El papel del catalizador usado de craqueo catalítico (fcc) como material puzolánico en el proceso de corrosión de armaduras de hormigón

El principal problema que afecta a la vida útil de las estructuras de hormigón armado es la corrosión de las armaduras. Los riesgos más importantes para dicha corrosión son la carbonatación del recubrimiento de hormigón y el ingreso de iones cloruro. Por estas razones, la investigación en nuevos conglomerantes que favorezcan la resistencia a la corrosión de las armaduras es un campo de interés para la ingeniería civil. El catalizador de craqueo catalítico (FCC) es un material de base solicoaluminosa y estructura zeolítica que se usa en las refinerías de petróleo. Cuando el FCC pierde sus propiedades catalíticas, se debe reemplazar por nuevo catalizador. En estudios previos se ha comprobado que el FCC mejora las propiedades emcánicas de los morteros y hormigones debido a una densificación de la matriz cementicia causada por la reacción puzolánica. No obstante, existe una carencia en el conocimiento de cómo la incorporación de este residuo puede afectar a la corrosión de las armaduras de hormigón frente a los clásicos ataques por dióxido de carbono y cloruros. Este trabajo se ha llevado a cabo para estudiar el papel del FCC en el proceso de corrosión de las armaduras de hormigón, cuando se emplea como sustitución parcial del cemento, sometiendo probetas de mortero con FCC a ambientes agresivos (Co2 y Cl-). Se realizaron ensayos de carbonetación acelerada de morteros con distintos niveles de sustitución de cemento por FCC y varias relaciones agua/material cementante (a/mc). En los mismos se puedo comprobar que el consumo de parte de la reserva alcallina de los morterios debido a la reacción puzolánico del FCC, puede ser un factor que acelere el proceso de carbonatación de los recubrimientos de hormigón. No obstante, este fenómeno sólo se puso de manifiesto para relaciones a/mc elevadas. Para relaciones a/mc inferiores a 0,5, la incorporación de FCC en cantidades de hasta el 15% con respecto al peso de cemento, no ofrecía diferencias significativas en cuento a la velZornoza Gómez, EM. (2007). El papel del catalizador usado de craqueo catalítico (fcc) como material puzolánico en el proceso de corrosión de armaduras de hormigón [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1843Palanci

Función de calefacción en pastas de cemento con adición de nanofibras de carbono

The viability of carbon nanofiber (CNF) composites in cement matrices as a self-heating material is reported in this paper. This functional application would allow the use of CNF cement composites as a heating element in buildings, or for deicing pavements of civil engineering transport infrastructures, such as highways or airport runways. Cement pastes with the addition of different CNF dosages (from 0 to 5% by cement mass) have been prepared. Afterwards, tests were run at different fixed voltages (50, 100 and 150V), and the temperature of the specimens was registered. Also the possibility of using a casting method like shotcrete, instead of just pouring the fresh mix into the mild (with no system’s efficiency loss expected) was studied. Temperatures up to 138 °C were registered during shotcrete-5% CNF cement paste tests (showing initial 10 °C/min heating rates). However a minimum voltage was required in order to achieve a proper system functioning.En este artículo se estudia la viabilidad del uso de matrices cementicias con adición de nanofibras de carbono (NFC) como elementos calefactores. Esto permitiría aumentar la temperatura de estancias en edificación o el deshielo de pavimentos en obras civiles. Se han fabricado pastas de cemento con distintas dosificaciones de NFC (0, 1, 2 y 5% respecto masa del cemento) y sometidas al paso de corriente continua a distintos potenciales fijos (50, 100 y 150 V), mientras se controlaba la temperatura en distintos puntos. Se ha estudiado la viabilidad de utilizar la proyección de la pasta fresca como método de puesta en obra, sin perjudicar la eficiencia del sistema. Se consiguieron temperaturas de hasta 138 °C (con velocidades iniciales de 10 °C/min) para pasta proyectada con 5% NFC. Además se ha detectado la necesidad de un potencial mínimo para que la densidad de corriente resultante sea suficiente para producir el efecto esperado.The authors would like to thank the Spanish Ministerio de Ciencia e Innovación for its financial support (ref: Mat2009-10866)

Optimization of the alkali activation conditions of ground granulated SiMn slag

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%.The authors wish to thank the Spanish Ministry of Economy and Competitiveness and European Union (FEDER) for project funding (BIA 2014-58194-R)

Composition of Corroded Reinforcing Steel Surface in Solutions Simulating the Electrolytic Environments in the Micropores of Concrete in the Propagation Period

In the present work, the composition of a corroded reinforcing steel surface is studied at different pH values (related to different degrees of development in the corroding zones of the corrosion process) in solutions simulating chloride-contaminated environments. The media considered consist of saturated calcium hydroxide solutions, progressively neutralized with FeCl2 or by adding 0.5 M NaCl to the solution. The results found in present work confirm higher levels of acidity in the solutions with higher concentrations of Fe2+.In the present work, emphasis is given to the composition of the oxides in solutions that simulate the conditions that exist inside of a localized corrosion pit as a consequence of the reaction of chloride on reinforcing steel. The oxides were studied using Raman and XPS techniques; the results obtained with both techniques are mutually coherent. Thus, in the passive state, the oxides found are those reported previously by other authors, while in the corroding state, the present results are more comprehensive because the conditions tested studied a variety of pore solution composition with several pH values; we tried to reproduce these values inside the pits in conditions of heavy corrosion (very acidic). The oxides found are those typically produced during iron dissolution and seem not the best route to study the corrosion process of steel in concrete; the electrochemical tests better characterize the corrosion stage

