Sara Tominc

Low-carbon binders are the cornerstone of a sustainable society as they drastically reduce the carbon footprint of the construction industry

The construction sector is a major contributor to global carbon emissions, due largely to high-temperature processing and extensive use of raw materials. Alkali-activated materials (AAMs), synthesised below 100 °C from secondary raw materials such as fly ash (FA), offer a more sustainable solution. This study investigates the mechanical performance and radionuclide presence of theoretically chemically optimal alkali-activated FA, based on three conditions: untreated FA, FA sieved below 200 µm, and FA treated thermally at 550 °C. Both the sieving and thermal treatment removed the organic content from the FA, resulting in a significant improvement in the compressive strength of the AAMs. However, in the absence of organic content, microwave irradiation no longer improved the compressive strength. Additionally, sieving below 200 µm also reduced the radionuclide content, offering a simple and effective pathway to high-performance, radwaste-safe building materials, even though the radionuclides themselves did not affect the mechanical performance of the AAM.

The construction industry contributes over 40% of human-made carbon emissions, due mainly to the high consumption of raw materials and energy-intensive processing. Alkali-activated materials (AAMs) prepared from secondary raw materials such as fly ash (FA) offer a sustainable alternative that is synthesised at low temperatures. In this study, the FA was milled and sieved to achieve higher reactivity and compressive strength (CS). To increase the bending strength, keratin-based fibres from a Chesapeake Bay Retriever were added in mass percentages (m%) of 1 and 5. While the addition of 1 m% of fibres improved the overall mechanical performance and increased the CS by ~20 MPa, the addition of 5 m% resulted in a soft composite that deforms under pressure but returns to its original shape cyclically as the fibres act as an internal cushioning network. However, the CS of the 5 m% fibre-rich composite decreased to 20 MPa, which is 30 MPa lower than the fibre-free sample.

As clay deposits become scarce, the brick industry is increasingly seeking additives or substitutes. This study investigates a high-plasticity clay that requires an opening agent, for which was used 10 wt.% of processed steel slag Ekominit S1. Ceramic-technological tests were performed to determine the compressive strength, density and porosity. The addition of Ekominit S1 increased the total porosity by 5% in samples fired at 950 °C, and by 4% in samples fired at 1050 °C, while the compressive strength decreased by 36% in the samples fired at 950 °C and by 38% in those fired at 1050 °C compared to the reference material. Although the mechanical properties were lower than those of the reference, the benefit is reduced shrinkage. The processed steel slag could be incorporated successfully into bricks, which would also reduce the environmental impact of this sector by using secondary products instead of virgin materials.

Mineral CO2 sequestration is a promising approach for reducing greenhouse gas emissions by storing CO2 in stable forms permanently. This process involves capturing CO2 and converting it into solid carbonates through mineralisation. Waste ashes and slags, by-products of waste incineration and steel production, are promising materials for CO2 sequestration, due to their high alkalinity and reactive mineral phases. In this study, the CO2 sequestration potentials of different metallurgical slags and incineration ashes from Austria and Slovenia were analysed using Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and calcimetry. Biomass ash (A1) showed the highest sequestration capacity of 153.7 g CO2 per kg of ash.

Alkali-activated materials (AAMs) are being envisaged as a future alternative to cements because AAMs (i) can be made solely from locally available secondary raw materials and (ii) because of their low energy consumption during synthesis. However, products that cannot be reused should never enter the market. Therefore, the circular economy viability of AAMs was studied by reusing (alkali-activated) pulverised rock wool in alkali activation synthesis until the theoretically determined limit leading to the efflorescence would be reached. While 25% of the maximal allowed liquid alkali did not offer significant mechanical performance, 50, 75 and 100% of allowed alkali ended up with 15, 20 and 20 MPa, respectively, which is only 5 MPa lower than AAM made with the same technique (pressing) in a one-step approach. Therefore, if the initial AAM does not reach the efflorescence limit, it could still be used as a precursor before it becomes waste.

There are several ways to utilize as-received or pre-treated waste ash, one of the most promising is by accelerated mineral carbonation. Ashes with a high content of Ca and Mg compounds, such as ashes from wood biomass, are ideal candidates for sequestration. Due to the shift toward renewable fuels, ash from biomass as a by-product of solid fuel combustion is therefore available in huge quantities. As part of the EU AshCycle project, we have analyzed ashes from different incineration and thermal power plants to determine their carbon sequestration potential. These include various waste ashes from Slovenia, which were subjected to accelerated carbonation in a closed carbonation chamber with a CO2 concentration of 4% (v/v), 80% relative humidity and a temperature of 40 °C until maximum CO2 uptake was reached. CO2 quantification was performed using calcimetry (pressure calcimeter) and thermogravimetry. We have shown that ash from wood biomass and the co-combustion of wood waste and paper sludge have a high CO2 sequestration potential in comparison to others. The direct use of wood biomass ash for CO2 sequestration in carbonated building products could significantly benefit the circular economy, especially since 70% of wood biomass ash is still landfilled.

6. mednarodna konferenca Tehnologije in poslovni modeli za krožno gospodarstvo: Zbornik konference. Fakulteta za kemijo in kemijsko tehnologijo Univerze v Mariboru je v sodelovanju s Strateško razvojno-inovacijskim partnerstvom – Mreže za prehod v krožno gospodarstvo (SRIP – Krožno gospodarstvo), s katerim upravlja Štajerska gospodarska zbornica, organizirala 6. mednarodno znanstvenoraziskovalno, strokovno in razvojno konferenco Tehnologije in poslovni modeli za krožno gospodarstvo (Technologies & Business Models for Circular Economy; TBMCE), ki je potekala od 6. do 8. septembra 2023 v Grand hotelu Bernardin. TBMCE 2023 je bila namenjena predstavitvi konceptov krožnega gospodarstva in tehnologij ter metodologij, ki prispevajo k preusmeritvi gospodarskih subjektov in družbe kot celote k bolj odgovornemu, krožnemu ravnanju z viri. Konferenčni program je vključeval okroglo mizo z naslovom Standardizacija za krožno gospodarstvo – varnejše in manj zapleteno zapiranje snovnih zank, 6 panelnih diskusij, plenarno in 3 uvodna predavanja, ustna predavanja in predstavitve v obliki posterjev. Dogodek je potekal pod pokroviteljstvom Ministrstva za gospodarstvo, turizem in šport in Ministrstva za kohezijo in regionalni razvoj. Kot država partnerica se nam je pridružila Nizozemska.