APPLICATION OF GRAPHENE OXIDE IN CEMENTITIOUS COMPOSITES FOR CEMENT CONTENT REDUCTION

oai:ojs.journaluts.emnuvens.com.br:article/6Cement production is responsible for 5% of CO2 emissions worldwide. The concern about the pollution derived from the construction industry has brought attention to the need of developing more sustainable construction materials and processes. Admixtures based on nanometric graphene oxide have the potential to enhance mechanical properties and durability of cementitious composites. In this context, an experimental program was conducted to evaluate how the addition of graphene oxide may be used to reduce cement content in concretes, maintaining the same mechanical properties of conventional concretes (control matrices, with no graphene oxide additions). Kinetics of hydration of cement pastes was evaluated by isothermal calorimetry, phase evolution during hydration was determined by X-ray diffraction coupled with quantitative Rietveld analysis and mechanical properties were evaluated by compressive strength. Results indicate that graphene oxide additions provide a faster hydration rate until 24 h and generate a larger amount of C-S-H gel, increasing mechanical strength of the matrix. By the addition of graphene oxide dispersion (0.4% of solid content) at 0.02% by cement weight, cement content reductions of up to 15% may be achieved, maintaining the same compressive strength as the control matrices. From this research, a reduction in cement content to obtain more sustainable construction materials and processes may be achieved

Accelerated cementitious matrices : hydration, microstructure and mechanical strength

Sprayed concrete is widely used as structural support for the stabilization of tunnel walls and underground constructions. The performance of sprayed cementitious matrices containing accelerators is strongly related to their mechanical properties at early and late ages. In practice, this is the main parameter that governs their mix design and applicability. Mechanical strength development results from the combination of several factors associated with mix composition, application method and microstructure of the matrix. The compatibility between cement and accelerator is one of the most important parameters that control kinetics of hydration and the rate of mechanical strength gain. The spraying process also needs to be taken into account, since it leads to faster reaction rates and directly affects the porosity of the matrix. Although the sprayed concrete technology advanced considerably over the past years, questions continue to arise regarding its performance, efficient use and optimized mix design. One of the main subjects that requires further research is the characterization of the early age hydration behavior of accelerated cementitious matrices. The influence of accelerator reactions on the mechanical properties of the matrix at short and long term and the influence of spraying on kinetics of hydration also need to be evaluated. In this context, a study covering these demands is proposed in this doctoral thesis. The first subject contemplates the characterization of the kinetics and mechanisms of hydration and the microstructure development of accelerated cement pastes. The early age hydration behavior of different mix composition was analyzed and compared. Results obtained allowed the elucidation of the main chemical processes occurring during accelerator reaction and further cement hydration and their influence on the microstructure of the matrix. The second subject comprises the parametrization of the early age hydration behavior of cement pastes containing accelerators. The main chemical properties of cements and accelerators were evaluated, explaining their influence on accelerator reactivity and on cement hydration. By doing so, the mix design of accelerated cementitious matrices may be optimized and unpredictable hydration reactions and their consequences may be avoided. The third subject deals with the characterization of the setting and hardening processes of accelerated cementitious matrices by ultrasound measurements. This subject was proposed because the current standard methods to characterize early strength evolution are discontinuous and have a limited application range. By using the ultrasound technique, a more complete characterization of the evolution of mechanical properties of the matrix was assessed. The fourth subject aims at evaluating how spraying affects accelerator reactivity and further cement hydration. A small-scale spraying equipment was used to simulate real life applications of sprayed concrete. A significant influence of the mixing process on reaction rates and on microstructure of the matrix was observed. The last subject of this thesis focuses on the evaluation of how accelerated hydration reactions influence the mechanical strength development of the matrix. The main chemical processes influencing the mechanical properties at early and late ages were determined. Results provided the characterization of the mechanical performance of sprayed materials based on their chemical composition, in order to improve their mix design and quality control.El hormigón proyectado es ampliamente utilizado como soporte estructural para la estabilización de túneles y construcciones subterráneas. El desempeño de mezclas cementicias que contienen acelerantes está fuertemente relacionado con sus propiedades mecánicas a cortas y largas edades. En la práctica, este es el principal parámetro que gobierna su diseño y aplicabilidad. El desarrollo de resistencia mecánica resulta de la combinación de diversos factores asociados al diseño de la mezcla, método de aplicación y microestructura de la matriz. La compatibilidad entre cemento y acelerante es uno de los parámetros más importantes que controla la cinética de hidratación y la velocidad de ganancia de resistencia mecánica. El proceso de proyección también debe de ser considerado, puesto que la velocidad de mezcla influye en la cinética de reacción y afecta la porosidad de la matriz. A pesar de que la tecnología del hormigón proyectado haya avanzado considerablemente durante los años pasados, cuestiones relacionadas a su desempeño, uso eficiente y diseño optimizado siguen surgiendo. Uno de los temas que requiere investigación es la caracterización de la hidratación a cortas edades de mezclas cementicias aceleradas. La influencia de las reacciones causadas por el acelerante en las propiedades mecánicas de la matriz a corto y largo plazo y la influencia de la proyección en la cinética de hidratación también necesitan ser evaluados. En este contexto, un estudio que engloba estas demandas es propuesto en esta tesis doctoral. El primer tema planteado es la caracterización de la cinética, de los mecanismos de hidratación y de la microestructura de pastas de cemento aceleradas. La hidratación a cortas edades de diferentes mezclas cementicias fue analizada y comparada. Los resultados obtenidos permitieron la elucidación de los procesos químicos que ocurren durante la reacción del acelerante y en la posterior hidratación del cemento, analizando su influencia en la microestructura de la matriz. El segundo tema comprende la parametrización de la hidratación a cortas edades de pastas de cemento con acelerantes. Las principales propiedades químicas de los cementos y acelerantes fueron evaluadas, justificando su influencia en la reactividad del acelerante y en la hidratación del cemento. Por lo tanto, el diseño de mezclas aceleradas puede ser optimizado y reacciones de hidratación imprevistas y sus consecuencias pueden ser evitadas. El tercer tema trata de la caracterización de los procesos de fraguado y endurecimiento de mezclas cementicias aceleradas por medidas de ultrasonido. Este tema fue propuesto porque los métodos actuales para caracterizar la evolución de resistencia a corta edad son discontinuos y tienen un rango limitado para su realización. Utilizando la técnica de ultrasonidos, se puede obtener una caracterización más completa del desarrollo de propiedades mecánicas de la matriz. El cuarto tema tiene como objetivo evaluar como el proceso de proyección afecta la reactividad del acelerante y la posterior hidratación del cemento. Un equipo de proyección de pequeña escala fue utilizado para simular aplicaciones de hormigón proyectado en escala real. Una influencia significativa del método de mezcla en las velocidades de reacción y en la microestructura de la matriz fue observada. El último tema de esta tesis enfoca la evaluación de como reacciones de hidratación aceleradas influyen en el desarrollo de resistencia mecánica de la matriz. Los procesos químicos principales que afectan las propiedades mecánicas a cortas y largas edades fueron determinados. Los resultados obtenidos proporcionan la caracterización del desempeño mecánico de materiales proyectados basado en su composición química, con el objetivo de optimizar su diseño y controlPostprint (published version

