Przeglądaj wg Autor "Stephan, Dietmar"

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  • Miniatura
    PozycjaOpen Access
    Insight into the microstructural and durability characteristics of 3D printed concrete: Cast versus printed specimens
    (Elsevier BV, 2022-07-16) Sikora, Pawel; Techman, Mateusz; Federowicz, Karol; El-Khayatt, Ahmed M.; Saudi, H.A.; Abd Elrahman, Mohamed; Hoffmann, Marcin; Stephan, Dietmar; Chung, Sang-Yeop; Faculty of Civil and Environmental Engineering, West Pomeranian University of Technology in Szczecin, Poland; Department of Physics, College of Science, Imam Mohammad Ibn Saud Islamic University, (IMSIU), Riyadh, Saudi Arabia; Reactor Physics Department, Nuclear Research Centre, Atomic Energy Authority, 13759 Cairo, Egypt; Department of Physics, Faculty of Science, Al-Azhar University, Women Branch, Nasr City, Cairo, Egypt; Structural Engineering Department, Mansoura University, Mansoura City 35516, Egypt; Faculty of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology in Szczecin, Poland; Building Materials and Construction Chemistry, Technische Universität Berlin, Germany; Department of Civil and Environmental Engineering, Sejong University, Seoul 05006, Republic of Korea
    This study presents the comparison of microstructural and durability characteristics of 3D printed concrete (3DPC) depending on its production method (printing or casting). Printed samples with different numbers of layers, as well as a cast specimen with an identical mix composition, were produced and compared, with their microstructural pore and solid characteristics quantitatively and qualitatively investigated. For this purpose, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and X-ray micro-computed tomography (micro-CT) were utilized to evaluate the microstructures of the 3DPC. In particular, quantitative approaches using micro-CT data were newly proposed for a better understanding of the microstructural characteristics of 3DPC. Moreover, their durability-related characteristics and transport properties, including freeze-thaw and thermal resistance, were examined and compared. Despite noticeable differences between the microstructures of the printed and cast specimens, including their anisotropic and inter-layer porosity and heterogeneity, confirmed by MIP, SEM and micro-CT, no significant differences in the transport (capillary water porosity and water sorptivity) or durability-related properties (frost and thermal attack) were found. This was due to the dense and homogenous microstructure of 3DPC, which is attributable to the high binder content and low w/b of the mixture. Moreover, the newly proposed evaluation provided reasonable quantitative and qualitative characteristics, which can be used to demonstrate and predict the material properties of 3DPC.
  • Miniatura
    PozycjaOpen Access
    Seawater-Mixed Lightweight Aggregate Concretes with Dune Sand, Waste Glass and Nanosilica: Experimental and Life Cycle Analysis
    (Springer Nature, 2023-08-01) Sikora, Pawel; Afsar, Levent; Rathnarajan, Sundar; Nikravan, Morteza; Chung, Sang-Yeop; Stephan, Dietmar; Abd Elrahman, Mohamed; Technische Universität Berlin, Germany; West Pomeranian University of Technology in Szczecin, Poland; Technische Universität Berlin, Germany; West Pomeranian University of Technology in Szczecin, Poland; Technische Universität Berlin, Germany; Yonsei University, Seoul, South Korea; Technische Universität Berlin, Germany; Mansoura University, Mansoura, Egypt
    The use of alternative and locally available materials is encouraged in the construction industry to improve its sustainability. Desert regions with shortages in freshwater and river sand as fine aggregates in concrete have to search for alternative materials such as seawater, dune sand, and waste glass powder to produce lightweight concretes. The potential negative effects of adding these alternative materials can be reduced by adding nanosilica to the cementitious system at very low quantities. This study evaluates the feasibility of using these alternative materials and nanosilica (NS) in producing lightweight aggregate concretes (LWACs). A systematic study was carried out to understand the synergistic effect of nanosilica and seawater in improving the hydration characteristics of the developed cementitious systems. Also, the effect of these alternative materials on the fresh properties of the cementitious system was assessed by slump flow tests. The evolution of compressive strength at early ages was investigated after 2, 7, and 28 days of moist curing and an improvement in the strength development in concretes with seawater was observed. Furthermore, the integrity of the developed LWACs was analyzed using oven-dry density, thermal conductivity, water porosity and shrinkage measurements. Moreover, the capillary porosity and sorptivity measurements revealed the denser microstructure in the nano-modified seawater lightweight concretes. In the end, the life-cycle assessment study calculated the benefit of alternative materials in terms of carbon footprint and water consumption. As an outcome, a sustainable solution for producing LWACs containing seawater, dune sand or glass powder was proposed.
  • Miniatura
    PozycjaOpen Access
    A systematic experimental study on biochar-cementitious composites: Towards carbon sequestration
    (Elsevier BV, 2022-05-26) Sikora, Paweł; Woliński, Paweł; Chougan, Mehdi; Madraszewski, Szymon; Węgrzyński, Wojciech; Papis, Bartłomiej K.; Federowicz, Karol; Ghaffar, Seyed Hamidreza; Stephan, Dietmar; Department of Civil Engineering, Technische Universität Berlin, Berlin 13355, Germany; Faculty of Civil and Environmental Engineering, West Pomeranian University of Technology in Szczecin, 70-311 Szczecin, Poland; Faculty of Applied Sciences, Collegium Mazovia Innovative School, 08-110 Siedlce, Poland; Department of Civil and Environmental Engineering, Brunel University London, Uxbridge UB8 3PH, UK; Building Research Institute (ITB), 00-611 Warsaw, Poland
    The utilisation of biochar, the carbon negative product of pyrolysis, reduces the carbon footprint of the cementitious composites as it possesses the potential to replace the consumption of Portland cement. In a systematic investigation, biochar was used as a partial cement replacement for up to 20 wt% in both cement pastes and mortars. A comprehensive experimental framework was conducted to evaluate the impact of biochar replacement on the performance of (i) cement paste in terms of hydration kinetics, rheology, strength development, porosity, and (ii) mortars in terms of mechanical, thermal, and transport properties. In addition, the durability of developed mortars, including freezing and thawing resistance, thermal resistance, acid (corrosion) resistance, flammability, and smoke production, were examined. The results revealed that lower replacement rates of cement with biochar (up to 5 wt%) do not substantially change the performance of cementitious composites. However, incorporating biochar in higher dosages (i.e., 20 wt%) influenced the hydration process, reduced flexural and compressive strengths by 49% and 29%, respectively, and increased the water absorption coefficient by 60% compared to control specimens. The same cement mortar demonstrated the most promising freeze-thaw (i.e., 98% relative residual compressive strength), acid resistance as well as considerably lower thermal conductivity. In addition, regardless of biochar content, mortars did not exhibit flammability. Therefore, this study demonstrated that despite specific technical issues, biochar can be successfully incorporated into high dosage to cementitious composite as an alternative binder with minimum environmental impacts to improve durability and insulating performance of mortars.