Cement-based composites: optimization of basalt fibers-cement matrices interfaces

Mención Internacional en el título de doctorThe 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).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 <APTES. Se ha encontrado que el mecanismo del proceso de degradación es el mismo para los tres sistemas estudiados, por lo que se concluye que la velocidad de hidrólisis depende de la concentración inicial de los enlaces de siloxano (-Si-O-Si-) capaces de ser hidrolizados. Además, este estudio sugiere que, en compuestos a base cementosa reforzada con fibras, el uso de un poliórganosiloxano con un bajo grado de reticulación, como el correspondiente a un recubrimiento realizado con APDES, podría ser la estrategia más eficaz para resistir un posible ataque de agua, especialmente en el ambiente alcalino característico de la matriz de cemento. Finalmente, se prepararon los materiales compuestos reforzados con los distintos tipos de fibras según su naturaleza superficial. Por un lado, se prepararon morteros a base de cemento Portland y fibras cortas de basalto (comerciales y modificadas). Por otro lado, se prepararon morteros a base de cal hidráulica natural y fibras cortas de basalto (comerciales y modificadas). Se investigaron las propiedades mecánicas de los materiales compuestos a través de ensayos de flexión en tres puntos y de compresión. Los resultados obtenidos permitieron realizar un análisis y posterior discusión en función de las interacciones y compatibilidad entre el agente de refuerzo y la matriz considerando diferentes características de las superficie de las fibras intentando encontrar las mejores condiciones, en términos de preparación de materiales, para conseguir interfases cuyas características especiales contribuyan a mejorar las prestaciones de los materiales compuestos finales. En este sentido, además, se llevó a cabo un análisis fractográfico de imágenes obtenidas por microscopía electrónica de barrido (SEM) y perfilometrías láser y óptica respectivamente para estudiar la compatibilidad entre la fibra y la matriz. Para evaluar otras posibles interacciones entre la fibra y la matriz, se realizó un estudio de rugosidad de las superficies de las fibras a escala nanosópica por microscopía de fuerza atómica (AFM) y así entender la posible contribución de adhesión mecánica entre las fibras y la matriz. Además, también se estudió la posible contribución al comportamiento mecánico final de la estructura porosa de las muestras, utilizando para ello análisis BET-BJH por adsorción-desorción de N2. De estos estudios se encontró que la simple presencia de fibras de basalto junto con variaciones específicas de la naturaleza superficial de las mismas en general aumentan las prestaciones mecánicas de los materiales bajo estudio en comparación con los morteros de referencia, es decir, materiales sin fibras. Finalmente, se pudo concluir, independientemente de la matriz utilizada, que las mejores prestaciones mecánicas se asocian fundamentalmente a mejor adhesión en la interfase fibra-matriz que en particular se consigue en el caso de morteros reforzados con fibras de basalto tratadas con la mezcla de los dos silanos (APTES+APDES).Programa Oficial de Doctorado en Ciencia e Ingeniería de MaterialesPresidente: Miguel Ángel Martínez Casanova.- Secretario: Assunta Marrocchi.- Vocal: Cristina Marieta Gorrit

Reinforcement-matrix interactions and their consequences on the mechanical behavior of basalt fibers-cement composites

