The chapter delves into the potential of waste limestone powder as a filler for epoxy coatings in floors, focusing on its effects on mechanical properties such as hardness, tensile strength, and flexural strength. Previous research has shown that waste limestone powder can reduce the toxicity and cost of epoxy coatings, but the particle size of the powder has a significant impact on the properties of the coating. This study aims to extend previous research by analyzing the effects of smaller particle sizes on the mechanical properties of epoxy coatings. The results demonstrate that while smaller particle sizes improve hardness and reduce the disparity between top and bottom hardness, they also decrease tensile and flexural strength. However, the pull-off strength of the epoxy coating remains generally unaffected, meeting standard requirements. This research contributes to the circular economy by finding new applications for mineral waste and reducing the demand for harmful epoxy resin.
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Abstract
This paper presents an analysis of the mechanical properties of modified epoxy coatings used as epoxy floors. Waste mineral powder (limestone powder) was used as filler for the epoxy coating. Epoxy resin mixtures were made with waste limestone powder in amounts ranging from 0% to 29% of the mixture mass. Then, four mechanical properties were tested: hardness with the Shore D durometer, tensile and flexural strength with a standard testing machine, and pull-off strength by the pull-off method. The use of waste limestone powder as filler for epoxy coatings resulted in an improvement in hardness by 5%, does not significantly change the pull-off strength, but a deterioration of tensile strength by 6–27% and flexural strength by 18–38%. However, the modified epoxy coating still meets the standard requirements for epoxy floors. Therefore, waste limestone powder can be used in practice as filler for epoxy floor coatings. This solution allows the recycling of mineral powders, reduces the consumption of harmful epoxy resin and lowers the cost of the coating.
The search for fillers for building materials is a common practice among manufacturers and scientists. This is often dictated by the desire to reduce material costs and waste disposal. Civil engineering opens up wide possibilities for recycling and finding applications for materials previously considered waste. An example is waste fly ash, which is now widely used in the production of concrete. Epoxy floor coatings are a material worth looking for fillers. Epoxy coatings are readily used in industrial construction because they create a surface with high chemical resistance and are resistant to persistent dirt. However, epoxy resin is a relatively expensive material and its components are harmful to the environment. As shown by the toxicity analysis in [1], epoxy resin contains more than 50% of ingredients that are harmful to organisms, especially aquatic ones, can damage organs or may cause cancer. Epoxy resin ingredients are also irritating to the skin, eyes and respiratory tract. Adding 29% of mineral powder can reduce the content of harmful ingredients in the epoxy coating by 20 percentage points [1]. Therefore, it is worth looking for fillers for epoxy coatings, especially fillers that are currently waste.
In recent years, many studies have been carried out on the modification of epoxy resins with bio-waste. Among the tested bioadditives to epoxy resins, the following can be mentioned: coconut fibers [2] biosilica and biocarbon from rice husk [3], waste hemp-derived carbon fibers [4], waste eggshell [5], orange peel [6] and waste hemp fibres [7]. Some waste additives affect the properties of epoxy resin important for the coating, e.g. waste lignin and salicylate alumoxane nanoparticles improves tensile strength and hardness [8], walnut shells improve hardness and reduce tensile strength [9], and on the other hand in [10] it is shown that hazelnut shell improve tensile strength. Other waste epoxy resin additives tested include: polypropylene plastic wastes, which improved hardness [11], waste polycarbonate, which improved hardness, impact strength and wear resistance [12] and fly ash, which improved hardness, tensile strength, flexural strength and wear resistance [13]. The addition of fly ash can improve the bond strength of the epoxy resin [14].
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According to the latest waste statistics data published by Eurostat [15], more than 23% of all waste in the European Union is waste from mining and quarrying, most of which is major mineral waste. In order to contribute to the reduction of the amount of mineral waste stored in heaps, the possibility of their recycling should be sought. Ray et al. [16] added waste marble powder to the epoxy resin to improve the thermal conductivity of the epoxy resin. Sharma et al. [17] used waste granite powder and improved hardness, impact strength and wear resistance. Krzywiński et al. [18, 19] used fine aggregate from demolition waste, which improved the pull-off strength of epoxy coatings.
Chowaniec et. al. Have previously carried out research on the possibility of using waste quartz and limestone powders as fillers for epoxy coatings. These tests [1, 20] showed that all waste quartz powders in the amount of up to 29% by weight of the coating mass do not deteriorate the pull-off strength, reduce the toxicity and costs of epoxy coating.
