DESCRIPTION

Field of the invention

The present invention relates to a process for making cement mortar, as well as elements of cementitious material obtained from such mortar, containing a fiber reinforcement product (in particular comprising carbon fibers) deriving from the recycling of scraps of pre impregnated fabrics ("pre-preg") used in the production of manufactured goods using such pre-pregs. The invention also relates to a process for obtaining such fiber reinforcement product obtained by a secondary process starting from scraps of pre-impregnated composite material.

The invention can find advantageous application in the sector of the formulation of cementitious compositions to be used for example in the building sector, to bond masonry elements together, for the restoration/renovation of masonry structures, for structural reinforcement in general, and for the manufacturing of prefabricated elements or the like.

Background of the invention

Carbon fibers, by their nature, establish a good interfacial bonding with the concrete matrix. The high strength and stiffness properties of the fibers offer excellent advantages in the use of CFRC (Carbon Fiber Reinforced Concrete) compounds, including:

• high tensile and flexural strength, even after the cracking of the matrix;

• increased compressive strength;

• excellent resistance to abrasion, impact and dynamic loads in general;

• high-temperature tolerance and low thermal expansion;

• high resistance to corrosion.

CFRC is a material of great technological interest thanks to the combination of good structural properties and exceptional electrical properties. In fact, the use of an appropriate amount of carbon fibers in the concrete matrix can regulate electrical conductivity. A factor that limits the use of carbon fiber reinforced compounds in the construction sector is the high production cost compared to other synthetic materials, even if bituminous- based fibers, cheaper than those deriving from polyacrylonitrile (PAN), are more used in this sector.

The effective use of carbon fibers in cement requires a homogeneous dispersion in the concrete matrix, as it directly affects its mechanical and electrical properties. Appropriate amounts of additives, such as silica fume, are then added to the mix, to which a small amount of methylcellulose is generally added to promote the dispersion of the fibers and the workability of the mixture.

Recycled fibers, derived from waste produced during other processes, have also been used to decrease the cost of CFRC products. For example, the manufacture of concrete elements using recycled carbon reinforcing fibers is known from WO2013/079482. This document refers to processing fiber waste, without going into detail on the origin of such waste. Several other documents describe the use of carbon fibers for obtaining fiber reinforced concretes, including for example WO2012/174414 and US7341627.

The Applicant investigated particular scraps containing carbon fibers, namely pre impregnated fabrics used in the production of manufactured goods in various types of industries, for example the automotive or aerospace industries. Pre-impregnated or "pre- preg" fabrics mean carbon fiber fabric pre-impregnated with pre-catalyzed epoxy resin, which guarantees greater flexibility of the shapes in automated or manual lamination operations ("wet-lay-up") or automated lamination operations. These pre-impregnated fabrics guarantee greater flexibility in the choice of thicknesses, a decisive advantage when developing, for example, components for a sports car, wherein the stiffness of the different parts of the chassis is essential. An advantageous aspect is that the reinforcement already contains the matrix material, such as an epoxy resin, and is therefore ready for handling and depositing in a mold without the prior addition of impregnating resins through the classic production processes such as manual lamination, infusion or injection. Once inserted into the mold, the pre-preg is then consolidated by infusing further resin.

A significant amount of scraps is generated downstream of the production and treatment process of the shaped pieces of “pre-preg” fabric made by the industrial activity. Recycling such materials is inherently complex due to their composition (fibers, matrix and fillers), the cross-linked nature of the thermosetting resins (which cannot be converted back to the pre-polymerized state) and the combination with other materials (metal fixings, honeycomb material, hybrid composites, etc.). Currently, most of the waste deriving from CFRP “Carbon Fiber Reinforced Polymer” or from pre-preg of the same type is landfilled as special waste or sent for incineration. Several companies in various sectors are forced to face similar critical issues in relation to the disposal of a significant quantity of fibers, which has both an economic and an environmental impact. In general, these solutions are unsatisfactory for several reasons:

• Environmental impact;

• Legislation;

• Production cost;

• Resource management;

• Economic opportunity.

The Applicant has optimized a secondary process for reusing the scraps deriving from pre impregnated fabrics based on carbon fibers. The fiber reinforcement product thus obtained is then used by inclusion of such fibers in a cement mortar.

The purpose of the present invention is, therefore, to provide a secondary production method for reusing the scraps of pre-impregnated carbon fibers, the fiber reinforcement product thus obtained is to be used in the production of mortars and cementitious products deriving from said mortars, as well as to provide cementitious products obtained by this method.

