The influence of fly ash as substitute of cement in the durability of concrete

Limitation of transport capacity through the concrete is one of the key points in the improvement of the material’s durability. The use of fly ash as an admixture to concrete is widely extended; a general consensus has been established due to the pore size reduction produced by the ashes. Nevertheless, the importance of the micro-structural and composition changes in mechanical and durable properties is not well defined. In the present study the use of fly ash has been considered as substitute of cement in the design limits. The concrete mechanical properties and its porous structure were evaluated. The tests included porosimetry and water permeability tests. In order to characterize the hydration products and its evolution with time TG and DTA analysis were performed. This work studies the fly ash concrete hydration process, their influence in the porous distribution, and the mechanical and durable properties of the material

Síntesis hidrotermal de zeolita a partir de ceniza volante tipo F: influencia de la temperatura = Influence of temperature of alkaline hydrothermal treatment on the zeolite conversion of spanish coal class F fly ash

En este trabajo se presenta el papel que juega la temperatura durante el tratamiento hidrotermal en medio alcalino para convertir una ceniza volante de bajo contenido en cal (clase F, según la norma ASTM) en zeolita. Durante este tratamiento a la temperatura de 100 ºC se forma zeolita Na-P1 tipo gismondina (Na6Al6Si10O32.12H2O); al elevar la temperatura a 200 ºC, dicha zeolita se transforma en zeolita analcima C (Na(Si2Al)O6H2O) y en fase sodalita (1.08 Na2O.Al2O3.1.68SiO2.1.8H2O) junto con trazas de tobermorita-11Å (Ca5(OH)2Si6O16.4H2O). A esta temperatura y en estas condiciones se ha conseguido un 100% de reacción. Un estudio equivalente se ha llevado a cabo empleando agua como medio de referencia. La conversión de ceniza volante en zeolita se ha caracterizado mediante técnicas, como difracción de Rayos X (DRX), espectroscopia infrarroja por transformada de Fourier (FTIR) y análisis térmico (TG/ATD); asimismo los cambios en el área superficial se han llevado a cabo mediante la técnica BET-N

Effect of temperature on the durability of class C fly ash belite cement in simulated radioactive liquid waste: Synergy of chloride and sulphate ions

The durability of class C fly ash belite cement (FABC-2-W) in simulated radioactive liquid waste (SRLW) rich in a mixed sodium chloride and sulphate solution is presented here. The effect of the temperature and potential synergic effect of chloride and sulfate ions are discussed. This study has been carried out according to theKoch–Steinegger test, at the temperature of 20 ◦Cand 40 ◦Cduring a period of 180days. The durability has been evaluated by the changes of the flexural strength of mortar, fabricatedwith this cement, immersed in a simulated radioactive liquid waste rich in sulfate (0.5 M), chloride (0.5M) and sodium (1.5M) ions – catalogued like severely aggressive for the traditional Portland cement – and demineralised water, which was used as reference. The reaction mechanism of sulphate, chloride and sodium ions with the mortar was evaluated by scanning electron microscopy (SEM), porosity and pore-size distribution, and X-ray diffraction (XRD). The results showed that the chloride binding and formation of Friedel’s salt was inhibited by the presence of sulphate. Sulphate ion reacts preferentially with the calcium aluminate hydrates forming non-expansive ettringite which precipitated inside the pores; the microstructure was refined and the mechanical properties enhanced. This process was faster and more marked at 40 ◦C

Reduction of the calcium leaching rate of cement paste by addition of silica nanoparticles

Calcium leaching is a degradation process that consists in the progressive dissolution of the cement paste as a consequence of the migration of the calcium ions to the aggressive solution. Although it is a well known phenomenon, a way of reducing it has not been found yet. The aim of this work is to prove how the addition of small amounts of silica nanoparticles to the cement paste can induce such reduction. The experimental results obtained have shown that nanosilica increases the strength of the cement paste about a 30% in cured samples and more than a 100% in the asymptotically leached ones. At the level of the structure of the C–S–H gel, silica nanoparticles increase the average length of the silicate chains and reduce their polymerization as calcium dissolves. An important decrease of the degradation in terms of porosity and amount of calcium lost has also been observed

Cementos Belíticos: caracterización de los productos de hidratación por microscopía electronica de barrido y microanálisis de rayos x