In order to prepare basalt fibers-reinforced cement-based mortars with higher compatibility between reinforcement and matrix, basalt fibers with new surface treatments (sizing) were studied looking for enhanced interaction at the interphase between basalt fibers and cement matrix. As-received, calcinated, activated and silanized (by three silane aqueous solutions: i) aminopropyltriethoxysilane, APTES; ii) ¿-aminopropylmethyldiethoxysilane, APDES and iii) a mixture APTES APDES 50% by weight) basalt fibers were dispersed in Portland cement matrix. Performances of the composites were evaluated by mechanical tests. Final correlation between the fibers surface characteristics and mechanical performance was carried out considering the induced microstructural changes and adhesion at the interface. Fractographic analysis by SEM and laser and optical profilometry were performed. A clear improvement in mechanical properties was obtained when basalt fibers were dispersed in cement matrix. Results suggest that better behavior is achieved when basalt fibers modified with a complex mixture of silanes are dispersed in cement matrix.This work was financially supported by the Projects MAT2014-59116-C2 (Ministerio de Economía y Competitividad); 2012/00130/004 (Fondos de Investigación de Fco. Javier GonzalezBenito, política de reinversión de costes generales, Universidad Carlos III de Madrid) and 2011/00287/002 (Acción Estratégica en Materiales Compuestos Poliméricos e Interfases, Universidad Carlos III de Madrid). The research was financially supported also by the Project Bando per il Finanzia- mento di Progetti di Ricerca Congiunti per la Mobilit`a all Estero di Studenti di Dottorato prot. n 0051266 (Universit`a degli Studi di Roma, La Sapienza) in the frame the PhD Thesis of Morena Iorio. Finally, the authors would like to thank the group In-service Material Performance (Universidad Carlos III de Madrid) for supporting the project in the mechanical tests

Conformational changes on PMMA induced by the presence of TiO2 nanoparticles and the processing by Solution Blow Spinning

Poly(methyl methacrylate) (PMMA) films filled with titanium oxide (TiO2) nanoparticles were prepared by solution blow spinning (SBS). The influence of the presence of nanoparticles (up to 10% by weight) in the physicochemical properties of the material was studied, focusing on the morphology, structure, and thermal properties, by scanning electron microscopy (SEM), Fourier-transformed infrared spectroscopy (FTIR), thermogravimetry (TGA), and differential scanning calorimetry (DSC). It was demonstrated that SBS allows obtaining PMMA/TiO2 nanocomposites with a relatively high amount of nanoparticles uniformly dispersed within the polymeric matrix, varying the surface characteristics of the films according to the amount of TiO2. The results indicate that the flow of the macromolecules is somewhat forced by the SBS process, and that specific interactions between the ester group of the MMA and the surface of the nanoparticles induce a preferential conformation of the functional group with respect to the PMMA backbone, at least at low loads of nanopartciles, conditions in which the self-aggregation of the nanofiller takes place in a lesser extent

Metals and Environment: Chemical Outputs From the Interaction Between Gilded Copper-Based Objects and Burial Soil

Three-dimensional chemical mapping was adopted to investigate an ancient fire-gilded buckle found in Rome. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) were used to detect and locate degradation products aiming to identify the alteration processes. Inorganic and organic compounds present in the outermost part of such a class of cultural heritage objects can be considered the result of long-term interaction with the burial environment. ToF-SIMS depth profiling experiments can provide chemical information at the molecular level and high resolved spatial information (about 1 μm laterally, and 1 nm in depth). In this work, the attention was focused on the identification and localization of the ionic and molecular species involved in the degradation process. Results showed the presence of copper oxides, chlorides, and sulfides as common corrosion products but also the presence of species related to copper and bronze corrosion process such as atacamite and its polymorphs. 3D maps for all the relevant molecular species allowed to visualize at the same time the eruption of copper chlorides throughout the micro/nanochannels present on the gold surface, the recrystallization of compounds of minor elements from the substrate, a pathway followed by silico-aluminates from the surface towards the internal corrosion layers, but mostly the evidence of biological activity of Sulphur Reducing Bacteria (SRB) living in anaerobic conditions

Synthesis and characterization of TEOS coating added with innovative antifouling silica nanocontainers and TiO2 nanoparticles

We study the synthesis and characterisation of an innovative TEOS-based composite coating, which could improve previous formulations used in the field of monument conservation. The proposed coating is composed by a tetraethoxyorthosilicate matrix (TEOS), containing an elasticiser (hydroxyl-terminated polydimethylsiloxane (PDMS-OH)) and a non-ionic surfactant (n-octylamine). The specific self-cleaning and antifouling properties are obtained by the addition of different kinds of nanofillers: the commercial TiO2 nanoparticles, plus two different silica nanocontainers, loaded with the commercial biocide 2-mercaptobenzothiazole. Through a multi-analytical approach, we evaluate the effect of the nanoparticles concentration on the coatings drying rate, on the variation of their visual aspect and textural properties. Our results show that the addition of the silica nanocontainers at 0.05% (w/v) in sol does not change the colour of the coating and reduces the formation of cracks after drying. Moreover, the coating charged with nanocontainers undergoes slower drying, thus improving its penetration into the pores of the treated surface. Further tests of photocatalytic and biocidal properties of this new product on different lithotypes and their potential interactions are in progress

