Nikolaos Chousidis

This study explores the potential of calcium-rich fly ash from the Ptolemais region in Greece as a partial cement replacement for improving sulfate resistance in cementitious composites. An integrated experimental program, combining mechanical testing, electrochemical corrosion monitoring and microstructural characterization, was designed to capture the progression of material properties over time and their impact on performance. The experimental results proved that, at early ages, incorporation of fly ash led to reductions in compressive, tensile and bond strengths, attributed to delayed pozzolanic reactivity. However, over prolonged curing, secondary reactions consumed portlandite and generated additional calcium silicate hydrate, refining the pore network and reducing permeability. These microstructural improvements were associated with enhanced mechanical performance, improved durability indices and markedly lower reinforcement corrosion rates. Bond tests further revealed a shift from brittle to a more ductile response, offering advantages for repair applications. These findings establish calcium-rich Ptolemais fly ash as a as a sustainable and promising supplementary cementitious material that substantially enhances the long-term durability and sulfate resistance of cementitious systems.

Cement–based mortars are essential in both modern construction and heritage conservation, where balancing mechanical strength with material compatibility is crucial. Mortars containing ––binders with low hydraulic activity, such as CEM II/B–L, often exhibit increased porosity and diminished strength, limiting their suitability for structurally demanding applications. This study investigates the potential of multiwalled carbon nanotubes (MWCNTs) to enhance the mechanical and microstructural properties of mortars formulated with both CEM II/B–L and CEM I binders. The influence of CNT incorporation was systematically assessed through compressive and flexural strength tests, vacuum saturation tests, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and differential thermal analysis (DTA). The results demonstrate significant mechanical improvements attributable to nanoscale mechanisms including crack bridging, pore–filling, and stress redistribution. Microstructural characterization revealed a refined pore network, increased densification of the matrix, and morphological modifications of hydration products. These findings underscore the effectiveness of CNT reinforcement in cementitious matrices and highlight the critical role of binder composition in influencing these effects. This work advances the development of high–performance mortar systems, optimized for enhanced structural integrity and long–term durability.

The incorporation of industrial by-products such as fly ash (FA) into cementitious materials plays a vital role in promoting environmental sustainability during cement production. This study evaluates the feasibility of using siliceous fuel fly ash, sourced from thermal power stations in the Rhenish region of Germany, as a partial cement replacement in mortar formulations. Mortar specimens with FA replacement levels ranging from 5 wt% to 30 wt% were prepared and tested. Data were collected through standardized laboratory testing of mechanical properties (compressive and flexural strength), physical characteristics (porosity, sorptivity) and microstructural analysis via SEM and XRD. The results showed that increasing FA content generally led to reductions in strength and increases in porosity and sorptivity, due to the mineralogical composition and higher water demand linked to the porous FA structure. However, when FA was used at levels not exceeding 10 wt%, the physical and mechanical properties remained within acceptable limits for construction applications. Additionally, the use of plasticizing admixtures proved effective in mitigating workability and strength issues by reducing the water-to-binder ratio. These findings highlight that, despite certain limitations, siliceous FA can be safely and effectively used in low percentages, contributing to sustainable mortar production and reducing reliance on Portland cement. In addition, the use of fly ash contributes to reduced CO₂ emissions and lower production costs, promoting sustainable and cost-efficient construction solutions.

This study examines the effects of steel fibers (SF) and carbon nanotubes (CNTs) on the performance of cementitious composites. Three types of mixes were analyzed: a reference mix (REF), a steel fiber-reinforced concrete (SFRC), and a hybrid mix containing both CNTs and SFs. The investigation included physicomechanical property evaluations, microstructural analysis, and ultrasonic pulse velocity (UPV) tests. Results indicate significant improvements in performance across the mixes, with the hybrid mix achieving the highest flexural and compressive strengths, highlighting a synergistic interaction between CNTs and SF to enhance load-bearing capacity. Additionally, the mixtures displayed reduced porosity and water absorption, signifying improved density and lower permeability. SEM analysis further confirmed a denser microstructure with enhanced crack-bridging capabilities due to the presence of CNTs and SF. UPV measurements supported these findings, demonstrating superior internal integrity and stiffness in the hybrid mix. These experimental results underscore the potential of hybrid reinforcement strategies for producing high-performance fiber concrete with enhanced durability, making it suitable for demanding construction applications.

