Gregor Kravanja

The increasing accumulation of plastic waste and the depletion of natural mineral aggregates have driven interest in the use of recycled plastic particles as alternative aggregates in concrete. However, weak interfacial bonding between hydrophobic plastic particles and the cement matrix remains a critical limitation. This study investigates the effect of oxygen plasma surface modification on the mechanical performance of concrete incorporating recycled polypropylene flakes (PPF) and polyethylene agglomerates (PEA). Compressive and flexural strength tests, together with triaxial tests under confined conditions, were performed. Surface wettability was evaluated using contact angle measurements, and interfacial morphology was analysed by scanning electron microscopy (SEM). Although plastic particles reduced compressive strength compared to reference concrete, plasma treatment improved mechanical performance, particularly for PEA mixtures, with compressive strength increasing by up to 33% relative to untreated counterparts. Under triaxial loading, concrete specimens containing plasma-treated PEA particles exhibited reduced deformations and improved elastic recovery, indicating enhanced pressure transfer under confinement. SEM analysis confirmed improved interfacial bonding in treated specimens. The results demonstrate that oxygen plasma treatment effectively enhances interfacial properties and multiaxial mechanical performance of concrete incorporating plastic particles, providing novel insights into its behaviour under triaxial loading.

This study investigates the potential utilisation of industrial red gypsum (RG), a by-product of titanium dioxide production, as a sustainable component in cementitious and geopolymer composites. Cement pastes, mortars and metakaolin-based geopolymers were prepared with partial replacement of the main binder by RG (5–50 wt%) to evaluate its effects on workability, porosity, hydration and mechanical performance. The experimental results showed that small additions of RG (5 %) enhanced the mechanical strength of cement mortars slightly due to its micro-filling and nucleation effects, while higher contents reduced the strength and workability because of the increased water demand and porosity. In geopolymers, RG acted mainly as an inert filler, with strength decreasing significantly beyond 10 % replacement. Response Surface Methodology (RSM) confirmed an optimal composition range at low RG dosages. The results demonstrate that limited RG incorporation can contribute to a circular economy strategy in construction, promoting waste valorisation and reducing cement consumption.

The construction sector’s reliance on traditional cement significantly contributes to CO₂ emissions, underscoring the urgent need for sustainable alternatives. This study investigates fine (0–4 mm), rounded, uncoated, porous-surfaced lightweight expanded clay aggregate (LECA)-based geopolymers, which combine the low-carbon benefits of geopolymers with LECA’s lightweight and insulating properties. Geopolymers were synthesized using lignite-rich fly ash with varying ratios of LECA to aggregate. Mechanical testing revealed that the reference mixture without LECA (REF-GEO) achieved the highest compressive strength of 37.89 ± 0.75 MPa and flexural strength of 7.62 ± 0.11 MPa, while complete substitution of the aggregate with LECA (LECA-100%) reduced the compressive strength to 17.31 ± 0.88 MPa and flexural strength to 3.41 ± 0.11 MPa. The density of the samples decreased from 2.06 g/cm³ for REF-GEO to 1.31 g/cm³ for LECA-100%, and thermal conductivity dropped significantly from 1.15 ± 0.07 W/mK to 0.38 ± 0.01 W/mK. Microstructural analysis using XRD and SEM-EDX highlighted changes in the material’s internal structure and the increase in porosity with higher LECA content. Water vapor permeability increases over time, particularly in samples with higher LECA content. These findings suggest that LECA-based geopolymers are suitable for low-load or non-structural elements. They are ideal for sustainable, energy-efficient construction that requires lightweight, insulating, and breathable materials.

Supercritical CO₂ was utilized to accelerate the carbonation process, altering the microstructure and composition of Portland cement mortar blended with metakaolin. Mortar samples were cured in a high-pressure reactor at 323 K and 15 MPa under supercritical CO₂ conditions. The results demonstrate a significant enhancement in both compressive and flexural strength for all carbonated mortar samples treated with supercritical CO₂ compared to those subjected to ambient curing conditions. Furthermore, longer reaction times resulted in increased overall CO₂ utilization within the mortar matrix, with more pronounced effects observed in samples containing metakaolin. Hydrated mortar samples demonstrated a complex pore system characterized by a substantial presence of small gel pores, alongside large gel and mesopores. Upon exposure to supercritical CO₂, SEM micrographs revealed a rougher surface, along with the formation of CaCO₃ crystals.Comprehending high-pressure carbonation is vital for several applications, such as carbon capture and storage, improving the properties of recycling concrete, and safely managing toxic elements in construction waste materials.