A first objective of the present invention is therefore to provide a process for producing cement mortars wherein the reinforcement is obtained by a method of reusing carbon pre- preg scraps and used both in addition to the basic composition of a cement mortar (wherein base composition means only the relative solid components i.e. the mixture of said solid components with water), to increase the mechanical performance of the manufactured products obtained from this composition, and in partial substitution of cement or aggregates (sand or other fine-grained rock materials) used in a basic composition of cement mortar, to reduce the final cost of the manufactured products or the joints of structural elements without any decay or with an increase in the mechanical performance thereof or for an improvement in environmental performance (reduction of cement).

Further advantages of the process according to the invention derive from avoiding a treatment of industrial waste consisting of scraps of pre-preg fabrics downstream of the production of elements of polymeric material reinforced with carbon fibers. It is also an object of the present invention to provide structural elements deriving from the use of cement mortars together with scraps of fabrics pre-impregnated with carbon fibers and which have, on average, a higher mechanical strength than that of the elements without said reinforcing fibers. Or even the same performance, but with a lower quantity of binding phase (cement) which is known to be the component determining its mechanical strength.

The cement mortars according to the invention can be advantageously used for three- dimensional printing or 3D printing with which structural elements are made by means of a three-dimensional printer or 3D printer with an additive technology of material (Additive Manufacturing).

These and other objects, which will become mainly from the following detailed description, are substantially achieved by a process for producing cement mortars, as well as by structural elements obtained from mortars produced by means of this process in accordance with what expressed in one or more of the appended claims and/or the following aspects, taken alone or in any combination with each other or in combination with any one of the appended claims.

Summary

The aspects of the invention are described below.

In a first independent aspect, a process is provided for obtaining a fiber reinforcement product obtained by an industrial process of reusing fibers deriving from scraps of pre impregnated composite material ("pre-preg"), said fiber reinforcement product being reusable as reinforcing and/or filling material in a matrix of other material, in particular in a concrete matrix, and comprising the following steps:

• thermally treating by heating said scraps and/or pieces and/or fragments of said scraps at a temperature and for a time duration such as to polymerize at least partially a resin contained in said scraps;

• fragmenting, for example by grinding, said scraps and/or pieces of scraps to obtain fiber reinforcement fragments,

• separating, for example by mechanical sieving, said fragments in order to obtain a plurality of fragments in at least a predetermined dimensional range, the fiber reinforcement product comprising said plurality of fragments in the predetermined dimensional range. In a second independent aspect, a process is provided for obtaining a concrete matrix containing a fiber reinforcement product obtained by an industrial process of reusing fibers deriving from scraps of pre-impregnated composite material ("pre-preg"), characterized by the fact that it comprises a process for obtaining the fiber reinforcement product comprising the following steps:

• thermally treating by heating said scraps and/or pieces and/or fragments of said scraps at a temperature and for a time duration such as to at least partially polymerize a resin contained in said scraps;

• fragmenting, for example by grinding, said scraps and/or pieces of scraps to obtain fiber reinforcement fragments,

• separating, for example by mechanical sieving, said fragments in order to obtain a plurality of fragments in at least a predetermined dimensional range, the fiber reinforcement product comprising said plurality of fragments in the predetermined dimensional range; the process for making the concrete matrix further comprising mixing the fiber reinforcement product with at least water, cement and aggregates to obtain said reinforced concrete matrix incorporating the fiber reinforcement product.

In a further aspect according to the preceding aspects, the pre-impregnated fabric scraps, prior to heating, are subjected to a preliminary grinding in order to obtain scrap pieces of predetermined dimensions.

In a further aspect according to the preceding aspect, the pieces obtained from the preliminary grinding have an average length of less than 50 mm, in particular less than 30 mm, even more in particular less than 20 mm.

In a further aspect according to the preceding aspects, the heating temperature is greater than or equal to 80°, in particular greater than or equal to 90°C, and the time duration is equal to or greater than 20 minutes, in particular 30 minutes.

In a further aspect according to the preceding aspects, said mixing is carried out by adding the fiber reinforcement product to a predetermined composition of the concrete matrix, said predetermined composition comprising proportionally a part of cement, three parts of sand, in particular CEN-standard sand, and half a part of water with a water/cement ratio=0.50.