En este trabajo se presenta la caracterización microscópica de dos tipos de cementos belíticos de cenizas volantes (CBCV) y sus productos de hidratación, por medio de microscopía electrónica de barrido, microanálisis cuantitativo por energía dispersa de rayos X y difracción de rayos X. Los cementos se sintetizaron a partir de cenizas volantes de la combustión de carbón, de alto contenido en cal, empleando una ruta de síntesis de baja energía basada en la obtención de fases precursoras del cemento mediante un tratamiento hidrotermal de las cenizas en agua (CBCV-2-A) y una disolución de NaOH 1M (CBCV-2-N). La hidratación ha sido estudiada durante 180 días a la temperatura de 21º C y humedad relativa >95%. Los resultados mostraron una incorporación importante de Al en el gel C-S-H así como otros elementos minoritarios, con una composición cercana a la de la tobermorita sustituida por aluminio. El gel C-S-H formado durante la hidratación del cemento belítico (CBCV-A) es más estable durante el tiempo de hidratación que el gel formado en el caso del cemento CBCV-N. Trazas de Portlandita (Ca(OH)2) se forman durante la hidratación de estos cementos

A research of the mineralogy phases of clinker in a spanish cement using the method of Rietveldd

[ES]: Con el objetivo de implantar un método de control de calidad de forma continua en una fábrica de cementos española, para mejorar las características del producto final, se están desarrollando nuevas metodologías de caracterización. El método de Rietveld [1] se ha utilizado con éxito para analizar la composición de las fases principales del clínker. Utilizando esta metodología, se ha llevado a cabo un estudio para cuantificar adecuadamente la fase minoritaria de Cal Libre, de gran importancia en la calidad del clínker. El método ha permitido obtener unos resultados satisfactorios en muestras cuyos contenidos superan el 3% de CaO. Estos resultados son posibles combinando adecuadamente las técnicas de difracción y fluorescencia de rayos-X, así como los resultados del análisis por vía química.[EN]: In order to introduce continuously a quality control method in a Spanish cement factory to improve the final product feature new research methodology is being developed. The Rietveld method [1] has been successfully used to analyze the composition of the main phases of clinker. Using this methodologies, research has been carried out to quantize appropriately the minor phase of free lime which is extremely important in the clinker quality. This method leads to satisfactory results on samples with contents in CaO from 3%. These results are possible combining X- ray diffraction and fluorescence techniques as well as the chemical analysis data.Los autores agradecen a la empresa Tudela-Veguín las facilidades y la financiación para llevar a cabo este trabajo. Trabajo parcialmente financiado con el proyecto MAT2010-15094 y fondos FEDER.Peer reviewe

Modifications of the C–S–H gel by hydration at 40 ºC of belite cements from coal fly ash class C

The influence of the temperature on two types of hydrated fly ash belite cement (FABC) pastes were investigated at a nanoscale (1–100 nm) by measuring the specific surface area and pore-size distribution by the sorption isotherms of nitrogen gas and the BET method, and at a microscale from the pore-size distribution measured by mercury intrusion porosimetry. The two belite cements were fabricated by the hydrothermal–calcinations route of fly ash class C in NaOH 1M solution (FABC-2-N) and demineralized water (FABC-2-W). In the case of FABC-2-W, a densification of the C–S–H gel was produced at the temperature of 401C, which favored the formation of pores B3 nm in diameter leading to higher surface area values, compared with the C–S–H gel formed at 201C. At a microscale, the temperature led to an increase of capillary porosity (40.05 lm) at a later age of hydration and, consequently, a decrease of compressive mechanical strength. In the case of FABC-2-N, the densification of the gel was less evident, but the increase of capillary porosity (pores of diameter 40.05 lm) was higher. Significant direct linear quantitative correlations were found among these nanostructure characteristics of the C–S–H gel and macrostructural engineering property such as the compressive mechanical strength, for the two FABC-2-W and FABC-2-N cements under normal conditions. At 401C, the correlations were not so clear probably due to another microstructural factor such as the increase of the larger capillary porosity (40.05 lm)

