AGH UST researcher uses coffee grounds to create innovative ceramic materials with low thermal conductivity

According to the International Coffee Organization, each year the world produces about 10 million tons of whole bean coffee. If we had ground the entire year’s harvest and made coffee from it, we would have been left with an equal mass of coffee grounds, amplified by the volume of unfiltered water. This waste is by no means environmentally neutral. This is because when it is deposited in landfills, it emits methane, which belongs to the group of gases that contribute to global warming. Therefore, projects are being implemented around the world that aim to develop ways of secondary management of used coffee grounds waste. After proper preparation, it can be used as a fertiliser, biodegradable material for industrial production, or as a biofuel. However, Dr Ewelina Kłosek-Wawrzyn from the Faculty of Materials Science and Ceramics is working on using coffee grounds as an eco-friendly raw material to produce porous ceramic materials with low thermal conductivity, which could be applied in construction or thermal insulation of buildings.

The researcher explains why pores in materials influence their thermal properties: ‘Transfer of heat can occur by means of conduction, convection, and radiation. Usually, these mechanisms do not occur individually. In building materials, heat transfer occurs mostly by means of conduction. The heat moves from a higher-temperature zone to a lower-temperature zone. If the material is porous, the heat encounters obstacles on its way, which hinder conduction. To act efficiently, pores must be less than 0.5 mm in diameter to eliminate convection as an additional mechanism of heat transfer. Generally, closed pores are the best for this, but this doesn’t have to be a rule. Everything depends on their diameter and direction’.

Energy and water saving

Pores in ceramic materials are obtained by stoving clay mixed with additives. The latter simply burn out during the production process, leaving micro-spaces filled with gas that has a much lower heat transfer coefficient than a solid material. In traditional construction ceramics, the additive used to obtain the porous structure is usually cellulose pulp or sawdust. Replacing them with used coffee grounds, in addition to secondary waste management, will allow producers to save considerable amounts of energy and water used in the production process.

‘Traditional construction ceramics uses additives that burn out during the production process’, says Dr Kłosek-Wawrzyn. ‘In addition to porosity, by burning, they add extra heat, which reduces the use of gas in the production process. The used coffee grounds show a much higher calorific value than sawdust and other commonly applied additives used in construction ceramics, allowing us to minimise the amount of heat used in the production process.

Furthermore, used coffee grounds collected from coffee shops, for example, are wet and contain from 55 to 60 percent of water. Usually, when building materials are made with the use of clay, we have to add water to make it plastic. Here, however, if we can organise the entire production process appropriately, adding water won’t be necessary at all’.

The used coffee grounds under investigation come from two coffee shop chains, which are mixed together during the production process. Although they come from different sources – as the AGH UST researcher explains – the only difference between them is the content of water; whereas they are identical in terms of granularity and very similar in terms of calorific value.

Low thermal conductivity vs. mechanical resistance

The idea of using ground coffee waste as a clay additive in the production of porous ceramic materials is not new and has already been discussed in the literature. The challenge for engineers is to optimise this process in such a way that will allow them to obtain a material with desired thermal parameters, while simultaneously meeting the norms of mechanical resistence. The materials used in construction must remain rigid under various stresses; therefore, adding too many easily burning materials to clay reduces its resistance.

Dr Kłosek-Wawrzyn conducts her research together with her colleagues from the Department of Building Materials Technology. Under laboratory conditions, the scientist prepares ceramic material samples, choosing various proportions of used coffee grounds to achieve the desired porosity. Subsequently, she meticulously analyses the prepared samples.

 

High-temperature microscope used in the investigation of material behaviour during stoving,
Photo by Marianna Cielecka

‘Above all, we study the efficiency of the heat transfer coefficient using a stationary heat flow meter’, says the AGH UST scientist. ‘We test its compressive strength because building materials are exposed to various mechanical forces. We analyse its density and porosity, but we also apply other techniques, including scanning electron microscopy, to find out how the material under investigation looks like in microstructure, that is, what its distribution of pores is and whether there are cracks. As a result, we can regulate the production process and shape the direction of future research projects’.

Innovative approach to materials

Dr Kłosek-Wawrzyn has an innovative approach to the materials she designs: ‘The building materials currently manufactured show, in my opinion, too high strength parameters, which translates to higher thermal conductivity’, explains the researcher. ‘In my material, I want to find a balance between thermal conductivity and strength. However, I am not aiming for high constructions; one- or two-storey is enough, where the material strength doesn’t have to be that high, which makes it possible to increase porosity and, consequently, lower thermal conductivity’.

 

Samples for testing the efficiency of the heat transfer coefficient λ in a heat flow meter,
Photo by Marianna Cielecka

 

Equipment for testing the heat transfer coefficient of TA FOX50 materials,
Photo by AGH UST Professor Waldemar Pichór

Initial research results are very promising:
‘Due to the fact that I have completely modified the production process, I can put much more used coffee grounds into the material than has been described in the literature to date and obtain significantly better thermal parameters of about 0.15–0.25 watts per meter-kelvin’, says Dr Kłosek-Wawrzyn.

The project was funded by a university grant within the framework of the programme “Excellence Initiative – Research University”, AGH UST 2020–2022, PRA-2.