**1. Introduction**

The building materials demand is rapidly increasing, especially in developing countries, on account of the rapid urbanization and the improving life standart. Consequently, decorative stone mining has grown to be an remarkably prosperous activity [1]. Brazil is one of the world's largest ornamental stone producers, having produced nine million tons of stone for coating and decoration in 2020, despite the pandemic that devastated the world, only 2.2% lower than the previous year [2]. The large amount of waste generated by the ornamental stone industry produces several environmental impacts such as water, landscape and flora pollution, since most of this waste is improperly disposed of [3,4]. Furthermore, according to Cordon et al. about 75% of natural resources are consumed for the development of this activity [5].

In the current scenario, environmental preservation is crucial, bringing prominence to the development of sustainable building materials that contribute to its preservation while simultaneously fulfilling their functions. Several researches around the world have been carried out in order to reuse granite wastes, thus reducing their environmental impacts [1,3]. It is worth mentioning that, under regular working conditions, granite waste is not environmentally deleterious, being classified as a non-inert and non-hazardous waste, i.e., it does not react and does not dissolve in water [6].

Some specific granite waste characteristics favor its industrial use as raw material for novel products, such as: fine granulometry, predefined chemical composition and

**Citation:** Gomes, M.L.P.M.; Carvalho, E.A.S.; Barreto, G.N.S.; Rodriguez, R.J.S.; Monteiro, S.N.; Vieira, C.M.F. Development of Sustainable Artificial Stone Using Granite Waste and Biodegradable Polyurethane from Castor Oil. *Sustainability* **2022**, *14*, 6380. https://doi.org/10.3390/ su14116380

Academic Editor: Miguel Bravo

Received: 1 April 2022 Accepted: 9 May 2022 Published: 24 May 2022

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the absence of mixed grains between the basic components [1]. For instance, in mortar manufacturing, it is possible to use granite wastes to replace natural aggregates, saving natural resources and providing a good mechanical performance without harming the mortar's appearance, once granite contains in its composition [1]. For concrete, it is possible to reuse granite waste to partially replace the fine aggregates, which even increases its compressive and flexural strength [7]. For ceramic products manufacturing, fine granite waste can be mixed in the clay mass, producing ceramic bodies with promising properties, depending on the firing temperature, and without altering its color, due to oxides content in granite's chemical composition [8]. Thereby, granite wastes are able to partially replace the fine aggregates in concrete, increasing its bending and compression strength [1,7,8].

Another alternative for recycling granite waste is its reuse as a raw material in the development of other construction materials, such as artificial stone. Indeed, artificial stones are made of a high content of particulate aggregates bonded by polymeric resins. The aesthetics and properties of artificial stones are distinct since their manufacturing occurs under different conditions, from a wide range of compositions, different types of aggregates and resins binders as well as different types of processing [9,10].

Artificial stones are, in general, technically superior compared to natural ornamental stones. As the particles are bonded together by a polymeric resin, which results in a lighter material. In fact, the artificial stones' density is lower. To improve even more, the resin fulfills most of the voids and pores, shaping a material to be further resistant to liquid infiltration, granting its use in humid environments for a long term without its performance being impaired. Likewise, the mechanical strength is superior, once voids can work as stress concentrators and facilitate crack expansion [11].

Several researches have been carried out in order to develop the most differentiated artificial stones, varying the types of aggregates and polymeric resins [12–15]. Shishegaran et al. [12] evaluated the mechanical properties of three artificial stones containing: traveline powder and sludge, traveline powder and sand and traveline powder, sand and sludge, using two distinct epoxy resins. Sarami and Mahdavian [13] evaluated the properties of artificial stones using travertine, marmarit stone with cement, water and unsaturated polyester resin. Borsellino et al. [14] produced artificial marbles using polyester or epoxy resin varying the power concentration of marble particles. Carvalho et al. [15] developed an artificial stone with an epoxy matrix incorporated with sintering steel industry waste. Notwithstanding, the researches carried out until then have used synthetic resins, derived from petroleum, in their polymer matrices, mainly epoxy and polyester resins.

Petroleum-derived polymers consumption has been growing over time, also giving rise to a large amount of waste that is often discarded inappropriately and, in aggravation, takes years to degrade. To minimize this problem, it is possible to use biodegradable polymers as their technical and economic feasibility has great potential for expansion [16]. Polyurethane (PU) castor oil is a widely found low-cost renewable oil that is attracting attention from researchers due to its utilization in coatings, adhesives, paints, sealants and encapsulating compounds. Castor oil-based PUs ordinarily have low mechanical strength and limited ductility due to their flexible, highly crosslinked and permanent network structure. In a sustainability concepts, it is interesting to replace polymers by this type of PU from a renewable source [17,18].

This work's main goal is to develop and characterize a novel sustainable artificial stone that, together with the granite waste recycling, also proposes application of a biodegradable PU from castor oil, a renewable and non-toxic source, as matrix. The scope of this work is also the evaluation of whether it can be used as a coating in civil construction. The artificial stone waste/polyurethane resin (ASPU) developed composite allows the creation of sustainable material with suitable properties to be used as coating and reducing the harmful environmental impacts that would occur with the disposal of granite waste and the synthetic polymers utilization.
