The chapter delves into the development of advanced concrete coating systems utilizing secondary raw materials, specifically hazardous waste, to enhance chemical resistance and durability. It explores the use of epoxy resins and various fillers, including neutralization sludge and fly ash, to create composite materials that meet demanding industrial requirements. The research highlights the effective solidification and incorporation of hazardous waste into the polymer matrix, ensuring no release of pollutants. The study also assesses the adhesion and chemical resistance of these coating systems, revealing their performance under aggressive conditions. Through detailed analysis and microscopic examination, the chapter underscores the potential of these innovative coatings to contribute to both environmental sustainability and economic efficiency in the construction industry.
AI Generated
This summary of the content was generated with the help of AI.
Abstract
New types of highly chemically resistant coating systems, mainly developed for concrete and metal substrates were subject to experimental testing and evaluation within the project. Secondary raw materials, including solidified hazardous waste (neutralization sludge (NS)), were used as microfillers. The three-layer polymer coating systems, applied using spray technology, were tested at two quality levels – one with a high content of solidification products, and the other with a low content. The microstructure of the epoxy coatings, including an observation of the degree of contamination of the polymer matrix, was investigated using scanning electron microscopy (SEM). It was demonstrated that the substitution of some of the primary filler with a solidification product does not result in the deterioration of the properties of the coating system, such as its adhesion to concrete or chemical resistance.
1 Introduction
Today, the pressure on the quality and durability of building materials and construction works continues to grow. This creates space for the development of new, progressive materials such as, for example, polymeric coatings. Coating materials, intended for use in industrial operations, are subject to particularly demanding requirements based on the extreme conditions that exist within most industries [1]. Special coatings that can withstand extreme stress plus the requirements for high mechanical strength and chemical resistance are mostly created using a polymer matrix and a suitable filler to form a composite with excellent properties [2].
As industry continues to develop, there is also the generation of significant quantities of industrial waste and secondary raw materials. Many of these wastes contain a high proportion of pollutants and are classified as hazardous waste (HW). The continuous production of HW, has led to efforts to reuse these waste products. This is a more ecologically friendly solution to the issue rather than just sending it to landfill [3, 4]. Currently, there is pressure to re-use these wastes and efforts are under way to develop building materials that incorporate, to the greatest extent, waste and secondary raw materials while maintaining the existing properties of the materials, or improving them. This trend can have a positive effect from both an economic and an ecological point of view. HW and secondary raw materials, suitable for use in coatings, can be used as specially modified (solidified) fillers, which can be effectively used to replace primary raw materials [5]. There are a number of different types of hazardous waste that contain large amounts of pollutants and dangerous substances. These are, for example, cement dust, neutralization sludge (NS) generated by the surface treatment of metals, waste from the incineration of municipal solid waste, waste from the production of elements such as zirconium dioxide and many others. Through an appropriate special treatment of the HW (solidification), the resultant solidification products could then be used as a suitable filler in polymer coatings [6]. Epoxy systems are the most commonly used chemically resistant coatings, they have a long service life, adhere very well to most surfaces and are stable at temperatures up to 100 ℃ [7]. One disadvantage of solvent-free epoxy resins is that they must be applied to a dry surface [8].
Advertisement
The assumption that treated HW may be used in polymer protective systems (coatings) must first be subject to proper verification, which is the aim of this paper. In the future, it will be necessary not only to solve issue around the storage of these types of waste, but also to find further uses.