The properties of polyvinyl alcohol (PVA) films are intricately influenced by factors such as polymer structure, fabrication method, the addition of plasticizers and the molecular weight of monomers. This research, investigates the implication of PVA films using a solution casting method for crosslinking with boric acid (H₃BO₄), glycerol (C₃H₈O₃) and citric acid (C₆H₈O₇). This approach is compared with pure PVA films, establishing a valuable benchmark. For the experiments, tensile strength tests, physicochemical property measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were conducted to gain insights into the microstructure, surface characteristics and mineral composition of the films. This comprehensive approach aims to enhance our understanding of the intricate relationship between PVA, plasticizers and crosslinking agents, providing valuable insights for applications across diverse industries, including, construction and biomedical fields. The overarching objective of this research is to revolutionize the construction industry by developing polymer films that serve as the foundation for self-healing materials, fostering durability and innovation. The experiments revealed a significant influence of crosslinking agents on the properties of PVA films as measured.

This study examines the potential use of Electrolytic Manganese Dioxide (EMD) residue as a replacement of cement (20%wt by cement weight) in construction materials to provide anticorrosion protection on reinforcing steel and improve the strength of cementitious materials under sulfate attack. To assess the corrosion parameters, the constructed building materials were immersed in a 5%wt sulfate salt (Na₂SO₄), while concrete samples incorporating 20%wt EMD were prepared and subjected to mechanical, porosity and thermal tests. Moreover, SEM images were obtained in order to examine the microstructure of concrete and the extent of damage caused by sulfate ions. The results demonstrate that the inclusion of EMD caused a notable rise in the corrosion of steel bars within cement mortars, as well as a decrease in the mechanical strength of the building materials. Overall, the experimental outcomes of the study suggest that the incorporation of high volume (20%wt) EMD residue leads to the degradation of all measured properties.

Sustainability in construction is related to the use of industrial by-products, such as fly ash (FA). FA varies in chemical/mineralogical composition, depending on the raw materials burnt during its production. While FA produced from coal-fired power stations is extensively used in cementitious composites, heavy oil FA produced from the firing of heavy fuels (e.g., mazut or diesel) remains largely unused. This paper focuses on the novel use of heavy fuel fly ash (HFFA), as a replacement of Class F FA, in high-volume fly ash self-compacting composites. Two different grain size distributions of HFFA were used in quantities 5–15% w/w of cement and Class F FA for the production of the composites. The assessment of the physico-mechanical properties and microstructure of the end-products at different curing ages suggests that HFFA may be used at quantities ≤10% w/w, without any negative effects. In fact, depending on the quantity and grain size distribution of the HFFA, this may even improve some of the properties of the end-products in the long term, provided that a careful mix design is adopted. The findings show the potential of sustainable reuse of HFFA and are beneficial for its incorporation into design codes.

According to currently enforced Eurocode 2 for the design of reinforced concrete structures, it is essential to protect steel reinforcements from corrosion and concrete from degradation under aggressive environmental conditions such as marine, urban, industrial environments and soils, to which these are normally exposed. In this context, this experimental study investigates the enhancement of the physico-mechanical properties of common cement-based mortars and the electro-chemical properties of reinforcing steel, through the addition of nanomaterials in the mix. For the experimental set-up, cylindrical, prismatic, and cubic specimens of different dimensions were cast and were partially immersed in sodium chloride solution for eight months. To evaluate the corrosion of steel rebars, cylindrical cement mortar specimens were used in order to induce a constant cover between exposure and the reinforcement; the physical and the mechanical tests were carried out on standardized shape cubes and prisms, respectively. Two groups were considered: cement-based mortar composites with 0.5 wt.% CNTs addition and plain (reference) specimens without any addition of nanomaterials, for comparison. The influence of adding CNTs on chloride penetration resistance was subsequently evaluated using standardized and non-standardized testing techniques: tests such as flexural strength and porosity, mass loss of steel, electrochemical measurements (corrosion current, HCP) and total chloride content calculation. The test results showed that using CNTs as an addition in mortar production led to the protection of steel rebars against pitting corrosion; moreover, an improvement (almost 9% at 120 days) in flexural strength and reduced porosity of mortars was also observed compared to the reference specimens without CNTs.