Comparative analysis of test methods for the characterization of the mechanical behavior of fiber-reinforced concrete.

O emprego de fibras para reforço de concreto é uma técnica bastante conhecida e estudada mundialmente. As mais conhecidas e estudadas são as fibras de aço, embora utilização das macrofibras poliméricas esteja amplamente difundida no mercado brasileiro. Entretanto, a caracterização do comportamento mecânico de concretos reforçados com essas macrofibras ainda deve ser mais bem avaliada. Dentre os diversos métodos utilizados para essa finalidade, os ensaios realizados com sistema fechado de controle de velocidade de deslocamento fornecem resultados mais confiáveis. A principal vantagem conferida por esse sistema está na avaliação desses compósitos em baixo nível de fissuração com maior acurácia. O desempenho desses compósitos foi estudado segundo as normas ASTM C1609 (2010), ASTM C1399 (2010) e EN 14651 (2007) e o método JSE-SF4 (1984). Os quatro procedimentos prescrevem ensaios de flexão de corpos-de-prova prismáticos para determinação da resistência residual pós-fissuração e da tenacidade. Os procedimentos americanos e o método japonês prescrevem configuração de aplicação de carga por dois cutelos superiores, posicionados sobre o terço médio do corpo-de-prova. Já a configuração do método europeu é de aplicação de carga por apenas um cutelo superior, centralizado em relação ao vão de ensaio, com corpos-de-prova com entalhe na face inferior. Foram estudadas uma macrofibra de polipropileno, nas dosagens de 0,22, 0,33, 0,50, 0,66, 0,82 e 1,0% em volume em três matrizes de concreto, com resistências médias à compressão de 30, 35 e 40MPa, e uma fibra de aço, nas dosagens de 0,19, 0,32 e 0,45% em volume em uma matriz de resistência média à compressão de 35MPa. Foi observado que a utilização da velocidade de deslocamento do corpo-de-prova como parâmetro de controle do ensaio forneceu boas condições de avaliação do compósito, devido à redução da instabilidade pós-pico. Com isso, a determinação da resistência residual do compósito nos níveis iniciais de deslocamento e fissuração da matriz foi mais bem caracterizada. Pela análise e comparação dos resultados, foram formuladas equações para estabelecer correlações entre os diferentes métodos de ensaio. Com o modelo de regressão utilizado na análise estatística foi possível verificar que a resistência à compressão da matriz, o tipo e o teor de fibra são as variáveis independentes que mais influenciam os resultados de resistência residual. Foi necessário estabelecer correlações para cada tipo de fibra separadamente, pois o comportamento de slip-softening ou de slip-hardening influencia as funções obtidas.The use of fibers for concrete reinforcing is a very common practice, used all over the world. Steel fibers are the most common and studied, although synthetic macrofibers are in very common use in the Brazilian market. However, the evaluation of the mechanical behavior of concrete reinforced with this type of fiber must be evaluated. A broad range of tests is available for this purpose. Tests performed under closed-loop displacement control provide more reliable results. The main advantage of that system is in the evaluation of the composite at low levels of crack opening with higher accuracy. In this study, the performance of these composites was examined according to the standard test methods ASTM C1609 (2010), ASTM C1399 (2010), EN 14651 (2007) and JSCE-SF4 (1984). These four methods prescribe flexural tests in prismatic specimens for the determination of post-crack residual strength and toughness. The American and the Japanese test methods prescribe four-point bending tests, while the European test method prescribes three-point bending tests and specimens with a notch in the bottom face. Two fibers were analyzed: a polypropylene macrofiber, used in the dosages of 0.22, 0.33, 0.50, 0.66, 0.82 and 1.0 in volume percentage, in three concrete matrices with average compressive strengths of 30, 35 and 40MPa, and a steel fiber, in the dosages of 0.19, 0.32 and 0.45 in volume percentage, in one concrete matrix of average compressive strength of 35MPa. It was observed that the use of the net displacement of the specimen as the parameter to control the load application provided good conditions for the evaluation of the mechanical behavior of the composite, due to the reduction of post-peak instability. Therefore, the determination of the residual strength of the composite in the initial levels of displacement and cracking of the matrix was better achieved. From the comparative analysis of the results, equations were developed to establish a correlation among the test methods. Based on the model used in the statistical analysis, the main independent variables that influence the results of residual strength are the compressive strength of the concrete matrix, the type and the content of the fiber. The equations were obtained separately according to the fiber type, because the behavior of slip-hardening or slip-softening influence the correlations