In a further aspect according to the preceding aspects, said mixing is carried out by partially replacing a solid constituent of a predetermined composition of the concrete matrix, said predetermined composition comprising proportionally a part of cement, three parts of sand, in particular CEN-standard sand, and half a part of water with a water/cement ratio=0.50.

In a further aspect according to the preceding aspects, the fiber reinforcement product is in a volumetric percentage of the concrete matrix comprised between 1 and 10% and/or said fragments have a length of between 1 and 5 mm.

In a further aspect according to the preceding aspects, the fiber reinforcement product replaces at least a solid component of the original mortar composition in a volumetric percentage between 3% and 15% and/or said fragments have a length between 2 and 15 mm.

In a further aspect according to the preceding aspects, the fiber reinforcement product is obtained by an industrial process of reusing composite material including carbon fibers.

A further independent aspect concerns a composition for a concrete matrix, in particular cement mortar, comprising a reinforcement product constituted by a plurality of fragments with reinforcing fibers deriving from scraps of pre-impregnated carbon fibers fabric with fragmented and heat-treated to polymerize the resin of the fabric, said composition comprising sand and cement and wherein the volumetric percentage of said reinforcement product with respect to sand and cement is comprised between 1 and 15%, in particular between 1 and 4%, and wherein the length of said fragments is between 1 and 15 mm, in particular between 2 and 5 mm.

In a further aspect, a secondary industrial process is provided starting from pre-preg scraps to obtain a fiber reinforcement to be used in a cement mortar, comprising heating the scraps of pre-impregnated material to a temperature and for a duration such as to ensure the polymerization of the resin contained in said scraps, grinding said scraps to obtain fragments of reinforcing fibers, sieving said fragments in order to obtain fragments of a predetermined dimensional range (diameter and length), mix design of the mortar starting from the dimensions of the reinforcement and the required performance target and mixing of said fragments with the components of a cement mortar (cement, fine-grained rock material, water).

In a second aspect of the invention, the pre-impregnated fabric scraps, prior to heating, are subjected to a preliminary grinding in order to obtain fiber lengths of predetermined dimensions.

In a third aspect of the invention, the fiber lengths obtained from the preliminary grinding have a length of less than 30 mm. In a fourth aspect of the invention the heating temperature is greater than or equal to 90°C and the time is equal to or greater than 30 minutes.

In a fifth aspect of the invention the mixing is carried out by adding the fiber fragments to a basic cement mortar.

In a sixth aspect of the invention, the mixing is carried out by partially replacing a constituent of the cement mortar selected between sand and cement.

In a seventh aspect of the invention, the fragments are added to the basic cement mortar in a volumetric percentage of between 1 and 10% and have a length of between 1 and 5 mm.

In an eighth aspect of the invention, the fragments replace the sand or the cement of the basic cement mortar in a volumetric percentage between 3% and 15% and have a length between 2 and 20 mm.

In a ninth aspect of the invention, the elements of cementitious material obtained by the process according to the invention comprise fragments of reinforcing fibers deriving from scraps of fabric pre-impregnated with shredded carbon fibers and subjected to a heat treatment to polymerize the resin of the fabric, wherein the volumetric percentage of said fibers with respect to a basic cement mortar is between 1 and 7.5% and wherein the length of said fragments is between 1 and 5 mm.

In a tenth aspect of the invention, the elements of cementitious material contain fragments of carbon fibers added to a base cement mortar in a volumetric percentage comprised between 1% and 2%, and with a length comprised between 1 and 2 mm.

In an eleventh aspect of the invention, the elements of cementitious material contain fragments of carbon fibers which partially replace the sand of the basic cement mortar and are present in a volumetric percentage between 3% and 4%, preferably around 4%, and with a length between 2 and 10 mm. The term “around” here means an interval between - 1 % and + 1 % with respect to the identified value.

In a twelfth aspect of the invention, the elements of cementitious material contain fragments of carbon fibers which partially replace the cement of the basic cement mortar and are present in a volumetric percentage around 6.4%, and with a length between 2 and 5 mm.

In light of a thirteenth aspect of the invention, the fiber fragments have a length/diameter (L/D) ratio lower than or equal to 10 (in particular lower than or equal to 6). In this regard, it is pointed out that the pieces of shredded reinforcing fibers derive from fibers joined together during the preparation of the pre-impregnated fabric and which, therefore, do not usually have "regular" dimensions.

