1. Introduction
Preserving the environment requires the rational and intelligent use of available resources. In this context, the recycling and reuse of materials becomes essential for reducing pressure on natural resources and carbon emissions [
1]. Among the materials that need to be recycled and/or reused are Glass Fiber-Reinforced Polymers (GFRPs). These materials consist of glass fibers dispersed in a resin (e.g., polyester). These materials are widely used in several fields, such as construction, renewable energy (e.g., wind turbine blades), and boats. The recyclability of these materials poses several challenges due to the difficulty of separating the glass fibers from the resin matrix. In addition, due to aging and the different treatments that these materials may have undergone during their lifespan, their properties can vary considerably and can lead to very significant degradation, depending on the medium to which the materials were exposed. This is why these materials are generally landfilled or incinerated [
2]. Incineration, which is currently the most used, will no longer be possible in order to protect the environment, particularly to reduce greenhouse gases. In addition, a European directive (1999/31/EC) prohibits the landfilling of large composite parts. So, economically and environmentally viable recycling options are needed.
Currently, there are three main recycling methods, including thermal, chemical, and mechanical recycling [
2,
3,
4,
5]. There are also some reuse solutions, such as incorporation into thermoplastics, road applications, incineration with energy recovery [
6,
7], and use as powder or aggregates in cement-based materials [
8,
9,
10,
11]. Other reuse solutions could be implemented, such as their addition into the paste backfill intended for use in underground salt mines [
12]. The reuse of GFRP waste in cementitious materials is considered an economically and environmentally viable solution [
2]. GFRP waste is also used in the cement manufacturing process as a raw material and/or energy source. The size of the GFRP waste is first reduced. The mineral part of the GFRP is used to make the clinker, while the organic polymer matrix is converted into energy to heat the rotary kiln [
2].
The field of cementitious materials thus represents an interesting recycling option for GFRP waste. This option currently makes it possible both to treat large volumes of this waste and to reduce the extraction of natural aggregates (especially sand). In addition, cementitious materials that incorporate GFRP waste could present very interesting specific properties [
9,
10,
11,
13].
A large number of studies have examined the effects of glass fibers on the behavior of cementitious materials [
14,
15,
16,
17,
18,
19,
20,
21,
22]. These studies have highlighted the beneficial effects of the incorporation of glass fibers into cementitious materials, such as pozzolanic activity, good fire behavior, and the enhancement of toughness [
15,
16,
17].
In recent years, several studies have focused on the reuse of GFRP waste in construction materials [
8,
9,
10,
13,
21,
23,
24]. It should be noted that GFRP waste is also incorporated into polymer concrete materials, in which a thermoset polymer entirely replaces the cement paste [
11,
25,
26]. However, this type of “concrete” has a relatively high cost and high sensitivity to creep and shrinkage [
11,
27,
28].
Several studies on the incorporation of GFRP waste into cementitious materials are available [
8,
9,
10,
13,
21,
23,
24]. The data vary depending on the particle size of the waste incorporated (fine or coarse) and the substitution rate [
2]. Depending on the grinding process, GFRP waste is incorporated in the form of a powder to replace the cement or fine particles of sand, or in the form of a fiber–resin to partially replace the aggregates. Generally, the incorporation of GFRP waste leads to a loss in the workability of the cementitious material. This is attributed to the flocculation of particles (or fibers), which increases water demand [
29,
30]. However, the effect on the workability strongly depends on the content, size, and morphological properties of the GFRP waste. The workability is generally reduced with a longer fiber length and higher fiber content. Farinha et al. [
31] observed that replacing 20% of fine aggregates by weight with GFRP waste (less than 63 µm) in mortars with the same water content improved workability. In fact, GFRP particles absorb less water than the replaced fine aggregates, thereby reducing water demand. Furthermore, the replacement of coarse aggregates with needle-shaped GFRP waste does not significantly affect the workability. In addition, the water to cement ratio (w/c) gradually increases with the GFRP waste content [
32].
The effect of GFRP waste on the mechanical properties should consider the type and size of the substituted waste (fine or coarse aggregate replacement, or supplementary cementitious material). When replacing fine aggregates, some results have shown an increase in the compressive strength, while other studies have reported a decrease in the compressive strength. The reduction in the compressive strength may be due to the lower compressive strength of GFRP particles compared to that of natural aggregates, the weaker bonding of GFRP particles/cement paste, the increase in voids, and the heterogeneous distribution of the waste in the material [
8,
9,
10,
32]. Moreover, an increase in the compressive strength when GFRP waste is used in powder form has also been reported [
11,
31]. This positive effect can be explained by the reduction in voids and the chemical effects induced by the fine particles of GFRP waste. GFRP waste powder can thereby induce pozzolanic activity and a nucleation effect, leading to the improvement of the mechanical strength [
33,
34].
Concerning the flexural strength, according to Ribeiro et al. [
11], a partial replacement rate of sand aggregates with GFRP waste, not exceeding 8% by weight, has a positive effect. Farinha et al. [
31] reported an increase in the flexural strength of 155% at 365 days; the reference mortar had a flexural strength of 1.94 MPa and the mortar with 50% (by volume) GFRP waste had 4.95 MPa.