Empleo de morteros ecoeficientes en las explotaciones agropecuarias

En los ambientes agrícola-ganaderos, como las explotaciones de ganado porcino, la gran producción de purín, que puede ocasionar importantes deterioros, particularmente en suelos, comederos y estructuras de almacenamiento de este subproducto. El objetivo principal de este trabajo es estudiar, en una balsa experimental de purín, procedente de una explotación integral, la durabilidad de dos morteros de cemento: un portland resistente a sulfatos y otro en el que se ha sustituido un 16% de portland por ceniza volante, ambos pertenecientes a la clase resistente 42,5. El ensayo se ha realizado durante un tiempo prolongado de 60 meses, en los que los morteros han estado sumergidos permanente en purín de cerdo, con disponibilidad de oxígeno (ambiente aerobio) y CO2. Se evalúa el comportamiento resistente y los cambios en la microestructura. Los resultados de este trabajo muestran que el mortero de CEM I tiene una porosidad total significativamente mayor, por lo que es un material más susceptible a la degradación. Se recomienda, para ambientes agrícolaganaderos, el uso de cementos con cenizas en baja proporción, ya que ofrecen excelentes prestaciones resistentes y un buen comportamiento durable, además de obtenerse a menores costes y caracterizarse por su mayor sostenibilidad

Influence of cement properties in the reaction rate and mechanical behavior of concrete with high fl y ash content

The use of fly ash (FA) as an admixture to concrete is broadly extended for two main reasons: the reduction of costs that supposes the substitution of cement and the micro structural changes motivated by the mineral admixture. Regarding this second point, there is a consensus that considers that the ash generates a more compact concrete and a reduction in the size of the pore. However, the measure in which this contributes to the pozzolanic activity or as filler is not well defined. There is also no justification to the influence of the physical parameters, fineness of the grain and free water, in its behavior. This work studies the use of FA as a partial substitute of the cement in concretes of different workability (dry and wet) and the influence in the reactivity of the ash. The concrete of dry consistency which serves as reference uses a cement dose of 250 Kg/m 3 and the concrete of fluid consistency utilized a dose of cement of 350 Kg/m 3 . Two trademark of Portland Cement Type 1 were used. The first reached the resistant class for its fineness of grain and the second one for its composition. Moreover, three doses of FA have been used, and the water/binder ratio was constant in all the mixtures. We have studied the mechanical properties and the micro-structure of the concretes by means of compressive strength tests, mercury intrusion porosimetry (MIP) and thermal analysis (TA). The results of compressive strength tests allow us to observe that concrete mixtures with cements of the same classification and similar dosage of binder do not present the same mechanical behavior. These results show that the effective water/binder ratio has a major role in the development of the mechanical properties of concrete. The study of different dosages using TA, thermo-gravimetry and differential thermal analysis, revealed that the portlandite content is not restrictive in any of the dosages studied. Again, this proves that the rheology of the material influences the reaction rate and content of hydrated cement products. We conclude that the available free water is determinant in the efficiency of pozzolanic reaction. It is so that in accordance to the availability of free water, the ashes can react as an active admixture or simply change the porous distribution. The MIP shows concretes that do not exhibit significant changes in their mechanical behavior, but have suffered significant variation in their porous structur

Durabilidad de morteros de cemento portland sometidos a ambientes agrícola-ganaderos: caso real

En los ambientes agrícola-ganaderos, los morteros de cemento Portland, son materiales ampliamente utilizados. Éstos se ven expuestos al contacto permanente con los purines del ganado con un deterioro que supone un elevado coste medioambiental y económico para las explotaciones. El objetivo principal de este trabajo es estudiar el comportamiento de los morteros sumergidos durante 60 meses en una balsa de purín de cerdo. Se han empleado cuatro tipos de cemento, un cemento Portland tradicional y tres cementos con adición de cenizas volantes. Se analiza el comportamiento resistente y los cambios microestructurales, en términos de porosidad y distribución de tamaño de poro, mediante porosimetría por intrusión de mercurio. Los resultados obtenidos indican que la utilización de cementos con características sulforresistentes no está justificada debido a que su comportamiento resistente es peor que el obtenido con cementos con bajo contenido de cenizas, y son más susceptibles a la degradación provocada por el purín. Para los ambientes agrícola-ganaderos, se aconseja utilizar morteros de cementos con cenizas volantes