Factibilidad en la fabricación de ladrillos no estructurales, a partir del reciclaje de las colillas de cigarrillo Practicability in the manufacture of non-structural bricks, from the recycling of cigarette butts

Cigarette butts are one of the most common waste worldwide. The toxic and non-biodegradable components make cigarette butts a hazardous waste and one of the causes of pollution. This paper presents some of the results of a study on the re-cycling cigarette butts in the manufacture of clay bricks. Four mixtures were made, these include a control clay brick, with a clay content of 100% (LADRICOL 0%) and three additional mixtures incorporating cigarette butts in different percentages by weight (LADRICOL 2.5%, LADRICOL 5.0% and LADRICOL 7.5%). The mixtures were fired at different temperatures and these were tested to determine the physical and mechanical properties of the bricks. The results showed that the samples incorporating 2.5% cigarette butts content and fired at 850 °C are in compliance with the standard normative for this type of product (NTC 4205-2). Furthermore, this brick can improve the environmental quality and can reduce energy consumption during firing, by 19.75%.


Introduction
Cigarette butts are undoubtedly the most common type of waste dumped on the streets. At present, approximately 6 trillion cigarettes a year are consumed worldwide, of which 4.5 trillion are dumped into the environment once consumed (De Granada et al., 2016). The high rates of waste dumped into the soil and water sources without control have become one of the causes of pollution, not only by the high volume of cigarette butts dumped but also by the toxic substances present in these products.
The materials used to make cigarette filters include cellulose acetate, a non-biodegradable material that can take between eighteen months and ten years to decompose depending on the environmental conditions to which it is exposed. Although the sun rays can break the filter down into small pieces, this material never disappears; instead, it passes into the soil and water sources, causing environmental pollution (Novotny and Slaughter, 2014) and (Mohajerani et al., 2016).
Besides cellulose acetate, the cigarette comprises about 4,000 chemical substances such as ammonia, nitrogen oxide, hydrogen cyanide, pesticides and some toxic metals such as cadmium and nickel that are part of the numerous chemicals considered carcinogenic (Castañeda, 2011) and (Monzonis, 2011). It is also composed of other toxic substances such as nicotine and tar, which are trapped in the cigarette butts. Only one cigarette can contaminate up to 50 liters of water (Guevara, 2010) and (Lozano et al., 2015).
Despite the risks that cigarette butts represent for the environment, it is frequent to see smokers throwing them on the ground, parks, bridges, streets and many other public places where they accumulate and become a source of danger for domestic animals, fish, birds, turtles, among others, which can ingest them causing their death due to their toxicity (Castañeda, 2011 and(Rath et al., 2012).
In Colombia,12.9% of the population over the age of 18 is a current consumer of cigarettes. This figure is equivalent to just over 3 million smokers in the country. (Gobierno Nacional de la Republica de Colombia, 2014). On the other hand, (Lozano et al. 2015) indicated that the number of cigarettes consumed in Colombia ranges from 412 to 600 cigarettes per smoker. This is why the country generates approximately 1,236 to 1.8 billion cigarette butts per year.
On the other hand, among the most used construction materials worldwide are clay bricks. During its manufacture, this material generates negative impacts on the environment due to the amount of energy it requires. Bricks are made from clay and are subjected to high temperatures during firing. Therefore, it is estimated that they contain approximately 2.0 kWh of incorporated energy and release around 0.41 kg carbon dioxide (CO 2 ), among other gases released into the atmosphere during their manufacture, such as carbon monoxide (CO), sulfur dioxide (SO 2 ), ammonia (NH 3 ), chlorine (Cl 2 ) and fluorine (F) (Zhang, 2013); (Abdul et al., 2015).
For this reason, this article presents the results of an investigation where cigarette butts were recycled by including them in the manufacture of clay bricks, evaluating the effect of the firing temperature on the physical and mechanical properties of the bricks, as well as its effect on the possible reduction of energy costs associated with their manufacture.

Preparation of the mixtures and processing of the specimens
The raw materials used were mainly clay and cigarette butts. In the case of cigarette butts, these were collected from streets and common areas such as bars, universities and business buildings. Additionally, a previous adaptation of these materials was carried out by grinding them using a domestic mill (Figure 1).

