I. Sánchez’s research while affiliated with University of Alicante and other places

The impedance spectroscopy technique has been used to study the microstructure of the binder resulting from the alkaline activation of SiMn slag. Two alkaline activators were used: waterglass and NaOH. Three different concentrations were analysed for both activators: 3.0, 3.5 and 4.0% Na2O for NaOH; and 4.0, 4.5 and 5.0% Na2O for waterglass with a constant SiO2/Na2O ratio of 1.0. The time evolution of the microstructure has been followed up using the non-destructive technique of impedance spectroscopy. This technique has been proved to be effective describing the microstructural changes in alkali activated pastes, and also can help predicting the mechanical behavior of mortars. The use of the resistivity itself seems to be deficient, but the analysis of the electrical parameters calculated from the impedance spectra measured gives a complete idea of the evolution in the material.

The influence of the type and concentration of the activator on the composition of the products formed and the microstructural development of alkali activated SiMn slag pastes and their evolution with curing time are studied. About 15–17 million tons of SiMn slag is produced per year, so valorising this residue is interesting for the economic and environmental scope. For this purpose, NaOH and WG with different concentrations of Na2O have been used as activators. The hydration products have been studied by X-ray diffraction analysis (XRD) and thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDX) has been used to study the microstructure and elemental composition of hydrated phases. The advance of the degree of hydration with time and with the activator concentration is consistent with the mechanical behaviour of these pastes observed in previous studies. All the pastes show high microcracking due to the stresses caused by the shrinkage, as it has been observed in previous studies. Elemental composition data do not show significant differences regarding the type of activator with respect to the main elements and their relationship in the reaction products except for Mn. The main reaction product formed in the alkaline activation of SiMn slag is C-S-H with a low Ca/Si ratio, this being higher for pastes activated with NaOH compared to those with WG. In addition, the modification of the C-S-H structure is demonstrated by the presence of small amounts of Al and Mn and that Mg is not incorporated into it. The presence of hydrotalcite is not detected by XRD, but small amounts have been confirmed by TGA/DTG and SEM/EDX.

The corrosion process of steel reinforcements embedded in alkali activated SiMn slag mortars was investigated. NaOH and waterglass were used as alkali activators of SiMn slag. The steel reinforced mortars were subjected to two aggressive environments: carbonation and chloride ingress. Carbonation progressed quicker for the binder activated with NaOH, decreasing the rate with the concentration. However, during the corrosion of the steels embedded in the mortars, both activators showed a similar behaviour. Mortars prepared with waterglass exhibited higher chloride migration coefficients, which decreased as the activator concentration increased. In a chloride contaminated ambient, steel embedded in mortars prepared with NaOH offered lower corrosion rate levels and it increased with the activator concentration.

This research investigates the mechanical performance and dimensional stability of mortars whose binder is prepared by alkali activation of ground granulated SiMn slag. Two types of activators have been used: NaOH and waterglass solutions. Three different types of aggregates have been tested: silica sand, limestone sand, and recycled sand obtained from recycled concrete. Three activator concentrations have been adopted in the binder design: 3.0, 3.5 and 4.0% Na2O for NaOH solution; and 4.0, 4.5 and 5.0% Na2O for waterglass solutions with a constant SiO2/Na2O of 1.0. The best mechanical performance was obtained for an aggregate/slag ratio of 2/1 when silica sand was used as aggregate: 68 MPa were obtained when waterglass was used as activator, and %Na2O of 4.5–5.0% at 90 days; 54 MPa were obtained for NaOH as activator and 4.0% Na2O at 90 days. Limestone sand also offered a good mechanical performance although the maximum compressive strength achieved was about 25% lower than values obtained with silica sand, but recycled concrete aggregate mortars exhibited poor results, probably due to the high absorption of recycled aggregates. The higher shrinkage was registered in mortars activated with waterglass, although autogenous shrinkage was low for all types of activators and aggregates, except for recycled concrete aggregate.

The viability of using a SiMn slag as raw material for the preparation of alkali activated binder has been analysed. The slag has been characterized from the physical, chemical, mineralogical and microstructural point of view. The solution used for the alkali activation process was waterglass, which was prepared by the mix of a sodium silicate solution with sodium hydroxide. The influence of the following design parameters in alkali activated pastes has been studied: %Na2O, SiO2/Na2O ratio and activating solution/slag ratio. All the prepared specimens have been cured under a 100% of relative humidity atmosphere and no additives have been included in the formulations. For the evaluation of the alkali activation process a response surface methodology has been adopted by modelling the obtained results for different control parameters. The proposed experimental methodology consisted of a composite cubic-type experimental design which follows 2nd degree polynomial model. The control parameters of the pastes used in this research have been: workability, compressive strength at 7, 28 and 90 days of curing time, setting times and shrinkage at 50% of relative humidity. The results show that it is possible to valorise the SiMn slag through its use as an alkali activated binder. In general, workability decreases as the %Na2O increases and SiO2/Na2O ratio decreases. Initial setting times ranges from 38 to 263 min depending on the activating solution composition, and workability time decreases for increasing %Na2O. A compressive strength higher than 45 MPa can be achieved by the following activating conditions: 4.0–4.5%Na2O, SiO2/Na2O = 1.00 and activating solution/slag = 0.35–0.375. Shrinkage values of pastes that were cured at 50% of relative humidity ranged from 1.5% to 3.0%.