**1. Introduction**

The civil construction sector accounts for approximately 10% of Brazil's gross domestic product [1]. In order to boost this growth in times of economic uncertainly, development of novel construction materials might be a solution to improve the process production and reduce costs [2]. Coating mortar, largely applied in all reinforced concrete carried out in Brazil, is a construction material with potential to be improved in a sustainable and cost-effective way [3]. Worldwide coating mortars are used, mainly, to protect the masonry, and represent a substantial cost to the total budget.

A conventional coating mortar is usually composed of cement, water, lime and fine aggregate. To optimize the coating mortar process and to achieve high performance in the building coating execution, a modern procedure is to use the mechanized projection method. Indeed, through this system it is possible to cover a substrate at a rate of 60/m2/day per worker, which is practically twice as much as would be achieved by conventional hand application [4]. For this reason, the interest in the use of mortars designed as an application system for these coatings has been increasing [5,6].The mechanized projection method,

**Citation:** Paes, A.L.; Alexandre, J.; Xavier, G.d.C.; Monteiro, S.N.; de Azevedo, A.R.G. Feasibility Analysis of Mortar Development with Ornamental Rock Waste for Coating Application by Mechanized Projection. *Sustainability* **2022**, *14*, 5101. https://doi.org/10.3390/ su14095101

Academic Editor: José Ignacio Alvarez

Received: 30 March 2022 Accepted: 20 April 2022 Published: 23 April 2022

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well established around the world, is an adequate solution to minimize the problems in the execution of internal and external coating of the buildings, by reducing human interference during the execution. So, it can be optimized by increasing productivity, and above all, improving the quality and gives uniformity to the coating [5,6]. There are variations of this method, for supplying the spraying equipment with fresh mortar. One system is known in Brazil as *canequinha* or compressed air spray. In more complex processes, there is a mixer coupled to the pump, so that the mortar is homogenized and sent directly to the pump from where it is projected to the substrate [4,5]. This last method was used in the present work.

According to Ribeiro et al. [5], an industrial company using this method can achieve a production up to 0.75 m2/h, while the average used to be 0.42 m2/h. With this in mind, a construction company of Campos dos Goytacazes, Brazil, chose this method and decided to replace the hydrating lime. However, it was observed the occurrence of cracks in the coating after drying and, as a reference regional center of study, the company came with this problem to be solved. The most common method of mortar application in Brazil, and in several countries with less industrialization of civil construction, is manual, which leads to higher costs and great waste. Thus, research involving the validation of mechanized projection is important in order to provide subsidies to the civil construction sector for the application of new technologies, which aim to provide greater rationality to the application of mortars. Currently, numerous companies have been using mechanized projection techniques in Brazil in order to increase their productivity. However there are still gaps related to the post-application condition of these mortars [6].

In order to mitigate such shortcomings, the present work has as its main objective to propose a new mortar mixture to be projected replacing the hydrated lime by ornamental rock waste (ORW), from Cachoeiro do Itapemirim, Brazil. There is a large amount of ornamental rock production worldwide, in the past six years, about 150 million tons were produced yearly, mainly in China, India, Turkey, Iran and Brazil [7,8]. China alone has produced in 2015 around 350,000,000 m<sup>2</sup> of marble, while the estimated export from Egypt is nearly 1,360,500 ton/year of stone, either processed or unprocessed [9,10]. In 2019 Brazil exported 2.16 million/ton [10,11]. The rock beneficiation process is divided into two stages: the blocks extraction and their beneficiation. According to Angelin et al. [12], in this last stage, the waste generation can reach 20 to 30% of the block volume. Furthermore, during the extraction of marble and granite, the loss can reach 60%, and this activity produces a fine dust that can cause health problems and damage the soil [10,11,13]. So, not only to reduce the consumption of hydrated lime, a material with high added value and a polluting production process, but also to decrease the amount of rock waste disposal at the cities, recent researches have incorporated this material into concrete, mortar, tile, pavement, and even used it for soil stabilization [10].

Some authors, such as Arce et al. [14], replaced lime by ornamental rock waste and concluded that, when finely ground, this type of waste gave mortars characteristics similar to those obtained with the lime used in fresh state. For Oliveira et al. [15], its use was interesting, since as a powdery material, there was an improvement in packaging. The choice to incorporate the ornamental rock waste into mortars contribute to mitigate the impact generated by its improper disposal. In this way, giving the ornamental rock waste an appropriate final destination by incorporating into mortars, the civil construction sector becomes more sustainable. In addition, its cost is about 90% cheaper than hydrated lime, being related only to its transport from one city to another [16]. The ornamental rock waste material had already been incorporated into self-compacting mortars and concretes by Corinaldesi et al. [17], who concluded that, owing to the waste high fineness, this material ensured cohesion and workability.

Other authors have studied the ornamental rock waste incorporation into mortar, Singh et al. [18], carried out an economic and environmental study of advantages of replacing cement by marble waste in concrete and produced three types of concrete. One type for reference and the other two with waste, from which they obtained compressive strength 20–25% higher than the reference mixture. The properties of porosity, resistance

to abrasion and carbonation, as well as resistance to sulfates and water penetration, were found superior for the modified mixtures, which can be justified by the filler effect due to the use of marble waste. In addition, they also report that the marble waste can be used to replace hydrated lime, since this rock is classified as limestone and the waste has a chemical composition similar to lime. Amaral et al. [19] studied the partial replacement of sand by rock waste. The authors defined the mixture in 1:1:6, and replacement percentages between 9 and 21% in sand weight. They analyzed their properties in the fresh state (density, incorporated air content and consistency) and in the hardened state (density, flexural and compressive strength), and concluded that these mortars showed satisfactory results, being the incorporation of 21% of residue the most recommended one. Leite et al. [20] studied mortars with the substitution of sand for residue generated in the cutting and polishing of rocks, in proportions of 0%, 10%, 20% and 30%. The authors tested their properties in the fresh and hardened state and concluded that based on physical and mechanical properties that there was not significant modification with the different incorporation level of residue. However, the sample composed of 20 wt.% of cutting residue showed the best performance for being the only one with an appropriate consistency index. This amount of residue did not significantly change the other properties, except capillarity.

Based on these preliminary considerations, the objective of this research is to evaluate the potential use of ornamental rock waste to replace hydrated lime, in order to develop a mortar to be applied by mechanized projection. Although many researcher works have already evaluated this incorporation, to our knowledge, no work exists specifically for mortar to be used by this method and on how the mechanized projection modifies the mortar properties. Indeed, modifying one component alters the entire mixture and its properties, such as void index, water absorption by capillarity and immersion, as well as the compressive, flexural and tensile bond strengths. Consequently, affects the durability and applicability of mortars. The investigation on how mortars properties were modified due to the replacing the lime by ornamental rock waste, and the creation of a superplasticizer to be used in mortar with ornamental rock waste, generating a new material to be applied by the mechanized projection method, is for the first time performed. It is also noteworthy that there are limited studies regarding the application of mechanized mortar with the addition of solid waste. Indeed, this collaborates in two ways for sustainability, either in the reduction of waste at the time of application, or in the reuse of other solid waste that would be discarded. This complementary approach will certainly fulfill a gap in the main studies already carried out.
