RT Dissertation/Thesis T1 Cement-based composites: optimization of basalt fibers-cement matrices interfaces A1 Iorio, Morena AB The main research activity carried out within the frame of the present PhD thesis was focused on the study and development of materials to be used as plasters in the building industry and in the restoration and conservation of Cultural Heritage. In particular, the aim of this PhD thesis was to optimize the fiber-matrix interface in fiber reinforced cement-based composites through specific surface treatments of the natural basalt fibers. Therefore, the compatibility, in terms of adhesion, between chopped basalt fibers (commercial and modified in the present work) and the selected matrices (Portland cement and natural hydraulic lime) was studied to understand and define possible improvements in the final composite materials. Therefore, different surface treatments were designed on the basalt fibers, to subsequently characterize them and to study the hydrolytic degradation phenomena respectively. With this information, composite materials reinforced with different types of fibers according to their surface nature, were finally designed and characterized. The first step of the project concerned the design and characterization of chemical coatings of basalt fibers with silane coupling agents. Surface treatments were carried out after a surface pretreatment through a calcination (elimination of the sizing applied during the production process on the commercial fiber) and an activation (treatment with chlorhydric acid to regenerate silanol groups on the fiber surface) process of the commercial fibers. Subsequently, the fibers were chemically treated with different silane aqueous solutions (aminosilanes): i) γ-aminopropyltriethoxysilane, APTES; ii) γ- aminopropylmethyldiethoxysilane, APDES and iii) mixture 50% by weight of both silanes, APTES + APDES. The commercial and modified fibers were characterized in terms of structure, composition and morphology through different instrumental techniques (DRX, FT-IR, TGA, SEM and AFM). From these initial results, it was observed that the calcination process was effective to remove the commercial sizing present on the fiber surface making the surface smooth. The activation process fully removed possible residues of the initial coatings, making completely smooth the fiber surfaces. In addition, this process regenerated silanol groups allowing the grafting of aminosilanes on the fibers surface through condensation processes with formation of siloxane bonds. Through the morphological analysis of the silanized fibers, it was found that the silanization process made the surfaces rough, showing higher heterogeneity due to the presence of the organic matter deposited on the fibers. It was found that the higher the amount of triethoxysilane, APTES, used in the composition of the solution, the higher the surface heterogeneity in terms of topography. In a second phase of the thesis project, the phenomena of hydrolytic degradation of the polysiloxane coatings were studied since the siloxane bonds (-Si-O-Si-) formed with the silanols of the fibers surface and the silanols of the silane molecules, as well as those formed between the silane molecules between them, are hydrolysable bonds dependent on pH. Therefore, it is considered that the study of possible surface degradation phenomena may be useful to understand similar phenomena that could occur at the fiber-matrix interface. These studies are considered of crucial importance since, during the preparation of cement-based composite materials (matrix characterized by alkaline pH), it is necessary to mix the components with water. The hydrolytic degradation processes of the siloxane coatings were studied by monitoring the pH of the aqueous solution where the silanized fibers were immersed, by steady-state fluorescence spectroscopy. After modification with silane coupling agents, the silanized fibers were chemically labeled with a fluorescent label (fluorescein isothiocyanate, FITC) to be immersed afterwards in different aqueous solutions (pH=7 and pH=10). The study was carried out at different temperatures to study the kinetics of the process. The kinetic study allowed to obtain information about the activation energy of the three studied systems (APTES, APTES+APDES, APDES) and to evaluate the equilibrium degradation times for the different silanes. The results indicated that the hydrolytic rate of the three coatings increased in the order: APDES < APTES+APDES < APTES. It was found that the mechanism of the hydrolytic process is the same for the three studied systems and it was concluded that the rate of the hydrolytic degradation process is related to the initial concentration of siloxane bonds (-Si-O-Si-) able to be hydrolyzed. In addition, this study suggests that, in cement-based fiber-reinforced composites, the use of a polyorganosiloxane with a lower crosslinking degree, such as the APDES coating, could be the most effective strategy to resist a possible attack of water, especially in the alkaline environment characteristic of the cement matrix. Finally, composite materials reinforced with different types of fibers according to their surface nature were prepared. Mortar samples based on Portland cement and chopped basalt fibers (commercial and modified) were prepared. On the other hand, mortar samples based on natural hydraulic lime and chopped basalt fibers (commercial and modified) were also prepared. Mechanical performances of the composite materials were evaluated by three-point flexural test and compressive strength test. An analysis and subsequent discussion on the interactions and compatibility between the reinforcing agent and the matrix were done. Different characteristics of the fiber surface were considered in order to find the best conditions, in terms of preparation of materials, to obtain interfaces whose special characteristics contribute to improve the performance of the final composite materials. Therefore, a fractographic analysis on the images obtained by scanning electron microscopy (SEM) and laser and optical profilometry