 0.5 and a curing time of 28 days. The same pozzolan and water/cement ratios were used in the case of the cement mortars, as described in the current norms [30], but in this case, the dimensions of the prismatic specimens were 4 \(\times\) 4 \(\times\) 16 cm. The specimens were cured under water for 28 days.