Effect of micro-encapsulated phase change materials on the mechanical properties of concrete

[SPA] Esta tesis doctoral está centrada en el estudio de las propiedades mecánicas de materiales de construcción basados en geopolímeros compuestos por cenizas volantes y escoria a los que se le incorporan materiales de cambios de fase microencapsulados (MPCM por sus siglas en inglés). Las investigaciones realizadas se enmarcan en un proyecto financiado por el Consejo de Investigación de Noruega. Se ha evaluado el efecto de estas microcápsulas tanto en estado sólido como líquido en las propiedades mecánicas y la microestructura de estos hormigones basados en geopolímeros (GPC, por sus siglas en inglés) y cemento Portland (PCC, por sus siglas en inglés). Se prepararon muestras de GPC y PCC con diferentes cantidades de MPCM, realizando curados a 20 y 40 ºC. Se registró un descenso de la resistencia a la compresión en ambos materiales, pero manteniendo valores suficientemente elevados para su uso como materiales de construcción. Las propiedades mecánicas de GPC no se vieron afectadas por la adición de MPCM ni en estado sólido (20 ºC), ni líquido (40 ºC); aunque en el caso del cemento Portland se pudo observar que las microcápsulas fundidas sí que provocaban un gran descenso en la resistencia mecánica. Se empleó la técnica de tomografía de rayos X para determinar el efecto de la porosidad de las microcápsulas tanto en las muestras de GPC como PCC. Mediante microscopía electrónica de barrido se pudo observar la formación de oquedades de aire entre las microcápsulas y la matriz de hormigón. Por otro lado, se ha desarrollado un método de diseño para las mezclas de GPC con el objetivo de maximizar la resistencia a la compresión tras la adición de MPCM. Se han utilizado dos tipos diferentes de microcápsulas para una mejor evaluación de su efecto en las propiedades de GPC. Se ha podido observar que el tiempo de fraguado de las pastas basadas en geopolímero depende tanto de la cantidad de agua adsorbida en la superficie de las microcápsulas, como de la viscosidad de las muestras, y posiblemente también de su calor latente. Al aumentar la concentración de MPCM se pudo observar que el tiempo inicial de fraguado aumentaba mientras que el tiempo final disminuía. Además, la adición de MPCM resultó disminuir tanto la trabajabilidadcomo la resistencia a la compresión de GPC. Estos efectos eran más pronunciados para el caso de MPCM con estructuras aglomeradas y que presentan en su superficiegrupos polares que en el caso de microcápsulas con menor cantidad de aglomeración, una estructura más esférica y una superficie completamente hidrofóbica. Aunque la adición de MPCM reduce la resistencia a la compresión de GPC, después de 28 días de curado, el desempeño mecánico fue mayor que en el caso del cemento Portland. Los estudios de SEM y tomografía de rayos X sugieren que la aglomeración de microcápsulas, los espacios vacíos generados con la matriz de hormigón, el incremento de aire ocluido y la rotura de microcápsulas al aplicar esfuerzos provocan la disminución de la resistencia a la compresión de GPC. También se ha realizado un estudio del efecto de las condiciones de congelación en las propiedades mecánicas de GPC y PCC con diferentes contenidos de MPCM. Cuando las microcápsulas se añaden al hormigón, el porcentaje de pérdida de masa tras los ciclos de congelación y descongelación se ve incrementado. La adición de MPCM proporciona una excelente durabilidad a la acción de estos ciclos con una mínima repercusión en la resistencia a la compresión. Se han realizado una serie de estudios microestructurales que han revelado que estos ciclos térmicos provocan un deterioro que se puede atribuir a la aparición de microgrietas en las zonas de interfase entre la pasta/agregados y la pasta/MPCM, y también a la formación de cristales de etringita. El efecto de la temperatura en el tiempo de fraguado de estas pastas también ha sido evaluado. A 0 ºC, el tiempo inicial de fraguado de las pastas basadas en cemento Portland se ven retrasados debido a la acción de la baja temperatura y la elevada viscosidad de MPCM. Sin embargo, las pastas de geopolímero muestran un tiempo inicial más corto debido a la separación de fases de la solución alcalina a bajas temperaturas. El tiempo final disminuye con la concentración de MPCM tanto para las pastas de GPC como para las de cemento Portland. Se ha evaluado también el efecto de dos tipos diferentes de MPCM en estado líquido (40 ºC) en las propiedades mecánicas y la microestructura de GPC y PCC. A esta temperatura, tanto el tiempo final como inicial de fraguado disminuyen hasta valores muy bajos debido a la aceleración de la reacción de geopolimerización. A 40 ºC, la resistencia a la compresión de ambos materiales es lo suficientemente elevada como para mantener sus aplicaciones hormigón estructural con la adición de las microcápsulas. El estudio de la microestructura reveló un aumento de los huecos de aire presentes en GPC y PCC cuando la temperatura de curado aumenta de 20 a 40 ºC debido a la aceleración de la reacción.