were performed to study the compatibility between fiber and matrix. To evaluate other possible interactions between fiber and matrix and to understand possible contributions in terms of mechanical adhesion between them, a study on the fiber surface roughness at nanoscopic scale by atomic force microscopy (AFM) was carried out. In addition, the possible contribution to the final mechanical behavior related to the porous structure of the samples was also studied through BET-BJH analysis by N2 adsorption-desorption. From these studies it was found, that, in general, the simple presence of basalt fibers as well as specific variations of the fibers surface nature, increased the mechanical performance of the materials under study compared to the reference mortars that is the materials without fibers. Finally, it was possible to conclude that, independently of the used matrix, better mechanical performances are mainly associated to the best adhesion at the fiber-matrix interface, which, in particular, is achieved in the case of mortars reinforced with basalt fibers treated with the mixture of two silanes (APTES + APDES). AB El siguiente proyecto de tesis doctoral se ha centrado en el estudio y desarrollo de materiales con potencial uso como revestimientos en el sector de la construcción y en la restauración de edificios históricos de gran interés cultural. En particular, como objetivo principal de la investigación se pretendió optimizar la interfase fibra-matriz de materiales compuestos reforzados con fibra de matriz cerámica (cementosa) a través de tratamientos superficiales de fibras naturales de basalto. Para ello, se estudió, en términos de adhesión, la compatibilidad existente entre las fibras cortas de basalto (comerciales y modificadas en el presente trabajo) y las matrices seleccionadas (cemento Portland y cal hidráulica natural) con el fin de comprender y definir posibles mejoras en las prestaciones o comportamiento en servicio final del material compuesto. Por ello, se diseñaron diferentes tratamientos superficiales sobre las fibras de basalto, para posteriormente caracterizarlos y estudiar fenómenos de degradación hidrolítica respectivamente. Con esta información finalmente se diseñaron y caracterizaron los materiales compuestos reforzados con los diferentes tipos de fibras según su naturaleza superficial. La primera fase del proyecto consistió en el diseño y caracterización de recubrimientos químicos de fibras de basalto con agentes de acomplamiento silano. Los tratamientos superficiales se realizaron después de un tratamiento previo por medio de un proceso de calcinación (eliminación del ensimaje superficial aplicado durante la fase producción en las fibras comerciales) y activación (tratamiento con ácido clorhídrico para promover la regeneración de grupos silanol de la superficie) de las fibras comerciales. Posteriormente, las fibras se trataron químicamente con diferentes disoluciones acuosas de agentes de acoplamiento silano (aminosilanos): i) γ-aminopropiltrietoxisilano, APTES; ii) γ-aminopropilmetildietoxisilano, APDES y iii) mezcla al 50% en peso de ambos silanos, APTES + APDES. Las fibras comerciales y modificadas se caracterizaron en términos de su estructura, composición y morfología a través de diferentes técnicas instrumentales (DRX, FT-IR, TGA, SEM y AFM). A partir de estos resultados iniciales, se observó que el proceso de calcinación fue eficaz para eliminar el ensimaje presente en la fibra comercial y suavizar su superficie. El siguiente proceso de activación eliminaba completamente los posibles residuos de recubrimientos iniciales, consiguiéndose así superficies de fibras completamente lisas. Además, este proceso favoreció la regeneración de grupos silanol que fueron fundamentales para el injerto de los aminosilanos en la superficie de las fibras a través de procesos de condensación con formación de enlaces siloxano. Gracias al análisis morfológico de las fibras silanizadas se encontró que el proceso de silanización generaba superficies en las fibras más rugosas mostrando mayor heterogeneidad debida a la presencia de la sustancia organica depositada sobre las fibras. Se encontró que cuanto mayor era la cantidad de trietoxisilano, APTES, utilizado en la composición de la solución, mayor era la heterogenidad superficial en términos topográficos. En una segunda fase del proyecto de tesis, se estudiaron los fenómenos de degradación hidrolítica de los recubrimientos de base organosiloxánica ya que los enlaces de siloxano (-Si-O-Si-) que se forman entre los silanoles de la superficie de las fibras y los silanoles de las moléculas de silano, así como los formados entre las propias moléculas de silano son enlaces hydrolizables dependientes del pH. Por lo tanto, el estudio de posibles fenómenos de degradación superficiales se piensa que podrían extrapolarse a fenómenos similares que pudieran ocurrir en la interfase fibra-matriz. Este tipo de estudios se considera que son de fundamental importancia pues durante la preparación de los materiales compuestos de matriz de cemento (matriz de pH alcalino) se necesita la mezcla de los componentes con agua. Mediante la utilización de espectroscopía de fluorescencia de estado estacionario se estudiaron los procesos de degradación hidrolítica de los recubrimientos siloxánicos controlando el pH de la disolución acuosa en la que se sumergieron las fibras silanizadas. Para ello, las fibras después de haber sido modificadas con los agentes de acoplamiento de silano, se marcaron químicamente con una especie fluorescente (isotiocianato de fluoresceína, FITC) y posteriormente se sumergieron en disoluciones acuosas a pH controlado (pH=7 y pH=10). El estudio se llevó a cabo a diferentes temperaturas para estudiar la cinética del proceso. El estudio cinético permitió obtener información sobre la energía de activación de los tres sistemas estudiados (APTES, APTES+APDES, APDES) y estimar los tiempos de degradación hidrolítica en el equilibrio para los diferentes silanos. Los resultados obtenidos indicaron que la velocidades de degradación de los tres recubrimientos aumenta según el orden: APDES < APTES+APDES