According to a further aspect of the invention, the recycled carbon fibers are added to standard cement mixture type, for example class M10, with fiber dimensions distributed in the diameter classes 0.7-1 mm, 1-2 mm and 2-5 mm.

According to a further aspect of the invention, in standard medium-strength cement mortar, for example R32.5 cement (Standard mortar UNI EN 196), the aggregate (sand) or the cement are partially replaced by recycled carbon fibers with a distribution of fiber diameters comprised between 0.2-0.425 mm, 0.425-0.85 mm, 0.85-1 mm, 1-2 mm and 2-5 mm or 5-10 mm.

According to a further aspect of the invention, the fibers obtained from pre-impregnated fabrics can be dry mixed with the aggregate and the cement of the matrix/cement mortar, in order to allow them to be transported to the place of use.

According to a further aspect of the invention, additives selected from the group consisting of plasticizers, fluidifiers, viscosifiers, fillers or their mixtures can be added to the matrix or cement mortar.

According to a further aspect of the invention, a composition for obtaining a concrete matrix/mortar which uses fragments of reinforcing fibers deriving from scraps of fabric pre impregnated with carbon fibers and subjected to heat treatment to polymerize the resin of the pre-impregnated fabric, comprises sand and cement in a volumetric percentage of said fibers with respect to sand and cement comprised between 1 and 4% and wherein the length of said fragments is comprised between 1 and 5 mm. Said composition is easily transportable in the dry state and immediately transformable into a ready-to-use cement mortar by adding and mixing with water and any additives if the latter are not already included in the initial composition.

According to a further aspect of the invention, elements of cementitious material herein mean both those obtained directly with the cement mortar according to the invention, and any complex structural element used in building whose elements are joined by means of a cement mortar according to the invention.

According to a further aspect of the invention, the elements of cementitious material obtained using a cement mortar according to the invention can be made by means of three-dimensional printing or 3D printing with a three-dimensional printer or 3D printer based on an additive material technology. The Applicant believes that the length ranges of the recycled carbon fiber fragments which offer the best results in terms of mechanical performance may result from a better interaction with the aggregate (standard grain size sand, 100% moisture content, with a maximum diameter less than 4 mm) of the cement mortar.

A correct balance must therefore be found between the grain size of the aggregates and the length of the fibers. In general, the maximum dimension of the aggregates must not exceed 0.5 times the length of the fibers used. There is, therefore, a correlation between optimal lengths of the carbon fiber fragments and dimensions of the aggregate.

Thanks to the characteristics of the invention, by adding the recycled fibers to a traditional mortar, significant increases are obtained especially in compression up to 30%.

By replacing the sand of standard mortar, significant increases are obtained especially in bending up to 30% and it is also possible to obtain an increase in the class of cement. Furthermore, it is also possible to obtain an increase in flexural strength (up to + 50%) and post-bending strength (i.e. softening).

According to the invention, it is therefore possible to formulate the cement mortar in relation to its final use, both as a structural element of conjunction between construction elements (wherein said conjunction is more subjected to compression or bending stress) and as an independent structural element.

By substituting the cement of standard mortar with the recycled fibers in accordance with the invention, increments are lower both in bending and in compression (same class), but with a lower quantity of cement (saving on the final cost of cementitious material products). According to the invention, a minimization of the environmental impact is also achieved by avoiding sending the pre-impregnated material scraps with carbon fibers to landfills.

Further advantages and characteristics of the invention will become evident from the following description, with reference to the attached drawings wherein:

• figure 1 is a graph illustrating the aspect ratio of the reinforcing fibers after heat treatment, grinding and subsequent sieving/screening;

• figure 2 is a graph showing the percentage variations of the flexural strength (on the left for each mixture) and of the compressive strength (on the right for each mixture) of a "standard" cement mortar to which, according to the invention, a predetermined volume percentage of recycled carbon fibers was added;

• figure 3 is a diagram of the flexural test of the best sample shown in figure 2 (on the left of the diagram), compared with a sample without reinforcement (on the right of the diagram); • figure 4 is a graph expressing the percentage variations of the flexural strength

(on the left for each mixture) and of the compressive strength (on the right for each mixture) of a "standard" cement mortar wherein, according to the invention, the aggregate (sand) has been substituted by a predetermined volumetric percentage of recycled carbon fibers;

• figure 5 is a diagram of the flexural test of the best sample shown in figure 4, compared with a sample without reinforcement; and

• figure 6 is a graph expressing the percentage variations of the flexural strength

(on the left for each mixture) and of the compressive strength (on the right for each mixture) of a "standard" cement mortar wherein, according to the invention, the cement has been substituted by a predetermined volume percentage of recycled carbon fibers.

