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

In the present moment, several industrial and technological applications require the use of new low cost and environmentally friendly materials, offering other properties that are not present in traditional materials. Due to this, special efforts should be addressed in the material science and engineering areas, trying to design and develop innovative composite materials. In this contest, the design of fiber-reinforced composite materials is especially interesting because they are widely used in different sectors such as aerospace, aeronautical, automotive, construction, sporting goods industries, among others. When fibers are incorporated to different matrices, they usually increase mechanical resistance and stiffness without compromising other characteristics as the low weight. Different types of fibers are available to reinforce polymeric, metallic and ceramic matrices. Nevertheless, the necessity of reducing risk for human health and environment impact, suggests using new materials that can be directly obtained from natural sources without reducing materials performances in certain applications. In the last decade, natural basalt fiber extruded from naturally fire-resistant basalt has emerged as a contender between the most used fiber reinforcements in composites. Basalt fibers are classified as natural fibers of mineral origin characterized by high performance and a great range of good mechanical (high-tensile strength and high E-modulus), thermal (high temperature resistance) and chemical properties. Furthermore, one of the great advantages of these fibers is the high availability of the raw material overworld and their low cost. Moreover, basalt fibers can be easily processed using conventional methods and equipments and they do not need any other additional step in the single producing process, making them to have an additional advantage in terms of costs. Finally, it is interesting to highlight that basalt fibers are safe for health because their diameters are larger than 9 micrometers and they are not subjected to longitudinal fracture, so they cannot be considered as inhalable materials. Due to the last characteristics mentioned basalt fibers have been recently classified as “The green industrial material for the twenty-first century”. This definition reveals the sustainability, point of growing interest in new worldwide productions. In many cases basalt fibers might be an extraordinary candidate to replace synthetic fibers in composite materials. Nowadays, the fiber market is mainly oriented to the use of glass and carbon fibers. Basalt fibers might be an extraordinary candidate to replace the last ones since they have better tensile strength than E-glass fibers, greater failure strain than carbon fibers as well as good resistance to chemical attack. Moreover, due to the high resistance against the action of fungi and microorganisms and their high mechanical properties, basalt fibers can be used for many applications as alternative to the other type of natural fibers of vegetable origin. The use of high-performance and eco-friendly materials is currently in high demand in different engineering and industrial fields. In the last years, a great interest of this type of materials is growing in the building industry where the concept of sustainability is becoming important. Moreover, special attention to fiber-reinforced materials is being developed in many countries where special climatic conditions and catastrophic events such as floods and earthquakes are becoming more frequent. These events, sometimes, contribute to the deterioration of the modern and ancient buildings. In Italy for example, the most important historical centres have been damaged as a consequence of these catastrophic events. In this way, the identification and use of new materials for the preservation and conservation of cultural heritage are very important issues that generate considerable interest. The necessity of maintaining the characteristics of ancient materials implies finding out innovative materials for restoring masonry and plasters, guaranteeing their properties especially in terms of mechanical performance. Moreover, a very important issue to be considered in a restoration intervention is compatibility with the original structure; therefore, an especial care should be taken into account when selecting materials. Sometimes, the use of non-compatible materials could enhance building deterioration processes leading to the necessity of continuous structure reparations. To satisfy the growing need of innovative and eco-friendly materials in modern building industry and in the restoration field, the main research activity carried out within the frame of the present PhD thesis was focused on the study of basalt fiber reinforced cement matrices. In particular, the aim of this work was to optimize the fiber-matrix interface in fiber reinforced cement-based composites through specific surface treatments of the natural basalt fibers. Especially, 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. Among the large variety of cement materials, Portland cement is one of the most used. It was developed in the mid nineteenth century and was rapidly become the main choice in the building industry. It is a hydraulic binder primarily used in mortar and concrete. It is largely used as construction and repair material in the modern building industry although in certain cases it was used to restore ancient masonry structures or buildings. Restoration interventions that have employed cement-based mortars have shown several incompatibilities (high mechanical strength, efflorescence phenomena owing to the formation of large amounts of soluble salts by migrations of alkaline ions, low permeability