1. Introduction
Plasterboard consists of a hardened gypsum core between sheets of paper on each side. Natural gypsum, or calcium sulfate dihydrate (CaSO
4·2[H
2O]), is widely used as a lightweight construction material in different types of products due to its low cost and ease of installation [
1]. The microstructure of plasterboards has a high porosity and high internal surface area due to interlocking crystals. The characteristics of the material’s microstructure are responsible for its physical properties [
2]; for example, properties like pore content, water solubility, and crystal size can have an effect on water affinity [
3].
The manufacturing process of CaSO
4·2[H
2O] includes the following stages [
4]. First, it must be dehydrated by heating, causing a phase change from CaSO
4·2[H
2O] to calcium sulfate hemihydrate (CaSO
4·0.5[H
2O]). Next, to prepare a pourable paste, CaSO
4·0.5[H
2O] is mixed with water and other admixtures, which is then poured onto a pre-sized sheet of paper or cardboard that is posteriorly molded. Finally, the molded board is set to harden and dry.
CaSO
4·2[H
2O] formation based on CaSO
4·0.5[H
2O] is a crucial manufacturing step. When water is added, CaSO
4·0.5[H
2O] hardens and remains in its original dihydrate state. Singh and Middendorf [
5] and Chen et al. [
6] found that CaSO
4·0.5[H
2O] hydration, leading to the formation of CaSO
4·2[H
2O], is due to the initial dissolution of CaSO
4·0.5[H
2O] particles in water, which leads to the precipitation of the less soluble CaSO
4·2[H
2O]. The particle size of CaSO
4·0.5[H
2O] controls the generation of mesopores; smaller particles promote a faster dissolution before the setting process [
7]. Crystal size depends on the duration of the set stirring time during the formation of CaSO
4·2[H
2O] [
8].
Li et al. [
9] and Roveri et al. [
10] found that the use of chemical additives could enhance the control of the final structure and properties resulting from CaSO
4·2[H
2O] formation based on CaSO
4·0.5[H
2O]. These studies showed that using a hydrophobic organic emulsion encased the CaSO
4·0.5[H
2O] particles during hydration, causing a change in the physical properties of the hydrated product; the result was a smoother surface of the product that prevented further hydration, and the crystal morphology of the intertwined CaSO
4·2[H
2O] changed from the classic needle structure to thicker and shorter column forms.
Interactions between CaSO
4·0.5[H
2O] and chemical additives depend on the nucleation process, which usually occurs through the blending of primary species by collision. However, chemical additives usually reduce the collision probability, resulting in more time for crystal nucleation and for the transformation into crystalline CaSO
4·2[H
2O] [
5,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21]. Additionally, Pan and Li [
22] proposed the use of an admixture containing fluorinated silicone to improve the humidity resistance. They found that the microscopic pores were covered by a film provided by the waterproof agent, turning the internal macropore surface from hydrophilic to hydrophobic, resulting in increased water repulsion and strength due to the prevention of water penetration in the matrix.
Wu et al. [
23] found that sodium methyl silanol increased water absorption by reducing the pore size (on average, from 500 μm to 100 μm) and increasing pore interconnectivity. Other researchers showed that some admixtures that act as hydration accelerators promote the formation of a denser and more compact crystal microstructure, boosting the impermeability of the CaSO
4·2[H
2O] matrix [
23,
24]. The type and dose of the additive, which influences the formation of hydration products and the final CaSO
4·2[H
2O] microstructure, ultimately influences the material’s porosity and could effectively reduce it. Furthermore, additives can be used to form fine structures, which are associated with higher mechanical strength and humidity resistance [
24,
25,
26].
The most widely used chemical additive in plasterboard to provide moisture resistance is polymethylhydrosiloxane (PMHS) [
27], giving the matrix of the plasterboard its hydrophobic property. PMHS is composed of a repetitive structure of silicon, oxygen, and hydrogen atoms and methyl groups linked by covalent bonds. The methyl groups are responsible for the high hydrophobicity of the material and reach a contact angle of 100 ± 2.0° on the plasterboard [
27,
28,
29].
Several recent studies have been carried out to improve the properties of plasterboards by reusing waste material. For instance, previous efforts to add plastic waste to the CaSO
4·2[H
2O] matrix include the research conducted by Pedreño-Rojas et al. [
30], who mixed recycled polycarbonate from waste compact discs (CDs) and digital versatile discs (DVDs) with CaSO
4·0.5[H
2O] and recycled CaSO
4·2[H
2O]. The mechanical strength increased due to the recycled CaSO
4·2[H
2O], which contained fiberglass remains, but the density and thermal conductivity decreased. Del Rio Merino et al. [
31] incorporated ceramic waste and extruded polystyrene with the objective of reducing the amount of raw material and improving the properties of traditional CaSO
4·2[H
2O]. The results showed that the water absorption decreased, while the surface hardness increased compared to the reference CaSO
4·2[H
2O]. Santamaria-Vicario et al. [
32] and Buggakupa et al. [
33] carried out experiments using polyurethane foam residues and used CaSO
4·2[H
2O] molds and glass remains to produce water-resistant CaSO
4·2[H
2O]-based products. In addition, previous studies [
34,
35] on the use of plastic residues in CaSO
4·2[H
2O] matrices showed that the addition of plastic residues improved the surface hardness and absorption and significantly reduced CaSO
4·2[H
2O] and water consumption without affecting the hygrothermal properties, while keeping their mechanical characteristics above the minimums required under current regulations.