Definitions

"Composite materials" means materials of a non-homogeneous nature, wherein it is possible to identify at least two fundamental elements, quite distinct from each other, each performing a specific function: the fibers (or reinforcement), which represent the structurally active part, and the matrix, usually made up of resins, which does not perform tasks of mechanical strength, but must ensure cohesion among the various layers of fiber.

"Pre-preg" or "pre-impregnated composite material" means a pre-impregnated fiber reinforcement composite material wherein a matrix material, such as epoxy resin, is already present; the fibers generally arrange themselves to form a fabric, while the matrix is used to fix them together and possibly with other components during production; the matrix is only partially cross-linked, so that it can be easily manipulated.

"Fiber reinforcement product" means the set of fibers obtained by the reuse process described herein starting from scraps and/or swarf of pre-preg composite material.

"Concrete matrix" means a mixture of water, cement and fine aggregates, for mortars, to which large aggregates are added, for concretes. The physical and mechanical characteristics are defined by specific regulations: UNI EN 206 with indications at a national level in UNI 11104:2016.

Detailed description The starting material for obtaining the fiber reinforcement product for the cement mortar according to the invention is derived from scraps or swarf of a pre-impregnated composite material based on reinforcing fibers, such as carbon fibers, and a thermosetting resin, for example a pre-catalyzed epoxy resin.

In greater detail, the starting material can be a fabric which is pre-impregnated in a machine with a pre-catalyzed resin system. For example, it is possible to use the scraps and/or swarf deriving from a 0-90 biaxial fabric of pre-impregnated carbon fibers (epoxy resin + carbon fibers), typically with a tensile strength of 620.5 MPa and a modulus of traction of 58.4 GPa.

Downstream of the pre-preg sheet production and treatment process carried out by the industrial activity (for example for components in the automotive or aerospace sector), a significant amount of scrap/swarf is generated, generally destined for disposal as special waste in landfills or incinerators.

In accordance with the process described below, the fibrous scrap/swarf material in turn requires a further preparation process in order to be reused. The fibrous material containing carbon fibers and deriving from composite materials, due to the technological complexity of the operation of separating the fiber from the resin matrix, presents various difficulties, mainly due to:

• a complex composition (fibers, resinous matrix and fillers used to reduce the price);

• the cross-linked nature of the thermosetting resins used (which once hardened cannot be re-plasticized and molded, as it is done with thermoplastics):

• the combination with other materials (metal inserts, honeycomb materials, hybrid composites).

This heterogeneity of the initial fibrous material therefore requires a technique for transforming the scrap material so as to allow a reuse, for example by mixing in matrices of other material. Specifically, the method consists of several steps that can be grouped into two main steps, namely a first step characterized by the collection of the scrap/swarf elements, grinding the material in order to obtain a plurality of fragments and finally a subsequent heat treatment of the fragments.

On the other hand, the second step (following the first) begins with grinding the fragments after the heat treatment in order to obtain so-called “chopped” fibers; the latter are then mechanically sieved in order to obtain fragments of more homogeneous length in a more defined range. The first and the second steps allow obtaining the fiber reinforcement product for subsequent reuse.

Finally, the fiber reinforcement product can be reused by incorporating it as a reinforcement and/or filler in a matrix of different material, for example a cement matrix, but also in other materials such as asphalt, artificial woods and plastics.

In greater detail, the initial fibrous material is subjected to one or more of (and in general to all) the following operations (in time sequence):

• Collection of scraps/swarf deriving from pre-impregnated composite material from processing lines intended to create carbon fiber reinforced products;

• Preliminary grinding scraps/swarf to obtain fragments of reduced length compared to scraps/swarf, for example a length not exceeding 30 mm;

• Cooking, such as cooking in a static oven or on a moving conveyor belt ("curing"), at a temperature such as to start the polymerization cycle of the "pre-preg" fibers, for example a temperature of at least 80° and preferably at least 90°C for a time (in particular a function of temperature) of at least 25/30 min;

• Grinding the fragments in order to obtain the discrete fibers (Note: in heterogeneous material comprising fiber + resin + additives) in the so-called “chopped” form;

• Sieving, for example with a percussion or frequency sieve, in order to have fragments of well-defined length (ranges between 0 and 10 mm measurements).