with excessive water retention) causing extensive damage to the ancient masonry structures. Due to these problems, in the last years lime-based mortars have received special attention for restoration activities, looking for higher compatibility (physical, chemical and mechanical) with the original components. In this contest, special attention is dedicated to the Natural Hydraulic Lime (NHL). This is a hydraulic binder highly compatible with structures of great cultural interest since lime based-mortar have been used as construction material since ancient time. In addition, the interest of using NHL for new construction is recently growing because it is responsible of reduction of CO2 emission in comparison with the common Portland cement since less energy is required in the production process. Nevertheless, due to its lower mechanical properties NHL is not being widely used in comparison with Portland cement. On the other hand, considering the potential applications of these materials it is important to take into account some disadvantages. First of all, it should be point out that cement materials are classified as brittle materials and great problems might be due to shrinkage cracking phenomenon. In general, random dispersion of short fibers within the cement-based matrices might contribute to reduce this problem since they should lead to more isotropic reinforcement limiting crack opening. Up to the present moment, several studies refer on the use of commercial short basalt fibers to reinforce cement-based materials. Among other interesting conclusions, experimental results pointed out that better performance of cement-based materials could be obtained by the optimization of fiber-matrix interface through surface treatments. In fact, it is well known that properties of composite materials are strongly dependent on the type of adhesion between the reinforcement and the matrix. One of the methods to improve the interface in composite materials is through especial surface treatments of the fibers. Among others, silane coupling agents are the most commonly used modifiers of fibers surfaces. However, most of the studies concerning the modification of fibers surface are mainly oriented to improve specific interactions with polymeric matrices. In the present work, it is proposed to perform chemical coatings and study the particular interaction with cement matrices since the research is currently less widespread. 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. A careful and detailed characterization of commercial and modified fibers, necessary to collect data in order to, subsequently, understand specific interactions with cement- based matrices, was done. The preliminary study about the structure, composition and morphology/topography of as-received basalt fibers by several analytical techniques (XRD, TGA, FT-IR, FE-SEM and AFM) confirmed that commercial fibers are characterized by an amorphous structure and a heterogeneous sizing of organic nature applied on the fiber surface during the production process. In addition, 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. Besides, other important issue that deserve especial attention is related to the effect of external agents or small molecules such as water since it may lead to materials failure by interphase hydrolytic degradation. When silane coupling agents are used to modify the fibers the siloxane bonds formed with fibers surface and/or with other silane molecules can be more or less easily hydrolyzed depending on pH. Therefore, taking into account that manufacturing of cement-based composite needs mixing with water, the study of possible hydrolytic degradation processes that might occur at the interface in these materials is very important to conveniently design these materials to improve their final performance. In addition, cement matrices are characterized by alkaline pH. Therefore, considering a possible application of these polyorganosiloxanes coatings in cement materials, the study of their behavior in alkaline environment is particularly important. Therefore, 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. The results demonstrate that the mechanism of the hydrolytic process is very similar for the three systems studied. Nevertheless, some differences in the rate of the hydrolytic degradation process are observed. Indeed, the hydrolytic degradation rate is related to the initial concentration of the Si-O-Si bonds and, consequently, to the number of the hydrolyzed siloxane bonds. This number is higher when a silane with a cross-linked structure is used. Furthermore, the hydrolytic degradation of the silane coupling agents quickens depending on the increase of the siloxane crosslinking degree. It could be assumed that one method to reduce the rate of hydrolytic degradation at the interface in the fiber reinforced cement-based composites would be to minimize the degree of coating. A polyorganosiloxane with a lower crosslinking degree and correspondingly reduced amount of Si-O-Si bonds, such as the APDES coating, could be the most effective strategy to resist possible water attack mainly in the alkaline environment characteristic of the cement matrix. Finally, 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. 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. The small differences shown by the BET-BJH results point out that the presence of fibers does not change significantly the micro and meso structure of the neat mortar. The fractographic study by laser and optical profilometry showed more tortuous fracture surface for samples containing fibers. Consequently, and being in accordance with the mechanical studies, the presence of the fibers confers a less brittle behavior. 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).