Recycled concrete aggregate (RA) from pavement demolition was used to make concrete. Ten concrete mixtures with different replacement percentages of RA (coarse and fine) were made. The corrosion rate of steel and the electrical resistivity of concrete were determined on reinforced concrete specimens subjected to wetting-drying cycles (3.5% solution of NaCl). Corrosion rate was determined using the electrochemical technique of linear polarization resistance, while the electrical resistivity was measured by electrochemical impedance spectroscopy. The results show that the use of RA introduces more interfaces in concrete, which accelerates the steel corrosion process because the porosity increases and the electrical resistivity decreases. However, steel corrosion and the electrical resistivity in concrete are not significantly influenced by replacing a maximum 30% of coarse aggregate or 20% of fine aggregate with RA.

Recycled aggregates (RA) from construction and demolition can be used in permeable concretes (PC), improving the environment. PCs have a significant porous network, their cement paste and the interaction between the paste and the RA establishing their strength. Therefore, it is important to evaluate the porosity in the interfacial transition zones. The porosity of the cement paste, the aggregate and the interfacial transitional zones (ITZ) of a PC with recycled coarse aggregates (RCA) and silica fume (SF) is measured by means of image analysis–scanning electron microscope (IA)-(SEM) and by mapping the chemical elements with an SEM-EDS (energy dispersive spectrometer) detector microanalysis linked to the SEM and, as a contrast, the mercury intrusion porosimetry technique (MIP). In the IA process, a “mask” was created for the aggregate and another for the paste, which determined the porosity percentage (for the anhydrous material and the products of hydration). The results showed that using SF caused a reduction (32%) in the cement paste porosity in comparison with the PC with RA. The use of RA in the PC led to a significant increase (190%) in the porosity at different thicknesses of ITZ compared with the reference PC. Finally, the MIP study shows that the use of SF caused a decrease in the micropores, mesopores and macropores.

Chemical properties of geopolymers were evaluated from the reduction of fly ash particle size by grinding. X-ray diffraction determined that at early curing ages new crystalline phases appear in the matrix of the geopolymer and they remain for 28 days, with increases in intensities up to 60%. In Fourier transform infrared spectroscopy, displacements were identified in the main band of the geopolymers at higher wavenumbers, attributed to the greater rigidity in the structures of the aluminosilicate gel due to the increase of the reaction products in the geopolymers obtained through fly ashsubjected to previous grinding, which is observable in the geopolymers matrix. Results indicate that the reduction of fly ash particle size by grinding has an influence on the chemical properties of geopolymers.Appl. Sci. 2018, 8, 365 2 of 12 and an alkaline solution at temperatures below 100 • C. This reaction results in the dissolution of reactive phases of aluminosilicates. With the progress of the reaction, water is gradually removed, and the tetrahedral groups of SiO 4 and AlO 4 form the polymeric precursors [18][19][20].According to various studies, the physicochemical properties of geopolymers depend mainly on the Si/Al, Na/Al ratios and water content [21,22]. Often, these ratios are investigated indirectly, for example, by varying the activation ratio and curing conditions [23][24][25]. The chemical composition in mass is largely related to the kinetics of the geopolymer matrix; therefore, FA reactivity and the geopolymers chemical behavior depend specifically on the particle sizes and surface area [26,27]. The activation of these materials is also promoted by mechanical means, such as grinding, where results that favor geopolymerization are obtained at low curing temperatures. Through FT-IR it is demonstrated that FA with finer particles produce high reactivity indexes in the geopolymers and, therefore, an increase in the reaction product of the alkaline aluminosilicate amorphous gel. This produces an increase in compressive strength [28,29].On the other hand, the use of X-ray diffraction (XRD) allows us to observe the formation of new phases which are characteristics of the FA geopolymerization process, such as semicrystalline and polycrystalline phases of inorganic polymers (especially zeolites) [30,31].In this work, the influence of three different particle sizes of Mexican FA were comparatively studied in the physicochemical behavior of the geopolymers, using XRD, FT-IR, and SEM.

Due to the environmental problem posed by the use of Portland cement as construction material, it becomes necessary the search for supplementary cementitious materials that mitigate the ecological damage caused by it. Because the chemical similarity and the high cementitious powers of the blast furnace slag, it is used in the generation of geopolymers in a cement total replacement. This research focused on the study of the influence of the curing conditions on the final properties of blast furnace slag establishing three variables: no cured process (N-C), cured of controlled temperature of 45°C (CT45-C) and room temperature cure (RT-C); evaluating the mechanical behavior until 28 days of age and the water porosity index. The results show that geopolymers based on blast furnace slag has a behavior similar to hydration maturity of Portland cement and curing process decreases the porosity; On the other hand, applying a controlled temperature generates densest resistant pastes such as the variable CT45-C which reach the highest value of resistance in all curing ages.

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