Cálculo del transporte de cloruros en la pasta de cemento

A method was developed to measure the diffusion coefficient of chloride ions in cement paste based on an analytical solution to Fick’s 2nd law in a cylindrical coordinate system. This natural method yielded diffusivity results within as little as a month. Testing time was reduced by exploiting the three-dimensional inward flux in the specimen. In an attempt to determine the saturation concentration, dense portland cement pastes were exposed to a concentrated chloride solution. The method proved to be useful for exploring cement hydration-induced changes in the diffusion coefficient of cement paste.Se ha desarrollado un método para medir el coeficiente de difusión de los iones cloruro en la pasta de cemento, partiendo de una aplicación analítica de la segunda ley de Fick en un sistema de coordinadas cilíndrico. Este método, que es natural, demostró ser capaz de producir resultados de difusividad en tan solo un mes. Se consiguió reducir el tiempo de ensayo mediante el aprovechamiento de la tridimensionalidad del flujo desde el exterior al interior de la probeta. A fin de determinar la concentración de saturación, se sometieron las pastas de cemento Portland a una disolución de cloruros concentrada. Este método resultó ser útil en el estudio de los cambios del coeficiente de difusión de la pasta de cemento provocados por las reacciones de hidratación que tienen lugar en esta

Advancements In Modelling The Development Of Microstructure In Cement Pastes

The time evolution of some microstructural parameters (porosity, composition) and physical properties (electrical conductivity) of young cement pastes have been modelled by means of a numerical code (chemhyd3d) based on the percolation theory. The computational results have been compared with experimental data (for the electrical conductivity) and porosities reckoned by the Powers equation. Very good agreement has been found which indicated that the effects of the foremost parameters on the properties of mortars and concrete can now accurately predicted

Cool Cementitious Materials for a More Sustainable Urban Environment

The temperature increase in cities called the "urban heat island" (UHI) depends on the local microclimate (e.g., solar irradiation, population, buildings density, industrial activities, traffic, emissions, heat sources) and results not only in increased electricity consumption for indoor cooling but also in decreased outdoor comfort, especially in summer periods or in warm climate zones. As the world's urban population continues to grow, there is an urgency to make buildings more ecoefficient and reduce the impact of cities on climate change and global warming. In this framework, the use of cool concretes can mitigate the UHI and improve energy saving and outdoor comfort: cool concrete roofs and facade cement-based tiles can provide valid solutions to decrease the energy demand for air conditioning in building envelops; moreover, cool concrete paving blocks and pervious pavements can improve urban comfort for outdoor wellbeing. The present work shows the improved solar reflective performance of three cool cementitious solutions compared with traditional ones, with both white and colored surfaces, to fulfill both architectural demand and sustainability issues without any additional post-treatments or after placing steps, such as surface coating. Finally, the study is completed with the durability evaluation of the products' solar reflective performance

Accelerated Curing Of Cement Based Materials

Portland cements are slow hardening materials and several techniques are used to expedite the attainment of a satisfactory strength, i.e., chemicals, reduction of particle size, heat. Increasing the temperature is by far the most effective. Dielectric heating is a well established technique in materials technology but the cement industry has so far expressed little interest in this technology, probably because the effects on the final properties of mortars and concrete are not well established yet and for the greater complexity of the process compared, for instance, to steam curing. Fear of radiation leakage from high power applicators not correctly designed and operated has placed a further constraint to the acceptance of this technique. Although early results of microwave heating were unsatisfactory, likely because of overheating effects, the availability of temperature controlled microwave applicators now makes it possible to choose temperature profiles that favourably influences the final properties of the materials. Here we report on the effects of microwave treatments on three types of modified portland cements: one contained microsilica and the others granulated slag or flyash from coal burning. In all cases an acrylic superfluidifier was used to keep water content low. The microwave treatment was given at 80°C for 0 to 16 h. The mechanical properties of the microwave treated samples were compared to those of samples treated in water at 80°C for 24 h. Mercury intrusion porosimetry, thermal analysis and SEM examination were also carried out to investigate the microstructural modifications brought about by the microwave treatment

Electrical properties of fluidified portland cement mixes in the early stage of hydration

The electrical conductivity and the dielectric constant of cement mixes have been studied in the initial stages of setting to correlate the time evolution of the electrical parameters with the chemical and microstructural modifications. The formulations, based on a mix of Portland cement and microsilica, were fluidified with various amounts of an acrylic polyelectrolite and had a w/c ratio of 0.24. Measurements were carried out at ambient temperature for times of up to 40 h, in the frequency interval 30 Hz–200 kHz. The conductivity showed a marked decrease correlated with the loss of connectivity of the macroscopic porosity (depercolation). The time to the drop of conductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenomenon was observed which vanished after the porosity depercolated. The temperature profiles of mixtures with different concentrations of the superplasticizer were drawn and a simple model was developed to compute the specific thermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat tests were also carried out

Metodi elettrici nella tecnologia di malte e calcestruzzi

The application of electrical methods to the technology of cementitious materials opens up new possibilities for monitoring setting and hardening processes of fresh, admixtured pastes or for tracking the invasion of aggressive chemicals in hardened structures. The electrical conductivity can be related, by means of percolation models, to the porosity of the material and, therefore, to its strength. Innovative formulations with unusual properties can also be produced by controlling the electrical conductivity of hardened mortars. Further, energy efficient accelerated hardening techniques and contactless breaking techniques for decontamination and decommissioning processes in nuclear power plants are being developed

Accelerated Direct Carbonation of Steel Slag and Cement Kiln Dust: An Industrial Symbiosis Strategy Applied in the Bergamo–Brescia Area

The carbonation of alkaline industrial wastes is a pressing issue that is aimed at reducing CO2 emissions while promoting a circular economy. In this study, we explored the direct aqueous carbonation of steel slag and cement kiln dust in a newly developed pressurized reactor that operated at 15 bar. The goal was to identify the optimal reaction conditions and the most promising by-products that can be reused in their carbonated form, particularly in the construction industry. We proposed a novel, synergistic strategy for managing industrial waste and reducing the use of virgin raw materials among industries located in Lombardy, Italy, specifically Bergamo-Brescia. Our initial findings are highly promising, with argon oxygen decarburization (AOD) slag and black slag (sample 3) producing the best results (70 g CO2/kg slag and 76 g CO2/kg slag, respectively) compared with the other samples. Cement kiln dust (CKD) yielded 48 g CO2/kg CKD. We showed that the high concentration of CaO in the waste facilitated carbonation, while the presence of Fe compounds in large amounts caused the material to be less soluble in water, affecting the homogeneity of the slurry