Cement-based compositesoptimization of basalt fibers-cement matrices interfaces

Resumen

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) 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. In a second phase of the thesis project, the phenomena of hydrolytic degradation of the polysiloxane coatings were studied since the siloxane bonds 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 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 able to be hydrolyzed. 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 commercial and modified fibers 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 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 is achieved in the case of mortars reinforced with basalt fibers treated with the mixture of two silanes (APTES+APDES).