M. Cabeza

Impedance spectroscopy study of hardened Portland cement paste

Abstract In this paper, the differential impedance analysis (DIA) has been applied to the study of the dielectric properties of hardened Portland cement paste. Two time constants are found in the impedance spectra obtained in the frequency region form 100 kHz to 15 MHz. One time constant has been attributed to the solid matrix and the other one to the liquid phase filling the pores. The effect of the cement paste–electrodes interface has been quantified using two different experimental set-ups. Measurements using direct contact between electrodes and cement paste have been compared with measurements using an air gap technique in which the specimen “floats” between the electrodes. The two referred time constants have been found in both types of measurements. The influence of drying on the dielectric parameters is also studied.

Electrical effects generated by mechanical loading of hardened Portland cement paste

Abstract This work presents results on the effect of applied load on the electrical properties (resistivity and capacitance) of hardened cement paste. Impedance spectroscopy measurements in the high frequency region (100 kHz–15 MHz) show two time constants, one associated to the solid phase, and the other to the liquid filling the pores. The time constant associated with the liquid phase is sensitive to external loads acting on the specimen. The observed variations are explained in terms of electrolyte drag from interlaminar spaces and structure pores and vice versa. The importance of those electrolyte movements concerning structure fatigue and rebars corrosion is also discussed.

Short-Term Behavior of Slag Concretes Exposed to a Real In Situ Mediterranean Climate Environment

At present, one of the most suitable ways to get a more sustainable cement industry is to reduce the CO2 emissions generated during cement production. In order to reach that goal, the use of ground granulated blast-furnace slag as clinker replacement is becoming increasingly popular. Although the effects of this addition in the properties of cementitious materials are influenced by their hardening conditions, there are not too many experimental studies in which slag concretes have been exposed to real in situ environments. Then, the main objective of this research is to study the short-term effects of exposure to real Mediterranean climate environment of an urban site, where the action of airborne chlorides from sea water and the presence of CO2 are combined, in the microstructure and service properties of a commercial slag cement concrete, compared to ordinary Portland cement (OPC). The microstructure was studied with mercury intrusion porosimetry. The effective porosity, capillary suction coefficient, chloride migration coefficient, carbonation front depth, and compressive strength were also analyzed. Considering the results obtained, slag concretes exposed to a real in situ Mediterranean climate environment show good service properties in the short-term (180 days), in comparison with OPC.

Microstructural characterization of laser surface melted AISI M2 tool steel

We describe the microstructure of Nd:YAG continuous wave laser surface melted high‐speed steel, namely AISI M2, treated with different laser scanning speeds and beam diameters on its surface. Microstructural characterization of the remelted surface layer was performed using light optical and scanning electron microscopy and X‐ray diffraction. The combination of the three techniques provided new insights into the substantial changes induced by laser surface melting of the steel surface layer. The advantage of the method is that it avoids the difficult and tedious work of preparing samples of this hard material for transmission electron microscopy, which is the technique normally used to study these fine microstructures. A melted zone with a dendritic structure and a partially melted zone with a heterogeneous cellular structure were observed. M2C carbides with different morphologies were identified in the resolidified surface layer after laser melting.

Modification of AISI M2 high-speed tool steels after laser-surface melting under different operating conditions

We applied a laser-surface melting treatment to AISI M2 high-speed steel – hardened and tempered – and studied the resulting surface characteristics (microstructure) and mechanical behaviour (hardness and wear performance). The steel was treated using a Nd:YAG continuous-wave laser with different operation conditions. The influence of the laser processing parameters on the individual beads and on the characteristics of the molten surface layer obtained using the multipass system with 50% overlap was studied. The microstructure characterization for all conditions is formed of MC- and M2C-type carbides, martensite, and retained austenite; the quantities of this phase depend on the operating conditions. It has been determined that low levels of power density and high scanning speeds of the beam lead to greater homogeneity in the microstructure with high hardness values and wear resistance.

Effect of Deep Cryogenic Treatment on the Microstructure of an Aerospace Aluminum Alloy

This paper describes how deep cryogenic treatment at 98K produces changes in the microstructure of a heat-treated aluminum alloy. It was observed how the sub-micrometric particles increased near and at the grain boundaries. This slight precipitation did not produce any modification in static mechanical properties. However, the compressive residual stresses of the material were higher after compared to before the treatment. Both these effects can enhance the life of this alloy through cryogenic treatment.

Optimization of Ageing Parameters of a Low Nickel Maraging Steel

In the present work the age hardening parameters of a 14Ni (200) maraging steel are studied in order to optimize mechanical properties of the steel. The initial characterization of the as received solution annealed steel has been carried out by optical and scanning electron microscopy (LOM and SEM), and hardness measurements. To identify the structural changes during ageing, differential scanning calorimetry (DSC) tests were performed. Different time-temperature combinations were considered for the precipitation hardening treatment of as-quenched material samples. After hardness measurements, three of these treatments were selected for an in depth study. The obtained microstructure at the maximum hardness peaks then analysed (LOM, SEM and TEM) and mechanical behaviour (strength, toughness and wear resistance) was studied for the final selection of the age hardening conditions.