The article presents the progress and applications of ultra-high-performance concrete (UHPC), a revolutionary material in modern construction that offers unparalleled strength, durability, and sustainability. The overview includes the historical development of UHPC, covering its production and design aspects, including composition and design methodology. It describes the mechanical properties and durability of UHPC and highlights recent innovations and research breakthroughs. The potential integration of multifunctional properties such as self-heating, self-sensing, self-luminescence and superhydrophobicity, is explored. In addition, advances in nanotechnology related to UHPC are addressed. Beyond the actual material properties, the article presents an environmental impact assessment and a life-cycle cost analysis, providing an insight into the wider implications of using UHPC. To illustrate the environmental aspects, the determination of CO₂ emissions is explained using three numerical examples. Finally, various applications of UHPC are presented, focusing on the construction of buildings and bridges. By synthesizing the above-mentioned aspects, this review paper captures the dynamic landscape of UHPC and serves as a valuable resource for researchers and engineers in the field of construction materials.

This study explores the potential of using basalt reinforced UHPC by incorporating simultaneously self-cleaning and self-luminescent features, paving the way for sustainable advancements in civil engineering. New green formulations of UHPC were developed by integrating supplementary cementitious materials and optimizing water to the binder ratio, followed by using basalt fibers to enhance strength and ductility. The fabricated samples with high particle-packing density exhibit sufficient workability and compressive strength up to 136 MPa, and, when incorporating basalt fibers, a notable reduction in brittleness. The inner microstructure of basalt fibers was observed to be smooth, homogeneously distributed, and well adhered to the UHPC matrix. To ensure the desired long-lasting visual appearance of decorative UHPC and reduce future maintenance costs, a time-effective strategy for creating a light-emitting biomimetic surface design was introduced. The samples exhibit high surface roughness, characterized by micro to nano-scale voids, displaying superhydrophobicity with contact angles reaching up to 155.45°. This is accompanied by roll-off angles decreasing to 7.1°, highlighting their self-cleaning features. The self-luminescence feature showcased intense initial light emission, offering a potential energy-efficient nighttime lighting solution.

There is a need to develop new construction materials with improved mechanical performance and durability that are low-priced and have environmental benefits at the same time. This paper focuses on the rheological, mechanical, morphological, and durability properties of synthetic and steel fiber reinforced self-compacting concrete (SCC) containing 5–15% metakaolin (M) by mass as a green replacement for Portland cement. Testing of the fresh mixes included a slump-flow test, density, and porosity tests. Mechanical properties were determined through compression and flexural strength. A rapid chloride penetrability test (RCPT) and the chloride migration coefficient were used to assess the durability of the samples. A scanning electron microscope (SEM) with energy dispersion spectrometry (EDS) was used to study the concrete microstructure and the interfacial transition zone (ITZ). The results show that a combination of metakaolin and hybrid fibers has a negative effect on the flowability of SCC. In contrast, the inclusion of M and hybrid fibers has a positive effect on the compressive and flexural strength of SCC. The fracture of SCC samples without fibers was brittle and sudden, unlike the fiber-reinforced SCC samples, which could still transfer a considerable load with increasing crack mouth opening deflection. Overall, the chloride migration coefficients were reduced by up to 71% compared to the control mix. The chloride reduction is consistent with the resulting compact concrete microstructure, which exhibits a strong bond between fibers and the concrete matrix.

Substantial growth in plastics consumption over the last decades has contributed to an increasing amount of plastic waste being deposited in landfills and in natural environments such as the oceans. The production of new materials from recycled plastics appears to be one of the best solutions for the management of plastic waste. The aim of this paper is to investigate the adequacy of using various recycled plastic waste in concrete. The effects of waste plastic aggregates or fibers on the physical, mechanical, thermal and durability properties of concrete have been investigated. In addition, several recommendations for future studies are provided.