Figure 1. Mill and ground cigarette butts
Revista Ingeniería de Construcción Vol 35 Nº3 Diciembre de 2020 www.ricuc.cl The clay was supplied by a local brick company and subjected to a milling and sieving process for its respective adaptation. Also, Atterberg limits were determined according to  and density  tests, as well as thermogravimetric (TGA) and thermo-differential (TDA) analyses were carried out at a heating speed of 10 ºC/min, from room temperature to 1000 ºC, in air atmosphere at a flow rate of 100 ml/min, chemical composition (X-ray fluorescence [XRF]), and mineralogical composition (X-ray diffraction [XRD]), scanned in the range of 2 = 5 -80° at a scanning speed of 0.02 sec/step, using Cu Kα radiation at 45 kV and 40 mA.
Four mixtures were made, including a standard mixture with 100% clay content (LADRICOL 0%) and three additional mixtures in which cigarette butts were incorporated in different percentages by weight, in relation to the clay content (LADRICOL 2.5%, LADRICOL 5% and LADRICOL 7.5%).
For the production of the specimens, these mixtures were molded into prismatic specimens of 150 x 50 x 25 mm. They were subsequently pressed with a Shimadzu machine at an average compaction pressure of 0.92 MPa.
The specimens were dried in an oven at 105 -110 °C for 48 hours. The firing was carried out at 800 °C, 850 °C and 900 °C to identify the optimum temperature at which the brick obtained the best physical and mechanical properties.

Determination of physical and mechanical properties
At the end of each of the molding, drying and firing phases, weights and dimensions of each specimen were recorded to determine important physical and mechanical properties of the brick and to determine the practicability of its manufacture and use as non-structural type M masonry, compliant with the requirements of NTC 4205-2. Considering non-structural type M masonry those units without perforations and those in which the perforations represent less than 25% of the total volume of the unit (Instituto Colombiano de Normas Técnicas y Certificación, 2019).
The firing shrinkage, volumetric density, water absorption percentage, and initial absorption rate were determined within the physical properties. In the case of the mechanical properties, the compressive strength and flexural strength of the finished product were evaluated, taking into account the methods described in NTC 4017. The reported results correspond to the average of three specimens tested in each mixture.
To determine the compressive strength of the finished product, it was previously submitted to capping to parallel the load faces and achieve a uniform distribution of the load on the specimen to be tested (Sanchez and Mejía, 2009 Finally, X-ray diffraction (XRD) tests were carried out on the conventional brick and the brick in which 2.5% of cigarette butts were incorporated to know the mineralogical phases present in each of them.

Energy saving
The assessment of energy costs associated with the production of each brick was carried out. Subsequently, with this information, the respective comparisons to determine the possible reduction in energy consumption were made.
To calculate the energy cost of the conventional brick, the average national energy consumption was taken as a reference, which is 2,405 MJ for each ton of bricks produced (Unidad de Planeación Minero-Energética, 2001). With this information and the average value of the kWh for the non-residential sector, the energy cost associated with the firing of the brick was determined.
Concerning the energy consumption for the manufacture of the brick in which cigarette butts were incorporated, the calculation was made taking into account the clay mass and the percentage of cigarette butts incorporated, as well as the calorific value of the cellulose acetate, which corresponds to 19 MJ kg -1 (Mohajerani et al., 2016).
To determine the energy saving generated by incorporating the cigarette butts, the energy used in the manufacture of the conventional brick was calculated (Equation 3.1) as well as the energy used in the brick with 2.5% cigarette butts (Equation 3.2). Finally, the energy saving percentage was calculated using (Equation 3.3), as mentioned by (Mohajerani et al., 2016).
Energy used in LADRICOL 0%, Energy used in LADRICOL 2,5%, Energy saving,  (Table 1) shows the physical properties of the clay used for the manufacture of the specimens.

