## 3 Results and Discussion  
### Matrix  
Figure 4 shows the X-ray diffractograms of the pastes obtained by alkaline activation of the reactive powders blend after 2, 7 and 28 days of curing at 20 \({}^{\circ}\)C.  
The main crystalline phases identified are aluminum containing C-S-H (C-A-S-H) (PDF 00-034-0002), Mullite (PDF 00-006-0259), Quartz (PDF 00-001-0649) and Hydrotalcite (PDF 01-089-5434). Quartz and Mullite are impurities of the reactive powders. Being crystalline phases, they do not participate in the alkaline activation and can be found unaltered among the final products. The main reaction products of the alkaline activation are the C-A-S-H and the Hydrotalcite [36; 37]. Hydrotalcite is commonly detected when a high degree of slag hydration is reached, and slag contains a sufficient amount of Mg [37; 38; 39]. C-A-S-H is responsible for the matrix compressive and flexural strength. It is less crystalline than the C-S-H produced during the hydration of Portland binder and it is also characterized by a lower Ca/Si molar ratio due the higher content in Al and Si of the used reactive powders. The intensity of the peaks associated with the reaction products is already remarkable after two days of curing and can be associated with a fast degree of reaction. This is consistent with the compressive and flexural strength values reported in the next paragraph. At longer curing age, the peaks associated with C-A-S-H and Hydrotalcite are more refined due to the higher degree of reaction. The thermogravimetric analysis of alkali-activated paste is shown in Figure 5.  
The mass loss up to around 100 \({}^{\circ}\)C can be attributed to the release of evaporable water. The dehydration of C-A-S-H(I) is responsible for the mass loss between 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, whereas the mass loss between 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C is associated with the decomposition of Hydrotalcite [40]. FT-IR spectrum of the paste is reported in Figure 6.  
Figure 4: X-Ray diffractograms of pastes after 2, 7 and 28 days of curing; M = Mullite; Q = quartz, H = Hydrotalcite. The wide and broad region between 2900 and 3700 cm\({}^{-1}\) can be attributed to hydrogen-bonded surface hydroxyl stretching vibrations of H\({}_{2}\)O molecules. In particular, the band centered at 3600 cm\({}^{-1}\) is attributed less strongly hydrogen-bonded interlayer water molecules. The broad peak centered near 3000 cm\({}^{-1}\), instead, is due to the O-H stretch of strongly polarized H-bonded water in the interlayer of the cementitious minerals. The band at 1650 cm\({}^{-1}\) can be attributed to O-H bending mode of molecular water. The net small peak at 2900 cm\({}^{-1}\) is due to the hydrogen bonding of H\({}_{2}\)O and interlayer of SO\({}_{4}\)\({}^{2-}\) anions in hydrotalcite. The broad band in the range of 1100-850 cm\({}^{-1}\) can be attributed to the stretching modes of the SiO\({}^{2}\)(mAl) units. The absence of distinctive sharp bands is indicative of widely distributed bonds stretching modes in the paste. Bands at -1485 cm\({}^{-1}\) at -1370 cm\({}^{-1}\) are produced by the splitting of band at 1415 cm\({}^{-1}\) that arises from the interaction of CO\({}_{3}\)\({}^{2-}\) with OH groups in Hydrotalcite. In Figure 7 the flexural and compressive strengths of plain mortar at 2, 7 and 28 days of curing are reported. The adopted mix design allows to obtain a fast development of the mechanical strength, with compressive strength value of 18.61 \(\pm\) 0.63 MPa after two days of curing. Figure 5: Thermogravimetric (TG) and Derivative Thermogravimetric (DTG) curves of paste after 28 days of curing. Figure 6: FT-IR spectrum of paste after 28 days of curing. The results for capillary absorption testing of the matrix are shown in Figure 8. Experimental data properly fit the model with a R\({}^{2}\) = 0.99. Sorpitivity (S), which is related to the water absorption and diffusion in the mortar by capillarity, is equal to 0.27 kg/(m\({}^{2}\) min\({}^{0.5}\)).