2 Materials
2.1 Polymer Binder
As the coating systems under test are designed to be used in demanding conditions, such as sewers, where there is a chemically aggressive environment, it is necessary to use the highest quality of polymer materials available. Used epoxy resin with technical abbreviation IN-ER (commercial name IN-EPOX 4090, manufactured by IN-CHEMIE Technology Ltd. (Olomouc, Czech Republic)) is a 2-component, colourless polymer. It stands out for its good mechanical and chemical resistance, high UV stability, fast polymerization, minimal odour and easy application. It does not contain thinners, benzyl alcohol or nonylphenol. The mixing ratio of the components, A (epoxy resin) to B (hardener) was 2.3: 1. The processing time at +25 ℃ is approximately 25 min and it is fully cured after 7 days. The humidity of the substrate during application cannot exceed 6% and the relative humidity of the ambient atmosphere cannot be higher than 75%. Component A (epoxy resin) contains: epoxy resin, (Alkoxymethyl)oxirane (alkyl C12-C14), formaldehyde and oligomeric reaction products with 1-chloro-2,3-epoxypropane and phenol products along with 1-chloro-2,3- epoxypropane and phenol. Component B (hardener) contains: Formaldehyde, polymer with N-(3-aminopropyl)-1-3-propanediamine, Carbo-monocyclic alkylated mixture of poly-aza-alkanes, hydrogenated and Polyamine adduct.
2.2 Materials Used for Microfiller Preparation
Pre-treated Hazardous Waste (Neutralization Sludge - NS). It is a by-product that arises from the surface treatment of metal elements. Neutralization sludge is produced by the neutralisation of acid waste water, in most cases a whitewash suspension (a product made from hydrated lime and water) [9]. According to the Waste Catalogue [10], this specific HW can be classified in the group, 19 02 05, sludges from physical/chemical treatment that contain dangerous substance. The NS needed to be treated, first by drying at 105 ℃ for 24 h, and then ground to a suitable particle size (< 100 µm) before its use as a microfiller. The NS contained high levels of lead and nickel. The specific weight of the treated sludge was 2850 kg/m3 and the specific surface area was 5400 cm2/g.
Fly Ash (FA). This is the ash that is generated by the fluid bed combustion process within a thermal lignite power plant. It is contaminated with ammonium ions as a result of the de-nitrification of the flue gases using the selective non-catalytic reduction method. The concentration of ammonium ions (NH3) was 30.110 ppm, the specific surface area was 627 m2/kg and the density was 2872 kg/m3. The chemical composition of the fly ash was: 36% SiO2, 20% Al2O3, 19% CaO and 6% Fe2O3. This secondary raw material served as a solidification agent during the dry solidification of NS.
Advertisement
Quartz Flour (QF). High-quality quartz flour with a SiO2 content of more than 99% is produced by grinding treated and dried sand in non-ferrous mills. Quartz flour has a high degree of purity, good chemical and mechanical resistance, good resistance to UV radiation and weathering, a consistent grain structure and a grain size of up to 0.1 mm (larger particle size than silica fume). Silica flour was used as the filler in the coating systems that were used as a reference within the study. It was also used in small quantities as a solidifying agent and in the top layer of the coating system.
Glass Flakes (GF). These are very thin flat plates with a smooth surface, a thickness of 7 − 100 μm and a length of 10 − 2000 μm. They are used as a filler in protective coatings to increase chemical resistance, hardness and abrasion resistance. The flakes, due to their shape, form dense obstacles in the material that prevent the penetration of water or chemicals. Glass flakes were used in the top layer of the coating systems to increase the chemical resistance. The specific weight of the flakes was 2470 kg/m3. They contain approximately 70% SiO2, 5% Al2O3, 10% Na2O, 6% CaO, and 3% MgO.
2.3 Preparation and Composition of Fillers
The first step was to solidify the HW through the dry homogenization method. This method mixes the dry components in closed containers at 80 revolutions per minute, they are then placed in a homogenizer for 24 h. Quartz powder and fly ash served as the solidifying agents. By using the dry homogenization method, the particles of treated NS were coated with the solidifying agents. The next step saw the milling of the homogenized mixture in a laboratory vibratory disc mill. Finally, the prepared filler was passed through a 63 µm sieve. In this way, fillers containing 10% and 50% of treated HW (F-NS10, F-NS50) were prepared, these were then used in the middle (second) layer of the polymer coating systems. The composition of the fillers used is shown in Table 1. The reference filler only contained quartz flour, the F-GF filler, used in the top layer, also contained glass flakes. The basic parameters of the fillers, such as the specific gravity and specific surface area, are presented in Table 2. The granulometry of the fillers is shown in Fig. 1 – it can be seen that the F-GF filler contained glass flakes with an edge length of up to 600 μm. SEM photomicrographs of the fillers can be seen in Fig. 2.