[ENG] This PhD Thesis focuses on the details of development of the mechanical properties of fly ash/slag geopolymer concrete incorporated with microcapsules for construction applications in the framework of a wide research program funded by the Research Council of Norway. The effect of MPCM in solid and liquid states on the mechanical properties and microstructure of geopolymer concrete (GPC) and Portland cement concrete (PCC) was investigated. GPC and PCC containing different amounts of MPCM were prepared and cured at both 20 °C and 40 °C. While the compressive strength of both GPC and PCC was found to decrease with the addition of MPCM, it is still sufficiently high for construction purposes. Whether the PCM is in solid (20 °C) or liquid (40 °C) state did not significantly affect the mechanical properties of GPC, while melting the PCM were found to reduce the strength of PCC. X-ray tomography imaging was utilized to examine the effect of MPCM on the porosity of the samples. SEM imaging revealed that air gaps were formed between the microcapsules and the surrounding concrete matrix. A mix design procedure for GPC was developed in order to maintain a high compressive strength after adding MPCM. Two types of MPCM were used for a better understanding the effect of different MPCMs on the properties of the GPC. The setting time of geopolymer pastes was found to depend on both the amount of water adsorbed by the microcapsules, the viscosities of the samples, and possibly the latent heat. Accordingly, the initial setting time increased and the final setting time decreased with MPCM concentration. The addition of MPCM was found to reduce both the slump and the compressive strength of GPC. These effects were more pronounced for the MPCM that form agglomerated structures and has a surface containing some polar groups, than for the more spherically shaped and less agglomerated MPCM with a hydrophobic surface. Although the addition of MPCM reduced the compressive strength of GPC, the mechanical performance was higher than that of PCC after 28 days of curing. A combination of SEM imaging and X-ray-tomography suggested that MPCM agglomeration, gaps between MPCM and the concrete matrix, an increased amount of entrapped air, and microcapsules that break under stress might contribute to the reduced compressive strength of GPC. The effect of frost conditions on the physical and mechanical properties of GPC and PCC containing different MPCM was examined. When MPCM was added to concrete, the percentage of mass loss after the freeze-thaw cycles increased. The addition of MPCM provided an excellent resistance against freeze-thaw cycles with a minor reduction of the compressive strength. Microstructural studies revealed that the freeze-thaw induced concrete deterioration could be contributed to microcracks appearing in the poor interfacial transition zones between paste/aggregate and paste/MPCM, and to the formation of ettringite crystals. The effect of temperature on the setting times of the corresponding pastes was also evaluated. At 0 °C, the initial setting time of Portland cement pastes had a delay due to the low temperature and the high viscosity of MPCM. However, for geopolymer pastes, the initial setting time became shorter due to phase separation of the alkaline solution at low temperatures. The final setting time decreased with MPCM concentration for both geopolymer and Portland cement pastes. The effect of two different MPCMs in the liquid state (at 40 °C) on the mechanical properties and microstructure of GPC and PCC was studied. At 40 °C, the initial and final setting times were very fast due to the acceleration of the geopolymerization reaction. At 40 °C, the compressive strength of both GPC and PCC with MPCM addition is sufficiently high for building applications. Microstructural studies showed that a higher number of air voids were present in GPC and PCC samples cured at 40 °C than at 20 °C. This is due to acceleration of the reaction rates.Escuela Internacional de Doctorado de la Universidad Politécnica de CartagenaUniversidad Politécnica de CartagenaPrograma de Doctorado en Tecnologías Industriales por la Universidad Politécnica de Cartagen