Exploring Manufacturing Process and Degradation Products of Gilt and Painted Leather

In this work, we studied the manufacturing processes and the conservation state of gilt and painted leather fragments from Palazzo Chigi in Ariccia (Italy) by using different analytical techniques. Leather fragments present a silver leaf superimposed onto leather support. A gold varnish and different painted layers decorate it all. A top-down analytical approach was used to investigate this complex multilayer structure, which adopted techniques with different sampling depths. Organic and inorganic constitutive materials together with related degradation products were studied by time of flight secondary ion mass spectrometry (ToF-SIMS), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and macro X-ray fluorescence (MA-XRF). The findings have revealed the presence of different elements and species as calcium and iron in the leather support, which was attributed to the un-hairing process in the leather tanning. Regarding what concerns the silver leaf, where the varnish cracked, silver chlorides, oxides, and sulfides were detected as degradation products of the silver leaf. Proteinaceous compounds were also identified where the silver leaf is unprotected by the varnish. These ion signals reveal a potential use of animal glue on both sides of the silver leaf to promote durable adhesion. In the gold varnish, the surface analysis revealed organic compounds such as resins and oils. In particular, the copresence of linoleic, arachidonic, and lignoceric acid ion signals in the yellow area suggests the use of aloe as a colorant. Lead ions in the same area were detected and attributed to the use of lead as siccative. Blue areas were obtained by using indigo and lead white in addition to an oil binder. This is confirmed by the detection of indigotin, fatty acid, and lead soap ion signals. A copper-based pigment was used to depict the green areas and copper oxalates were identified as its degradation products. Lastly, no significant information about the red colorant was obtained. Colophony is present as a component of the final varnish

Fluorescence study of the hydrolytic degradation process of the polysiloxane coatings of basalt fibers

Basalt fiber surfaces were modified using different silane aqueous solutions to generate a variety of poly-organosiloxane coatings. After removing the commercial coating of the fibers by calcination and subsequent activation processes, polysiloxanes were grafted on the fiber surfaces. Three aqueous solutions were used for the silanization: (i) gamma-aminopropyltriethoxysilane, APTES; (ii) gamma-aminopropylmethyldiethoxysilane, APDES, and (iii) a mixture of 50% by weight of both APTES + APDES. The silanized fibers were chemically labeled with fluorescein isothiocyanate to be immersed afterwards in different aqueous solutions (pH = 7) to study the hydrolytic degradation of the polysiloxane coatings. The hydrolysis phenomena were monitored by steady state fluorescence at different temperatures to subsequently study the kinetics of the process. The hydrolysis process was also studied by monitoring the pH of the solution in which the silanized fibers were immersed as a function of time. The data obtained from fluorimetry were fitted to an integrated expression arising from a first order kinetic process, which allowed estimation of the activation energies of the hydrolytic degradations. The results indicated that although the hydrolytic rate of the polysiloxane coatings increased in the order APDES < APDES + APTES < APTES, differences in the mechanism were not the cause of that order; the initial concentration of siloxane bonds able to be hydrolyzed was responsible.This work was financially supported by Projects MAT2014-59116-C2 (Ministerio de Economía y Competitividad), 2012/00130/004 (Fondos de Investigación de Fco. Javier González-Benito, política de reinversión de costes generales, Universidad Carlos III de Madrid) and 2011/00287/002 (Acción Estratégica en Materiales Compuestos Poliméricos e Interfases, Universidad Carlos III de Madrid). Furthermore, we would like to appreciate the Universitá di Roma, La Sapienza, for formalizing the contract of Morena Iorio in the frame of conducting a PhD thesis