According to an embodiment of the invention, a material having the following dimensional distribution is obtained during sieving: The aspect ratio of the reinforcing fibers after sieving/screening has the distribution shown in figure 1.

FiberApp software, which is a Matlab application used to trace and analyze polymers, filaments and fibrous objects, was used in the screening and characterization of the fibers. This software operates on images coming from various microscope sources (atomic force or transmission electron microscopy, optics, fluorescence, confocal, etc.), obtaining the spatial coordinates of the objects through a semi-automatic tracking procedure based on the "A * pathfinding” algorithm, followed by the application of active contour models and the generation of statistical, topological and graphical outputs, derivable from these coordinates.

The making of the cement mortar mixtures according to the invention was carried out starting from two macro-categories, i.e. cement mortars were composed with the addition of fiber reinforcement product (for example containing carbon fibers) in the mixture and others with the substitution of sand or cement, and the latter two, being the volume unchanged since the fiber reinforcement product replaces sand and/or cement, starting from standard cement mortar.

The concrete matrix is, in general, essentially composed of a mixture of water, cement and fine aggregates, for mortars (to which large aggregates are added, for concretes). The physical and mechanical characteristics are defined by specific regulations: UNI EN 206 with indications at a national level in UNI 11104:2016. The making of a cement conglomerate with good mechanical characteristics implies that, in the mixture, uniform distribution of the fibers and good workability of the mixture are guaranteed. In general, the addition of fibers to the mixture determines lower workability and consequently lower homogeneity of the mix. The results of experimental tests carried out by the Applicant on cementitious material reinforced with recycled fibers from pre-impregnated fabrics showed an increase in tensile strength, which in some cases exceeded 50% with respect to non- reinforced ones.

The production of the mixtures with substitution, starting from standard mortar, was carried out following the reference standards, in particular the standard UNI EN 196-1:2016, which indicates all the proportions between the constituents of the standard mortar. This standard also indicates the procedures for making the samples and the subsequent mechanical determination. As for the samples with addition, a traditional M10 mortar was used, moreover the added fibers were sieved in such a way as to insert them into the compound with a well-defined size range.

As regards the samples with substitutions, cement with a nominal strength of 32.5 kN was used and the fibers were sieved as well as being characterized, as seen for the fibers in addition.

The substitution was carried out according to the density of sand, cement and carbon fibers, respectively equal to 2640 Kg/m3, 1400 Kg/m3 and 1750 Kg/m3, and the quantity of sand and cement to be removed and the amount of fiber to be added in every single mixture were defined through simple proportions.

With this procedure, being the volume unchanged in the mortar made with substitution, standard mortar is obtained and so the quantities of sand, cement and water have been respected in accordance with the standard UNI EN 196-1:2016. The fibers replaced sand first, and then cement.

The production of the samples and the characterization of the resistance of the materials were carried out in accordance with the standard UNI EN 196 1:2005, paying particular attention to the homogeneous distribution of the fibers in the mixture.

To carry out the mechanical resistance tests of the samples, a 500 KN universal servo- hydraulic MTS machine was used.

Fibers in addition

As regards the samples with the addition of fibers, starting from a AM-10-0.33 mortar (wherein the water/cement ratio is indicated with 0.33) the following mechanical performances were obtained (with reference values of flexural strength without fibers equal to 6.01 MPa and compressive strength values without fibers equal to 22.5 MPa):

The graph in figure 2 shows the percentage variations of the flexural strength (on the left for each mixture) and of the compressive strength (on the right for each mixture).

In the graph in figure 2, it can be seen that the best results are obtained with a mixture with the addition from 1% to 2% of fibers with dimensions of 1-2 mm. In this way, by adding a low-performance mortar with recycled fibers according to the invention, it is possible to have mortar of a higher category, without an increase in cost.

From the diagram of the flexural test of the best sample shown in figure 3 (with 1% of fibers with a length between 1 and 2 mm) an increase can be noticed in the slope of the loading branch and therefore in stiffness quantified in 21.53 kN/mm with an increase of 86%.

The sample with a fiber length of 1-2 mm with 2% of the fibers is characterized by a flexural strength equal to 6.15 MPa while the one with 1% is characterized by a flexural strength of 6.29 MPa and therefore an increase of 2% and 5% is recorded respectively. Compressive strengths equal to 24.7 MPa and equal to 26.3 MPa respectively with an increase of 10% and 17% are registered.