Physical and chemical properties of clay
Revista Ingeniería de Construcción Vol 35 Nº3 Diciembre de 2020 www.ricuc.cl (Figure 2) shows the granulometric distribution curve of the clay used.
In the tests of thermogravimetric (ATG) and thermo-differential (ATD) analyses, a loss of weight and an endothermic peak ranging from 25 ºC to 110 ºC could be observed, which is possibly attributed to the loss of water from moisture. At approximately 520°C, structural water loss from kaolinite occurs (Figure 3). This is in line with authors such as (Joshi et al. 1994) and (Isel et al. (2017), who indicate that the dehydroxylation of this mineral generally occurs between 400 °C and 600 °C.
Regarding the chemical properties, (Table 2) shows the results obtained in the X-ray fluorescence (XRF) analysis, where it can be noted that the most present components are silica (SiO 2 ) and alumina (Al 2 O 3 ). As mentioned by authors such as (Barranzuela 2014) and (Santos et al. 2009), these are the two most essential components in the composition of clays. Therefore, they are usually the most abundant in this material, while others, such as iron, are usually found in lower percentages.     (Mohajerani et al. 2016) and(Fuentes et al. 2017) in the manufacture of bricks containing cigarette butts, biosolids and fly ash.
The apparent density results show that this property increases in direct proportion to the firing temperature, as observed in (Figure 5), (Figure 6), (Figure 7) and (Figure 8); although, it decreases when the amount of cigarette butts incorporated increases.    Regarding water absorption, this relationship is inversely proportional, as shown in (Figure 9), (Figure 10), ( Figure 11) and (Figure 12). In these cases, water absorption decreases as the firing temperature increases. However, it is directly proportional to the addition of cigarette butts since in LADRICOL 5% and 7.5% mixtures the absorption is higher than in the conventional mixture and in the mixture with 2.5% butts. This behavior is attributed to the increase in the brick porosity since the bricks in which the butts were added presented a greater porosity in their interior and consequently greater water absorption.

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Revista Ingeniería de Construcción Vol 35 Nº3 Diciembre de 2020 www.ricuc.cl In addition, the mixtures LADRICOL 5.0% and LADRICOL 7.5% were rejected, as they did not comply with the usual minimum density. In both cases, the densities did not exceed the minimum value of 1.75 g/cm 3 , thus affecting the strength of the material.
Based on the results shown above, the mixtures LADRICOL 0% and LADRICOL 2.5%, fired at a temperature of 850 °C, were chosen as suitable for the manufacture of non-structural type M masonry. The initial absorption rate was evaluated on these mixtures, obtaining the results shown in (Figure 13).
The result of the initial absorption rate increased for the mixture containing cigarette butts. This is mainly due to the porosity generated by the butts inside the brick, which in turn generates a higher initial absorption rate. However, this value does not exceed the 0.25 g/cm 2 /min established in NTC 4205-2.

Mechanical properties of fired bricks
The average result of the compressive strength for the mixtures LADRICOL 0% and LADRICOL 2.5% was 19, 03 ± 4.33 MPa and 18.85 ± 1.10 MPa, respectively. See (Figure 14). This is compliant with NTC 4205-2, which establishes a minimum compressive strength for non-structural type M masonry of 10 MPa. This result also complied with the Colombian regulations for seismic-resistant construction (NSR-10 title D,  Regarding flexural strength, the result obtained was 3.39 ± 0.24 MPa for the mixture LADRICOL 0%, and 1.28 0.24 MPa for the mixture LADRICOL 2.5%, as shown in (Figure 15).
There is a slight reduction in the physical and mechanical properties of the bricks containing cigarette butts (LADRICOL 2.5%). This reduction is associated with an increase in the porosity of the interior of the fired brick. The brick porosity increases in proportion to the percentage of butts added, as shown in (Figure 16).

Energy consumption
To calculate the energy consumption required for firing the mixture LADRICOL 0% and LADRICOL 2.5%, (Equation 3.1) and (Equation 3.2) were used. (Table 3) shows the results and reflects a higher energy consumption in the conventional brick (0.9160 MJ) than in the brick with 2.5% cigarette butts (0.7351 MJ).
With respect to the percentage of energy saving, the result reflects a saving of 19.75% by incorporating cigarette butts in the manufacture of the brick. This is mainly due to the cellulose acetate present in the cigarette butts, as it is organic matter, and when mixed with the clay, it contributes positively to the firing process, allowing the concentration of heat; thus reducing the amount of energy required for the firing of the brick (Jackson and Dhir, 1996) and (Mohajerani et al., 2016).