Furthermore, beyond the previous research mentioned regarding the reuse of waste material as additives, the research conducted by Zhu et al. [
36] studied the effect of incorporating polyvinyl alcohol and polypropylene fibers into CaSO
4·2[H
2O]-based compounds to influence properties such as workability, hydration kinetics, flexural strength, and hardness. The results obtained demonstrated that the inclusion of fibers significantly increased the flexural strength and hardness, but decreased the workability and hydration rate of the samples. Furthermore, using scanning electron microscopy (SEM), it was observed that the interfacial transition zone (ITZ) between the fiber and the CaSO
4·2[H
2O] was remarkably compact, and the space was much smaller, which is relevant when analyzing the effect of the plastic–CaSO
4·2[H
2O] matrix union.
Some previous work has been carried out regarding the use of polyethylene terephthalate (PET). Ali et al. [
37] studied the effect of incorporating lightweight PET waste into CaSO
4·2[H
2O] matrices under standard laboratory conditions and obtained improved physical, mechanical, and insulating performances. The research found that the best behavior was achieved in mixtures with an addition of 7% PET weight, while the flexural strength decreased by over 10%. In addition, Erdem and Arioglu [
38] produced a composite material by adding recycled PET fibers and an additive that improved adhesion in the CaSO
4·2[H
2O] matrix and properties. The test results showed that adding the fibers slightly decreased the flexural strength, but the admixture improved adhesion, producing less reduction in the flexural and compressive strength. Additionally, in another study, mixtures were used to analyze the influence of different amounts of solid residues such as recycled PET on the mechanical properties of CaSO
4·2[H
2O] at room temperature. Adding this residue to CaSO
4·2[H
2O] improved the compression strength compared to the reference mix [
39].
Some research has also been undertaken to evaluate the use of different fillers and fine materials on hydrated calcium sulfate preparations. Doleželová et al. [
40] studied the structure and behavior of CaSO
4·2[H
2O] compounds prepared with different fillers and fine materials such as silica sand, perlite, expanded clay aggregate, and residual polyurethane foam and the mechanical strength, thermal conductivity, and moisture were measured. The intrinsic properties of the aggregate type and their surface quality were found to affect the CaSO
4·2[H
2O] crystal size and shape significantly. Flexural strength increased with a higher surface roughness of the particles. The more porous the particle surface, the smaller the CaSO
4·2[H
2O] crystals in the ITZ and the more densely packed they were.
We can see that these conclusions are similar to the previously mentioned investigations regarding the properties of plasterboard with added plastics. In general, the water absorption capacity, the thermal conductivity, and the mechanical strength decreases [
34,
41] as the plastic weight percentage in the mixture increases. However, none of these investigations included the evaluation of the interaction between the effect of plastic waste and the preparation variables such as stirring time, PMHS dosage, and drying temperature after the setting of CaSO
4·2[H
2O]. Such an understanding could allow us to improve our understanding for the industrial preparation of plasterboards.
In addition, in our previous study [
42], we evaluated the effect of PMHS on the morphology and porosity of CaSO
4·2[H
2O] plasterboard. The results showed that the PMHS admixture caused changes in the morphology and porosity of the CaSO
4·2[H
2O] structure obtained, which decreased the moisture absorption and thermal conductivity without affecting the flexural strength.
In this investigation, we chose to evaluate the effect of a fine microplastic PET as a material filler in plasterboard. The reutilization of fine microplastics as a filler could help with the recirculation of a material not useful for recycling. The fine microplastic waste resulting from the plastic recycling process is usually sent to a final disposal landfill where it will stay for decades. This material is not selected to be recycled because it reduces the performance of the recycling process and generates operational problems in the whole recycling plant. These microplastics are flakes obtained after grinding and screening while the PET is prepared for recycling, and they have a particle size smaller than 5 mm. The addition of these recycled microplastics may cause positive changes in the morphology and porosity of plasterboard, affecting the water absorption, flexural strength, and thermal conductivity. To evaluate these effects, several tests were carried out under controlled conditions to which the boards were subjected during the manufacturing process. The plasterboard performance is related to the effects of the PMHS dosage, stirring time of the mixture, and the drying temperature after setting. This research contributes to improving our understanding of the effect of the addition of fine particles of PET to plasterboard and to the creation of a circular economy where PET can be used in plasterboard production.