Corrosion resistance of steel reinforcements embedded in alkali activated ground granulated SiMn slag mortars

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.The authors wish to thank the Spanish Ministry of Economy and Competitiveness and European Union (FEDER) for project funding (BIA 2014-58194-R)

Use of Impedance Spectroscopy for the Characterization of the Microstructure of Alkali Activated SiMn Slag: Influence of Activator and Time Evolution

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.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. The paper is founded by the research project granted by the Spanish Ministry of Science and Innovation for project funding (PID2020-118322RB-I00) through the “Programa Estatal de I + D + i Orientada a los Retos de la Sociedad” included in the “Plan Estatal de Investigación Científica y Técnica y de Innovación 2017–2020”

Corrosion Behavior of Steel Reinforcement in Concrete with Recycled Aggregates, Fly Ash and Spent Cracking Catalyst

The main strategy to reduce the environmental impact of the concrete industry is to reuse the waste materials. This research has considered the combination of cement replacement by industrial by-products, and natural coarse aggregate substitution by recycled aggregate. The aim is to evaluate the behavior of concretes with a reduced impact on the environment by replacing a 50% of cement by industrial by-products (15% of spent fluid catalytic cracking catalyst and 35% of fly ash) and a 100% of natural coarse aggregate by recycled aggregate. The concretes prepared according to these considerations have been tested in terms of mechanical strengths and the protection offered against steel reinforcement corrosion under carbonation attack and chloride-contaminated environments. The proposed concrete combinations reduced the mechanical performance of concretes in terms of elastic modulus, compressive strength, and flexural strength. In addition, an increase in open porosity due to the presence of recycled aggregate was observed, which is coherent with the changes observed in mechanical tests. Regarding corrosion tests, no significant differences were observed in the case of the resistance of these types of concretes under a natural chloride attack. In the case of carbonation attack, although all concretes did not stand the highly aggressive conditions, those concretes with cement replacement behaved worse than Portland cement concretes.Authors thank to University of Alicante and Generalitat Valenciana the financial support given to this research through projects GRE11-27 and GV/2013/021

Mechanical Properties and Durability of CNT Cement Composites

In the present paper, changes in mechanical properties of Portland cement-based mortars due to the addition of carbon nanotubes (CNT) and corrosion of embedded steel rebars in CNT cement pastes are reported. Bending strength, compression strength, porosity and density of mortars were determined and related to the CNT dosages. CNT cement paste specimens were exposed to carbonation and chloride attacks, and results on steel corrosion rate tests were related to CNT dosages. The increase in CNT content implies no significant variations of mechanical properties but higher steel corrosion intensities were observed.The authors would like to acknowledge the Spanish Ministry of Science and Innovation (Ref: Mat 2009-10866) and Generalitat Valenciana (PROMETEO/2013/035) for their economic support on this research

Carbon Nanofiber Cement Sensors to Detect Strain and Damage of Concrete Specimens Under Compression

Cement composites with nano-additions have been vastly studied for their functional applications, such as strain and damage sensing. The capacity of a carbon nanofiber (CNF) cement paste has already been tested. However, this study is focused on the use of CNF cement composites as sensors in regular concrete samples. Different measuring techniques and humidity conditions of CNF samples were tested to optimize the strain and damage sensing of this material. In the strain sensing tests (for compressive stresses up to 10 MPa), the response depends on the maximum stress applied. The material was more sensitive at higher loads. Furthermore, the actual load time history did not influence the electrical response, and similar curves were obtained for different test configurations. On the other hand, damage sensing tests proved the capability of CNF cement composites to measure the strain level of concrete samples, even for loads close to the material’s strength. Some problems were detected in the strain transmission between sensor and concrete specimens, which will require specific calibration of each sensor one attached to the structure.The authors wish to thank the Spanish Ministry of Science and Innovation for their economic support on this research (grant Mat 2009-10866) and Generalitat Valenciana (grant PROMETEO/2013/035)

Heating and de-icing function in conductive concrete and cement paste with the hybrid addition of carbon nanotubes and graphite products

This paper aims to study the viability of conductive cement paste and conductive concrete with the hybrid addition of carbon nanotubes (CNT) and graphite powder (GP) as a self-heating material for heating, ice formation prevention and de-icing in pavements. Different heating tests, ice-preventing tests and de-icing tests were performed with cement paste and concrete specimens. Results confirm that the conductive cement composites studied, with the addition of 1% CNT + 5% GP, exhibited heating, de-icing and ice-prevention properties, when applying constant AC/DC voltages between the two end sides of each specimen, with relatively low energy consumption. The main contribution of this work is to achieve a sufficient conductivity level for the development of the heating and de-icing function using this hybrid addition in concrete, which has not been used so far, in order to be applied in real concrete structures.The authors would like to acknowledge financial support received from European Union’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 760940 and from the Generalitat Valenciana (Spain) (AICO/2019/050)