Asokan et al. [
24] investigated the effect of the replacement of fine aggregates by GFRP waste powder and fiber in concrete and cement composites. The substitution rates ranged from 5 to 50 wt%. The results showed that the compressive strength decreased with the GFRP substitution rate, while the bending strength with 5% of GFRP waste was enhanced compared to that of the reference sample. Moreover, the density of concrete with 50% GFRP waste was reduced by about 12% [
24].
Correia et al. [
8] examined the substitution of sand by fine GFRP waste (about 96% of the particles measure less than 63 μm) with volume replacement rates ranging from 5 to 20%. For the 5% substitution, the tensile splitting strength and the modulus of elasticity of the concretes were slightly reduced by 2.7% and 3.0%, respectively. The compressive strength decreased by about 19.4%. At higher substitution rates (>5%), the mechanical properties were significantly decreased. It has to be kept in mind that the incorporation of GFRP waste can lead to a workability (or slump) loss that can be overcome with the addition of a superplasticizer or an increase in the water content (water to cement ratio). The increase in substitution rates significantly increases the amount of water needed to maintain the workability [
8,
24]. This is why concretes containing GFRP waste are often fabricated with an equivalent water to cement ratio.
Tittarelli and Moriconi [
10] investigated the possibility of reusing GFRP waste coming directly from a shipyard as a partial aggregate or filler replacement in cement-based composites. The authors specify that a high GFRP replacement rate (15% and 20%) significantly decreases the mortar workability, and a superplasticizer was added (0.25% and 1 wt% of cement) in order to maintain the workability. Tests were performed on cement mortars manufactured by replacing 0%, 10%, 15%, and 20% of the aggregate volume with a GFRP byproduct and on self-compacting concretes (SCC) manufactured through replacing 0%, 25%, and 50% of the calcareous filler volume with GFRP powder. GFRP powder is composed of 20% (v%) glass fibers and 80% (v%) polyester resin. A strong reduction in compressive strength (up to about 25%) was observed both in mortars and self-compacting concretes. The capillary water absorption and drying shrinkage of the GFRP cementitious composites are decreased by about 70% and 50%, respectively, compared to the reference samples without GFRP waste.
Furthermore, it is interesting to note that some studies have reported an improvement in the toughness of cementitious materials incorporating GFRP waste. Overall, the use of recycled GFRP waste in concrete/mortar has contributed to the reduction in crack propagation [
29,
35,
36,
37]. This improvement in toughness is very interesting because it can limit the effects of restrained shrinkage. In addition, it is possible to further improve the toughness of the cementitious material and its mechanical properties by improving the dispersion and orientation of the recycled GFRP fibers [
37].
All these studies show that the reuse of GFRP waste in cement-based materials can be interesting. Generally, the partial substitution of sand by GFRP waste results in a reduction in the compressive strength, which can be explained in part by the increase in water demand to maintain satisfactory workability. Additionally, it appears that the incorporation of GFRP waste slightly affects the flexural strength [
24]. It is even thought that incorporation of GFRP waste could reduce the brittleness of the cementitious material.
The aim of this study is to examine the effect of the partial substitution of sand with GFRP waste from a shipyard on the compressive and flexural strength of cement mortars. Particular focus will be placed on improving the toughness of the mortar with the incorporation of GFRP waste. In addition, the reduction in mechanical strength (notably, the compressive strength) is partly due to the increase in water demand. This work also aims to examine the contribution of an addition of superplasticizer, instead of an increase in the w/c ratio, on the loss of compressive strength.
4. Conclusions
This study aimed to investigate the effect of the incorporation of glass fiber-reinforced polymer waste (GFRP), coming from a shipyard, on the mechanical strength and workability of cement mortars.
The substitution of 3% of sand volume with GFRP waste does not affect the workability and mechanical strengths. However, increasing the GRFP content results in workability loss due to the flocculation of the GFRP particles, which increases the water demand. This workability loss requires an adjustment of the water to cement ratio, but the mechanical strengths are reduced. Adding superplasticizer can improve the workability without increasing the water to cement ratio, but the required dosage is relatively high. Moreover, the addition of the superplasticizer does not significantly limit the loss of strength. The compressive strength appears to be more affected by the incorporation of GFRP waste than the flexural strength. The decrease in the flexural strength of 15% GFRP mortar is only 37%, while the compressive strength decreases by 54%. The decrease in the mechanical strengths can be explained by a poor bonding of the GFRP particles/cement paste, an increase in voids, and a heterogeneous partitioning of waste in the mortar.
Furthermore, it seems that the incorporation of GFRP waste can enhance the flexural toughness, which increases from 0.351 N.m for the reference sample to 0.642 N·m for the sample with 15% GFRP. This improvement in the toughness can be very interesting from a practical point of view, particularly to limit the effect of the restrained shrinkage. The optimization of the dispersion and distribution of GFRP waste should be performed in order to improve the mechanical behavior.