In the work [21], the influence of waste limestone powder on the epoxy coating was investigated, where 85% of the powder particles had a size of 0.1 to 1.2 mm. This limestone powder sedimented a lot. The addition of limestone powder did not affect the hardness of the coating, it caused a decrease in tensile strength of the coating by 60–62%, flexural strength by 10–59% and a decrease in pull-off strength of coating by 1–7%.
2 Research Goals and Significance
The motivation of this work was the desire to extend the research on the possibility of using waste limestone powders as fillers for epoxy coatings. These activities support: the idea of a circular economy (recycling of mineral waste), the natural environment (reducing the demand for harmful epoxy resin) and economy (lowering the cost of the floor). As mentioned in the introduction, the adhesion tests of the epoxy coating with waste limestone powders were previously carried out [21]. As the next stage of research, it was decided to analyze epoxy coatings with waste limestone powder, but with a much smaller particle size. The specific objectives of the research are:
characterization of the effect of waste limestone powder on the hardness, tensile strength and flexural strength of epoxy coating by making standard samples,
broadening the knowledge about the adhesion of the epoxy coating with waste limestone powders by making fragments of the floor,
the comparison of the research results with the standard requirements for epoxy floors will allow to verify the applicability of the developed solution in real civil engineering areas.
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3 Materials
3.1 Materials
The epoxy coating was obtained by mixing two components: epoxy resin and phenalkamine curing agent. Waste mineral powder (limestone powder) was selected as filler. The chemical composition and the particle size distribution of the powder was determined by spectronometric analysis using the SEM JEOL model JSM-6610A. All particles were smaller than 0.03 mm. The results of both analyses are presented in Table 1 and Fig. 1a.
Table 1.
Chemical composition of the waste limestone powder.
Compound
CaCO3
MgO
SiO2
Fe2O3
Al2O3
%
97.12
1.00
1.50
0.08
0.30
Fig. 1.
(a) Particle size distribution curves for waste limestone powder, (b) Mixing ratio of epoxy coating (ration in g/100g of the mixture)
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As in the previous tests [21], five mixtures of epoxy resin with a curing agent and with limestone powder were prepared. One reference mixture was also prepared without powder. In each case, the weight ratio between the epoxy resin and the curing agent was the same (100: 50). The composition of each mixture in grams per 100 g of the mixture was shown in Fig. 1b.
3.2 Preparation of the Samples
Samples for testing hardness, tensile and flexural strength.
The dimensions of the samples were determined on the basis of the PN-EN ISO 527–1:2020–01 standard for tensile samples and according to PN-EN ISO 178: 2019–06 for flexural samples (see Fig. 2). The dimensions of the samples and the process of their preparation were prepared in the same way as in the last tests [21]. Five samples of the mixtures were made for each test. The samples were made at room temperature.
Fig. 2.
Shapes of the samples (in mm) for: (a) flexural, and (b) tensile strength tests
×
Samples for the pull-off strength tests.
The epoxy coating for the pull-off strength tests was made on the same substrate and in the same way as in the previous tests [21]. The thickness of the bonding agent layer was about 0.1–0.2 mm, the epoxy coating was 1.4 mm and the gray finish was 0.5 mm (see Fig. 3).
Fig. 3.
View of the substrate
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4 Methods
The hardness test procedure was carried out in accordance with ISO 868: 2003, using a Shore D durometer. The tensile strength tests were carried out in accordance with PN-EN ISO 527–1: 2020–01, using a universal testing machine with a load range of up to 2 kN. The rate of application of the axial force was 2 mm/min. The flexural strength tests were carried out in accordance with the PN-EN ISO 178: 2019–06, using the same machine as in the tensile tests. The rate of force application was 1 mm/min, and the support spacing was 64 mm. For each test, the average of the measurements was calculated from five samples.
The pull-off strength tests were performed in accordance with ASTM D4541 using a DY-216 machine (Proceq, Switzerland). The rate of force application was 0.050 MPa/s. The average pull-off strength was calculated from the 3 results for each area as in Fig. 4.
5 Results
5.1 Hardness
Studies with previous waste limestone powder [21] showed that limestone powder sediment in the epoxy coating. This creates a layer on the bottom of the samples with a higher powder content than on the remaining height of the samples, which affects the hardness results. Therefore, hardness tests were performed on the top and bottom of each sample.