Investigation of severe lunar environmental conditions on the physical and mechanical properties of lunar regolith geopolymers

3D-printing of geopolymers produced from lunar regolith is an interesting option for space in situ habitats. In this study, the influence of the severe lunar environmental conditions such as extreme temperature variations and vacuum on the physical and mechanical properties of lunar regolith geopolymers were investigated. Additionally, the effect of different amounts of urea as a geopolymer superplasticizer was evaluated. Utilization of urea was found to reduce the water needed to reach the same workability by up to 32%. Extrudability tests showed that mixtures containing 3 wt.% urea could be continuously extruded, and built up into a five layer structure without any noticeable deformation. Addition of urea decreased the compressive strength after exposure to the temperature variations of one lunar day–and–night cycle during curing. However, urea can prevent concrete degradation after the lunar cycle by increasing the amounts of air voids. X-ray tomography showed that the porosity became higher when urea was added to the samples, and increased markedly when the samples were cured in vacuum.publishedVersio

Utilization of urea as an accessible superplasticizer on the moon for lunar geopolymer mixtures

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Effect of freeze-thaw cycles on the mechanical behavior of geopolymer concrete and Portland cement concrete containing micro-encapsulated phase change materials

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Effect of temperature on geopolymer and Portland cement composites modified with Micro-encapsulated Phase Change materials

Abstract To reduce pollution and global warming, the energy consumption needs to be decreased. Incorporation of Phase Change Materials (PCMs) into building materials can help lower the energy needed to cool and warm buildings, while keeping the indoor temperature at a comfortable level. However, incorporation of PCMs into construction materials alter their performance. In this study, the effect of temperature and addition of two different Micro-encapsulated Phase Change Materials (MPCM) to geopolymer concrete (GPC) and Portland cement concrete (PCC) and pastes was investigated. The samples were examined both below (20 °C) and above (40 °C) the melting points of the PCMs. While the MPCM is not damaged by the alkaline solution, a few microcapsules are broken during the mixing process. Isothermal calorimetry shows that MPCM addition slows down the reaction rate of both geopolymer and Portland cement paste. The setting times were faster when the temperature was increased. The mechanical properties are reduced when MPCM is added to GPC and PCC, although the compressive strength is adequate for building applications. Microstructural studies show more uniform and undamaged edges in the shell-concrete matrix transition zone of GPC than PCC. The samples cured at 40 °C exhibits more air voids in both GPC and PCC than at 20 °C

The effect of microencapsulated phase change materials on the rheology of geopolymer and Portland cement mortars

The effect of microencapsulated phase‐change materials (MPCM) on the rheological properties of pre‐set geopolymer and Portland cement mortars was examined. Microcapsules with hydrophilic and hydrophobic shells were compared. The shear rate dependency of the viscosities fitted well to a double Carreau model. The zero shear viscosities are higher for geopolymer mortar, illustrating poorer workability. The time evolution of the viscosities was explored at shear rates of 1 and 10 s−1. New empirical equations were developed to quantify the time‐dependent viscosity changes. The highest shear rate disrupted the buildup of the mortar structures much more than the lower shear rate. Microcapsules with a hydrophobic shell affect the rheological properties much less than the microcapsules with a hydrophilic shell, due to the higher water adsorption onto the hydrophilic microcapsules. Shear forces was found to break down the initial structures within geopolymer mortars more easily than for Portland cement mortars, while the geopolymer reaction products are able to withstand shear forces better than Portland cement hydration products. Initially, the viscosity of geopolymer mortars increases relatively slowly during due to formation of geopolymer precursors; at longer times, there is a steeper viscosity rise caused by the development of a 3D‐geopolymer network. Disruption of agglomerates causes the viscosities of portland cement mortars to decrease during the first few minutes, after which the hydration process (increasing viscosities) competes with shear‐induced disruption of the structures (decreasing viscosities), resulting in a complex viscosity behavior.publishedVersio

Microencapsulated phase change materials for enhancing the thermal performance of Portland cement concrete and geopolymer concrete for passive building applications

Concretes with a high thermal energy storage capacity were fabricated by mixing microencapsulated phase change materials (MPCM) into Portland cement concrete (PCC) and geopolymer concrete (GPC). The effect of MPCM on thermal performance and compressive strength of PCC and GPC were investigated. It was found that the replacement of sand by MPCM resulted in lower thermal conductivity and higher thermal energy storage, while the specific heat capacity of concrete remained practically stable when the phase change material (PCM) was in the liquid or solid phase. Furthermore, the thermal conductivity of GPC as function of MPCM concentration was reduced at a higher rate than that of PCC. The power consumption needed to stabilize a simulated indoor temperature of 23°C was reduced after the addition of MPCM. GPC exhibited better energy saving properties than PCC at the same conditions. A significant loss in compressive strength was observed due to the addition of MPCM to concrete. However, the compressive strength still satisfies the mechanical European regulation (EN 206-1, compressive strength class C20/25) for concrete applications. Finally, MPCM-concrete provided a good thermal stability after subjecting the samples to 100 thermal cycles at high heating/cooling rates.publishedVersio

Physical and mechanical properties of fly ash and slag geopolymer concrete containing different types of micro-encapsulated phase change materials

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Mechanical properties and microscale changes of geopolymer concrete and Portland cement concrete containing micro-encapsulated phase change materials

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Influence of microcapsule size and shell polarity on thermal and mechanical properties of thermoregulating geopolymer concrete for passive building applications

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