Relation between ultrasound measurements and phase evolution in accelerated cementitious matrices

This paper focuses on the characterization of setting and hardening of accelerated cementitious matrices by ultrasound propagation velocity, correlating these processes with chemical parameters and the phase evolution obtained by in situ XRD. Evolution of temperature and determination of setting times complemented this analysis. The technique employed provided a continuous monitoring of the setting and hardening of the hydrating matrix and was susceptible to changes in accelerator reactivity and phase composition. Results showed that ettringite formed by accelerator reaction improves the solid-phase interconnectivity and increases initial ultrasound velocity. P-wave propagation during the acceleration period is directly proportional to alite and C3A degrees of hydration. The influence of AFm phases to increase ultrasound velocity is stronger than ettringite and C-S-H. Based on an extensive statistical analysis, multivariate linear regressions were established between ultrasound velocity and the main chemical properties influencing its evolution, leading to a better comprehension of how these parameters are related.Peer ReviewedPostprint (author's final draft

Effect of gypsum content in sprayed cementitious matrices: early age hydration and mechanical properties

Sprayed materials must present short setting times and a fast early strength development for safety and productivity reasons. In order to improve these characteristics, the construction industry has focused on the development of new formulations of accelerators. Research and improvement of other components of the mix, such as cement or additions, have not advanced at the same rate despite being also crucial for the reaction kinetics. The objective of this work is to evaluate the influence of gypsum content on the hydration and mechanical strength development in sprayed mixes. Sprayed pastes and mortars were prepared with one type of cement, two types of accelerators and different gypsum contents. Kinetics, mechanisms of hydration and mechanical properties were evaluated. Results showed a better performance in sprayed mixes that contain ideal doses of gypsum. Such approach provides valuable information for the improvement of the formulation of cement used in sprayed concrete applications.Peer Reviewe

Effect of gypsum content in sprayed cementitious matrices: early age hydration and mechanical properties

Sprayed materials must present short setting times and a fast early strength development for safety and productivity reasons. In order to improve these characteristics, the construction industry has focused on the development of new formulations of accelerators. Research and improvement of other components of the mix, such as cement or additions, have not advanced at the same rate despite being also crucial for the reaction kinetics. The objective of this work is to evaluate the influence of gypsum content on the hydration and mechanical strength development in sprayed mixes. Sprayed pastes and mortars were prepared with one type of cement, two types of accelerators and different gypsum contents. Kinetics, mechanisms of hydration and mechanical properties were evaluated. Results showed a better performance in sprayed mixes that contain ideal doses of gypsum. Such approach provides valuable information for the improvement of the formulation of cement used in sprayed concrete applications.Peer Reviewe

Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed concrete

The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42- ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical.compositions are explained.Peer Reviewe

Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed concrete

The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42- ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical.compositions are explained.Peer ReviewedPostprint (author's final draft