Fig. 4.
The dependence of the hardness of the epoxy coating on the content of waste limestone powder for: (a) top side, (b) bottom side
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Analyzing the results for the top side (see Fig. 4a) it can be seen that the waste limestone powders had little effect on the hardness of the epoxy coating. All results are between 79 and 81. Standard deviation (SD) for all results ranged from 1.6 to 2.1. The coefficient of variation (CV) ranged from 2.0 to 2.6. The bottom side of the samples gave slightly better results than the top side (on average by 1). Epoxy coating hardness results ranged from 80 to 82. SD for all scores ranged from 0.7 to 1.5, CV ranged from 0.9 to 1.9.
According to the PN-EN 1504–2 standard, the minimum Shore D hardness of the floor coating is 60. Analysing the product data sheets of other epoxy floor coatings available on the European market, their Shore D hardness is usually in the range of 65 to 80. Thus, the tested epoxy coating with waste limestone powders has a hardness well above the required value.
5.2 Tensile and Flexural Strength
The waste limestone powder at all percentages reduced the tensile and flexural strength of the epoxy coating (see Fig. 5). The reduction in tensile strength ranged from 28% to 47% compared to the reference sample. The reduction in flexural strength ranged from 36% to 39% compared to the reference sample. SD and CV significantly worsened. The deterioration of the SD and CV parameters may result from the accidental distribution of the powder particles in the samples and hence the lower reproducibility of the results.
Fig. 5.
The dependence of strength of the epoxy coating on the content of waste limestone powder for: (a) tensile strength, (b) flexural strength
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No standard specifies the tensile and flexural strength values required for floor coatings. Therefore, the decision to use the developed solution depends on the designer and the expected loads on the floor. However, it can be assumed that the tensile strength of the epoxy coating must be the same or greater than the tensile strength of the substrate material. Fiber-reinforced concrete is most often used as the substrate for the floor. According to Marcalikova et al. [22] the tensile strength of the fiber-reinforced concrete substrate usually does not exceed 13 MPa. Therefore, it is likely that the reduction in tensile strength should not be a barrier for most floors.
Some European companies provide the declared values of the flexural strength of the epoxy coating in the product sheets. On the basis of these product sheets, it was estimated that usually epoxy coatings of floors have a flexural strength in the range of 20–60 MPa. Thus, the obtained values of flexural strength for epoxy coating with the addition of limestone powders are within the limits of standard epoxy coatings found in Europe.
5.3 Pull-Off Strength
The pull-off strength results are shown in Fig. 6. As the content of limestone powder increased, the pull-off strength decreased or increased slightly. Thus, no unequivocal downward or upward trend was observed. For the reference coating without powder, the pull-off strength was 2.9 ± 0.2 MPa for the raw surface and 2.9 ± 0.3 MPa for the ground surface. The pull-off strength results for the coating with the addition of limestone powder ranged from 2.3 MPa to 3.2 MPa for the raw surface and from 2.5 MPa to 3.1 MPa for the ground surface.
Fig. 6.
The dependence of the pull-off strength of the epoxy coating on the content of waste limestone powder
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The PN-EN 1504–2 standard characterizes that the floor coating should have an average pull-off strength of min. 1.5 MPa. All the obtained results are significantly above the standard requirements. In summary, the addition of waste limestone powders does not generally deteriorate the pull-off strength of the epoxy coating.
6 Conclusions
Based on the research, the following conclusions were made:
The addition of waste limestone powder slightly improves the hardness of the epoxy coating (by about 1–2). In previous studies, in which limestone powder with larger particles was used, the improvement was greater (by about 1–8), but only for the bottom side of the samples. The use of a powder with a smaller particle size reduced the difference between the results for the top and bottom sides.
The addition of limestone powder significantly deteriorated the tensile and flexural strength of the epoxy coating. However, the powder with smaller particles showed higher results compared to the previously used powder with larger particles [21]. The tensile strength of the powder with smaller particles was 18–24 MPa, and that of the powder from previous tests was 13–14 MPa. The flexural strength for the powder with smaller particles was 36–38 MPa, and for the powder from previous tests it was 25–54 MPa.
The addition of waste limestone powder in an amount up to 29% generally does not deteriorate the pull-off strength of the epoxy coating. All samples obtained results above the standard required value of 1.5 MPa.
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