Table 1.
Composition of the fillers in wt.% used in the layers of coating systems.
Mark of filler
Neutralization sludge
Fly ash
Quartz flour
Glass flakes
F-NS10
10
40
50
–
F-NS50
50
30
20
–
F-GF
–
40
50
10
QF (REF)
–
–
100
–
Table 2.
Specific parameters of the fillers.
Parameter
F-NS10
F-NS50
F-GF
QF
Specific gravity [g·cm−3]
2.89
2.86
2.72
2.68
Specific surface [cm2·g−1]
10950
13260
6380
4660
Fig. 1.
Particle size distribution of the fillers.
Fig. 2.
SEM photomicrographs of the fillers: a) F-NS10; b) F-NS50; c) F-GF.
×
×
2.4 Composition of Coating System’s Layers
The pre-treated HW was used as a filler in the lower layers of the coating system, so that in the event of a breach of the top coat, there would be no possibility of a release of pollutants into the surrounding environment. The upper layers (top coats) only contained fillers based on QF, GF and FA. As the filler is perfectly incorporated into the polymer matrix, there is no release of hazardous substances into the surrounding environment. The CS-HW10 coating system used a filler with an NS content of 10% in the second layer, and the CS-HW50 used a filler with an NS content of 50%. The same IN-ER epoxy resin was used for all the coatings tested. The composition of the coating systems is shown in Table 3. Preferred dry thickness of the base layer was 200 – 250 µm, and for the top coat it was 150 – 200 µm.
Table 3.
Composition of the layers of the coating system.
Type of coating system
Coating system layer
Filler
Binder
CS-HW10
1 (primer)
5% NS
95% IN-ER
2 (base coat)
30% F-NS10
70% IN-ER
3 (top coat)
30% F-GF
70% IN-ER
CS-HW50
1 (primer)
5% NS
95% IN-ER
2 (base coat)
30% F-NS50
70% IN-ER
3 (top coat)
30% F-GF
70% IN-ER
CS-REF
1 (primer)
5% QF
95% IN-ER
2 (base coat)
30% QF
70% IN-ER
3 (top coat)
–
100% IN-ER
3 Methods
3.1 Adhesion
Testing of the adhesion of the coating systems to a dry and clean concrete surface was performed according to EN ISO 4624 [11] using a pull off adhesion tester, Elcometer 506-20D. The coating systems were applied evenly with a suitable brush on the concrete paving block measuring 30x30 cm. The concrete surface was smooth out of formwork. The individual layers of the coating system were applied after the previous layer had hardened (24 h). After polymerization of the entire coating system, metal dolls with a diameter of 20 mm were glued using a two-component epoxy glue on the surface of top coat. Pull off tests were performed 7 days after the application of the coating materials to the substrate, and again after the chemical resistance test. The adhesion was tested from each coating system in three places.
3.2 Chemical Resistance
To test the chemical resistance of the coating systems, 4 different aggressive liquids were applied to their surfaces: 15% HCl, 15% CH3COOH, 30% H2SO4 and 30% HNO3. The liquid solutions were poured onto the surface of the coating systems using a plastic funnel, which were then sealed to prevent any leakage. The area of top coat, on which the aggressive medium acted, was 80 cm2. The chemically aggressive environment was left on the surface of the test coating for 28 days. The samples were then visually assessed. The pull-off test was performed and any disruption of the individual layers of the coating systems was also investigated using a Keyence VHX-950F digital microscope.
3.3 Microstructure (SEM)
The degree of incorporation of the NS particles into the polymer matrix was investigated using scanning electron microscopy (SEM), with a TESCAN MIRA3 XMU microscope. The cohesion of the individual layers of the coating system was also investigated using SEM.
