Effect of Polymer Mortar Modification Using Eco-friendly Biochar on Microstructure

As climate change is threatening the world and society grows exponentially, more and more waste and greenhouse gases being generated, environmental sustainability is being questioned. It is fundamental and urgent to develop a sustainable economy, actively promoting the efficient use of resources, highlighting product, component and material reuse. The construction sector should have much to offer in terms of helping to achieve circular economy goals, among others the use of waste materials – byproducts originating from industrial processes [1‐3]. The example of such materials is biochar, a black porous and carbon-rich matter could be converted from various waste biomass, such as wood waste, agricultural waste, food waste, manure waste, and municipal/industrial sludge [Biochar as construction materials for achieving carbon neutrality]. According to the guidelines of the European Biochar Certificate [4] biochar is defined as charcoal resulting from biomass pyrolysis, a process in which organic substances decomposed at temperature ranging from 350 ℃ to 1000 ℃ in a low oxygen environment. Biochar has attracted the attention of researchers due to its unique properties, such as high surface area, high porosity, functional groups, high cation exchange capacity and stability, making it suitable for various applications [5, 6].

Numerous studies have concerned incorporation of biochar into cement composites in terms of cement hydration, mechanical properties, durability, microstructure, rheological properties, and carbon sequestration etc. [7‐12], and the results of these studies have been very promising.

A biochar could be also utilized as microfiller in polymer concretes. Powder waste materials are often used as an essential microfiller in polymer concretes [13‐15]. This application of biochar is promising due to good interfacial bonding with polymer, like other carbon-based fillers [16]. It is also well known that usually there is no reactivity between surrounding polymer matrix and filler particles [17]. In contrast to cement composites, even fillers with irregular particles and large specific surface area could be utilized in polymer concretes [18, 19]. These create real opportunity to effectively dispose waste materials as a replacement of natural aggregates in polymer concrete technology.

Important aspect of polymer concrete modification with biochar or any other material is its influence on the essential properties of fresh and hardened composite. Modification cannot negatively influence basic properties of hardened composite, what could threaten is quality and possibility of its application as a construction material. The influence of biochar incorporation as a partial replacement of quartz powder and/or resin has been already investigated by the authors [20]; tests conducted included course of setting, consistency, flexural and compressive strength. The results have shown that the curing time of two composites containing resin, quartz powder and biochar in vol. proportions 75:20:5 and 75:15:10 was the longest - the curing time about 5h in comparison to about 2h for pure resin, and about 3h for resin containing 25% by vol. of quartz powder. The highest temperature was noticed in case of composition with 10% by vol. of biochar, and it was just few degrees of Celsius higher than temperature measured for being next in line composition with 5% by vol. of biochar. To assess the consistency of mixes the flow table method acc. to EN 1015-3 was used. It was observed that increase in level of quartz powder substitution with biochar caused decrease in flow diameter. Despite a drop in flowability designed mixes could be considered as sufficiently workable, as it was possible to tightly fill molds. In terms of flexural strength, the composites have not shown statistically important differences. The obtained results were within standard deviations of the measurements made for individual compositions. Compressive strength have appeared more sensitive for presence of biochar. It has been concluded that the introduction of biochar into the polymer matrix yields a slight reduction in compressive strength. This effect is less pronounced when biochar replaces part of the polymer (a decrease of 5–7%) than when it replaces part of the quartz filler (a decrease of 10–15%). The dosage level of biochar up to 5% does not result in the significant reduction of compressive strength.

The presented paper is a second part of the research concerning the utilization of ecofriendly biochar in polymer composites conducted by authors. To better understand the impact of modification by biochar, authors decided to supplement already performed tests by measurements of ultrasonic pulse velocity and quantitative analysis of microstructure by image analysis method.