For all the mixtures an overall increase in the compressive strength is observed, while for the flexural strength the variations are more contained.

Fibers substituting sand

In a similar way, samples were made starting from a R32.5 cement mortar (Standard mortar UNI EN 196) and partially replacing the sand in the mixture with recycled carbon fibers with a predetermined size distribution.

The following results were obtained (with reference values of flexural strength without fibers equal to 7.53 MPa and reference values of compressive strength without fibers equal to 38 MPa):

The graph in figure 4 shows the percentage variations of the flexural strength (on the left for each mixture) and of the compressive strength (on the right for each mixture).

Referring to the results obtained, summarized in the preceding table, it can be seen that the best sample of the series is the one with a percentage of fibers around 4% with a size of 2-5 mm.

From the force displacement graph of this sample (flexural test) shown in figure 5 (compared to a sample without reinforcement), an increase can be noticed in the slope of the loading branch and therefore in the stiffness quantified in 14.07 kN/mm with an increase of 27%. Furthermore, from the flexural test it can be noticed a less inclined descending branch and, therefore, greater ductility.

Of particular interest is the increase of the sample with 4% of the fibers with the size of 0.85-1 mm, wherein it has almost been reached a higher category of cement classification, as it passes from 32.5 MPa to almost 42.5 MPa.

Another fact emerging from the results is that in substitution of sand, unlike the mixtures with addition where there is a general increase in compression, in this case the overall improvement occurs in bending.

Fibers substituting cement

Starting from the same cement mortar mixture, partial substitutions of cement with fibers were carried out, which gave the following results (the sample of reference of the flexural and compressive strength is the same as in the series in substitution of sand):

The variations in flexural and compressive strength are shown in the graph in figure 6.

From these data it is clear that the best sample of the series happens to be the one with about 6.4% of fibers with a size of 2-5 mm. From the force-displacement graphs of this sample (flexural test, not shown in the drawings) it is possible to notice an increase in the slope of the loading branch and therefore in the stiffness quantified in 13.83 kN/mm with an increase of 25%. Furthermore, from the flexural test, it can be noticed a less inclined descending branch and, therefore, greater ductility.

It can also be noticed that with higher values of fibers in substitution of cement, a decay of the mechanical performances is noticed, evidently due to the lower quantity of cement. From the data recorded during the tests (load-deformation graphs) it is possible to deduce the behavior of fiber reinforced mortars, which is characterized by an increase in stiffness compared to samples without fibers for almost all the mixtures analyzed. Furthermore, still following the analysis of the results obtained from the tests, it is reasonable to state that the insertion of the fibers in the mortars has meant that the behavior of the mortar after reaching the break load is more ductile than what happens in mortars where fibers are absent.

Finally, thanks to the high values of the maximum load found in fiber reinforced mortars, the increase and the improvement in mechanical performances are clearly deducible.

The results of the experimental investigation highlighted that the carbon fibers obtained by the process described contribute to an improvement in the performance of cement mortars.

In almost all of the analyzed samples, there was an increase in the slope of the loading branch, in the case of force-displacement graphs, and therefore in the stiffness of fiber reinforced mortars compared to mortars without fibers, with an increase ranging from 25% in the mixtures with substitution up to 86% for mixtures with addition.

Furthermore, according to the observation of the behavior of fiber reinforced mortars in compression, the post-peak section exhibited by the slope presents a greater deformation at break, which leads to believe that the aforementioned mortars are more ductile than the reference ones.

With reference to the flexural and compression test, the best results were found for the samples with fibers substituting sand, in particular for the mixtures with a fiber content of about 4% and a length of 2-5 mm where there is an increase 18% of the flexural strength and 10% of the compressive strength compared to the sample of reference in the absence of fibers.

A further fact emerging from the production of the samples is the increase in the workability of the mixtures as the size of the fibers increases, also due to a possible portion of small-sized fibers that tend to thicken inside the mixture.

The results obtained appear satisfactory for the use of carbon fiber scraps, in the production of cementitious materials, in particular mortars, as optimal ranges have been identified both as regards the number of fibers and their dimensions.

Further tests carried out by the Applicant have shown how the improved mechanical properties are reflected in the use of the mortars according to the invention both for the production of independent structural elements, for example by means of three-dimensional printing, and for the union of structural construction elements such as for example stones, bricks, masonry elements or the like.