4 Results and Discussion
4.1 Adhesion and Chemical Resistance
Figure 3 presents a graphical representation of the adherence of the coating systems to the concrete substrate. It can be observed that after the surface of the coating had been exposed to 15% HCl and 30% H2SO4 for 28 days, there was no significant reduction in the adhesion to concrete. For the samples exposed to a 15% solution of CH3COOH (acetic acid), it was not possible to determine an adhesion value, the coating system showed such significant signs of degradation that it was not even possible to attach the necessary test target. A significant decrease in adhesion was recorded for samples exposed to the 30% solution of HNO3, which showed an approximate decrease in adhesion of 80% in comparison to those samples that had not been exposed to an aggressive environment. The 30% HNO3 solution penetrated the structure of the concrete and the lack of cohesiveness of the coating systems was evident. In all cases, except for those samples exposed to 30% HNO3, the failure mode was seen in the underlying concrete. The bonding strength between the coating and concrete substrate is an important indicator of the service performance of a coating system [12].
Fig. 3.
Results of coatings system’s adhesion on concrete substrate.
×
Based on an evaluation of the images from the digital microscope (Fig. 4), it can be concluded that the coatings demonstrated good resistance to inorganic acids (15% HCl and 30% H2SO4), where only minimal penetration of the aggressive solutions into the top layer was visible and there was no disruption of the cohesion between the individual layers or with the concrete - see Fig. 4a. The most pronounced degradation of the epoxy-based coating systems was caused by a 15% solution of CH3COOH. Formic acid and acetic acid are known to be aggressive to epoxy coatings. The immersion of epoxy coatings in acetic acid significantly affects the chemical resistance of the coating - bubble formation associated with undercoat corrosion [13]. As can be seen in Fig. 4b, the coating system was severely damaged and the acetic acid solution penetrated into the concrete substrate. Obvious signs of degradation could also be seen after exposure to the 30% solution of HNO3, for comparison see Fig. 4c. Nitric acid was attacking the same phase of the coating (epoxy matrix) like the acetic acid, but the solution of HNO3 reacted only on the surface of top coat. To allow a comparison, Fig. 4d shows an undamaged coating system that has been stored within a laboratory environment and not exposed to a chemically aggressive environment.
Fig. 4.
The CS-HW50 coating system after 28 days of chemical exposure: a) 15% HCl; b) 15% CH3COOH; c) 30% HNO3; d) reference – laboratory conditions.
×
4.2 Microstructure (SEM)
In Fig. 5a, it is possible to observe a perfect connection between the first layer of the CS-HW50 coating system (primer) and the base layer. The secondary raw material used, in the form of a microfiller, including solidified HW, did not have a negative effect on the cohesion between the individual layers of the coating system. The successful incorporation of the NS particles in the epoxy matrix can be seen in Fig. 5b. A strong contact zone is visible between the NS particles and the epoxy binder, which should guarantee that pollutants from the filler are not released into the environment, even after the coating system has been in service for a long period of time in demanding conditions.
Fig. 5.
SEM photomicrographs of the coating system CS-HW50: a) transition between the primer and base coat; b) incorporation of the NS particles in the epoxy matrix.
×
5 Conclusion
As part of the research, it was proven that the particles of the treated HW (solidified neutralization sludge) were perfectly incorporated into the structure of the epoxy matrix and in practice there should be no release of hazardous substances into the surrounding environment. The coating systems tested showed a high degree of adhesion to the concrete substrate and good cohesion between the individual layers. This was confirmed using a digital microscope and SEM. The coating systems under test were found to have poor chemical resistance to acetic and nitric acids. The chemical resistance of the coating systems that contained solidified hazardous waste was not inferior to the reference CS-REF coating system that only contained primary raw materials. The coating system that included a higher HW (CS-HW50) content demonstrated almost the same properties as the CS-HW10 coating system.
Acknowledgements
The paper was prepared with the financial support of The Technology Agency of the Czech Republic (TA CR) project No. FW03010107 “Development and research of new materials for polymer rehabilitation sprays”.
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.
The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