Composites in the study were polymer mortars prepared using synthetic vinyl-ester resin of low viscosity (350 ± 50 mPa·s at 25 ℃), high flexural strength and high tensile strength (110 MPa and 75 MPa respectively, as declared by manufacturer). The chemical formula of vinyl-ester were presented in Fig. 1. Quartz powder and biochar were used as a microfillers. The analysis with a laser particle size analyzer (Horiba, Irvine, CA, USA) showed that about 85% of the particles of biochar were below 120 µm in diameter [20], and could be considered a microfiller particles. The biochar grain refinement differed significantly from that of the quartz powder, a traditional filler of polymer composites. Its grains were mostly not spherical in shape but had smooth surface without much branching that could increase specific surface area and resin demand (Fig. 2). Final mortars ingredient was fine aggregate in the form of CEN standard sand EN 196-1.

To determine the effect of biochar on polymer composites an experimental plan basing on the standards for ternary mixtures was prepared. The experimental plan was intended for the component proportions in the micro slurry. Therefore, the amount of resin (R), biochar (B), and quartz powder (Q) were applied as input variables. The mass of sand was constant for all mixes. While, the maximal and minimal masses of the individual components were determined in liminary studies, so to ensure the mixes will be workable enough to form samples. Consequently, the experimental plan had limitations (Fig. 3). In the experimental plan, the volumetric variation in the dosage of components was assumed. The detailed composition of composites used is given in Table 1.

The process of sample preparation was divided into four steps: (1) manual mixing of the microfillers - quartz powder and biochar; (2) adding the microfillers mix to the resin and manual mixing; (3) mechanical mixing of the fillers and resin mix with quartz sand; and (4) forming samples by placing the ready composite mix in two layers in molds and compacting on the vibrating table by 5s.

To fully assess the influence of biochar on the polymer mortars the measurements already made in the first part of the research [20], were supplemented by measurements of ultrasonic pulse velocity and quantitative analysis of microstructure by computer image analysis.

The procedure of samples preparation for image analysis involved (1) cutting out slices of dimensions 10 mm x 40 mm x 40 mm from samples of each of 9 composites, (2) cold mounting in colored epoxy resin under lowered pressure, (3) three steps grinding and (4) final polishing. After this procedure, 2D images of microstructure were acquired on a scanner with a resolution of 800 DPI (1 pixel equals about 0,02 mm). The images were subjected to computer processing using photo editing software with a purpose to obtain the most precise binary image of black pores, which are of interest, on white background of aggregate and cement paste. In this study preparation of images for the analysis was the combination of contrast, brightness, gamma modulation and color saturation, followed by selection of pores base on the color and ending with binarization. The computer aided quantitative description of the microstructure of concretes was achieved by specialized software, developed at the Faculty of Materials Science and Engineering of Warsaw University of Technology. The image analysis was applied to calculate the volume fraction of air voids in material – V_V, and the surface area of pores in a volume unit of a material – S_V. The detailed information about the image analysis method and stereological parameters is presented elsewhere [23].

The ultrasonic testing was done by direct method (transmission method) using a digital ultrasonic flow detector and piezoelectric transducers of 100 kHz central frequency. To ensure adequate acoustic coupling between the concrete surface and the special head, commercial coupling gel was applied. The ultrasonic measurements included the determination of the propagation time of ultrasonic impulse between emitter and receiver, and then calculation of the wave velocity by dividing the distance between transducers by the time of travel. The signals were registered using specialized program and to reduce random error each signal was averaged 10 times. The UPV was determined 2 times per sample and including fact that 2 samples per each composition were tested, finally 4 UPV results per composition were computed. The full description of the ultrasonic methods and UPV calculations are presented elsewhere [24].

The aim of this paper was to analyze how the increase in content of biochar in polymer mortar may influence the ultrasonic pulse velocity as well as stereological parameters of microstructure calculated by image analysis method. It was concluded that the increase in biochar content in fillers mix led to decrease in ultrasonic pulse velocity, the most significant in case of composites containing 75% of resin by vol. The increase in content of biochar in composites containing 85% of resin by vol. Led to increase in volume fraction of pores, and thus to increase in relative surface area of pores. When the total content of microfillers increases above 15%, the stereological parameters do not exhibit any relation with biochar content. Consequently, these could suggesting that microstructure became more heterogeneous.

Summarizing the results obtained in both parts of the research, the addition of biochar does not cause a significant deterioration of mechanical and physical properties, so its utilization in polymer mortars should not be an obstacle in application of such the composites in construction sector.