2.2. CFRP Cementitious Composites: Production Route and Mixing Process

As mentioned above, the CFRP recycling process (green flow in Figure 1) aims to produce a better performing mortar in terms of mechanical properties, meaning an increased value of compression/tensile strength. Indeed, the addition of carbon fibers in the mortar potentially leads to (i) a reduction of micro-cracks caused by plastic shrinkage due to evaporation of the water into the mix, and (ii) a bridging effect over the crack itself. Hence, a production process for CFRP cementitious composites was developed, consisting of the steps summarized in Figure 2.

Recycling process and fiber reinforced mortar production.

Figure 2 shows the recycling process in more detail and with some illustrations. The first step is the collection of pre-preg scraps/waste from industry storage. These items are no longer useful for the purpose for which they were designed, i.e., automotive. Then, in order to eliminate the sticky coating resulting from the resin pre-impregnation (which makes these fibers difficult to process), these scraps are heated in an oven at a temperature of about 90 °C for 30 min. This step allows the polymerization of the pre-preg sheets, resulting in a hardened product. This process takes place in the company’s autoclave, benefiting from the curing of CFRP profiles that are manufactured for the company’s appropriate commercial purposes. This is an important aspect because it avoids adding energy burdens to the recycling process for CFRP waste.

After heating, the stabilized CFRP wastes are ready to be chopped up into suitably smaller fragments that can be easily worked into mortars. In fact, the third illustration of Figure 2, refers to the grinding of CFRP heated pre-pregs, a pre-treatment operation consisting of reducing the size of the material into fragments using a mechanical grinder. Afterwards, the fibers are of different sizes, and therefore it is necessary to use a sieve in order to select fibers of the desired size range. Thus, by using a series of vertical sieves, with the opening gradually decreasing from top to bottom, the separation of grain sizes according to the selected openings is obtained. This procedure allows for the identification of the length of fiber fragments in order to define optimal CFRP mortar mixes. One of the most sensitive aspects is related to the production of a residual dust (usually around 4%) during the grinding process; nevertheless, this residual component could be introduced into the mortar as a filler. Once the fibers of the desired size have been selected, they are dry-mixed with the sand and cement; after a homogeneous dry mixture is obtained, the necessary water is added, and it is mixed again. Afterwards, the mortar is ready to be cast into molds to obtain test specimens for measurement of the mechanical properties.