Papers by Keyword: Cement Paste

Abstract: With a growing global focus on sustainability, the construction industry is steadily replacing conventional materials with environmentally friendly alternatives. Plant-based fibers, particularly cellulose fibers, are increasingly considered as reinforcement in cement-based materials due to their favorable mechanical properties and renewable origin. Cement-based materials, despite their strength, are prone to porosity and moisture absorption, which compromise long-term durability. To address these challenges while promoting sustainable material use, this study investigates the effects of micro and nanoscale cellulose fibers extracted from sugarcane bagasse on the performance of cement-based materials. Cellulose fibers were obtained through a chemo-mechanical process and surface-modified to achieve hydrophilic and hydrophobic properties. The produced fibers were characterized using SEM and XRD, confirming predominantly amorphous structures and smooth, rod-like morphologies Mortar samples with hydrophobic cellulose microfibers (0.5% and 1%) and cement paste samples with hydrophilic cellulose nanofibers (0.1, 0.25, 0.5 and 1%) were prepared and tested for fresh and hardened properties. Results demonstrated that hydrophobic cellulose microfibers improved mortar workability by up to 13.6% and reduced water absorption by 31.5%, while also enhancing compressive strength (4.7% increase) and density (3.8% increase) at 0.5% dosage. However, higher fiber content (1%) led to entrapped air voids, reducing strength and density. In cement paste, hydrophilic cellulose nanofibers exhibited dual behavior: small dosages had negligible effect, 0.5% significantly improved density (4.9% increase) and compressive strength (38–40 MPa at 7–14 days), while higher dosages caused strength reduction and increased absorption due to fiber agglomeration. Overall, 0.5% fiber incorporation at both scales provided the optimal balance of strength, durability, and workability.

Abstract: Retracted paper: Evaporation of a saline solution from a porous medium often leads to the formation of salt efflorescence at the surface of the medium. We look at the special case where the medium is formed by the assembly of fine and coarse medium vertical columns perpendicular to the evaporation surface and where there is a continuous wicking of the solution into the medium in combination with evaporation. Experiments lead to distinguish two main cases depending on the development or not of the efflorescence at the surface of the coarse medium. On the contrary, the presented analysis suggests that the coarse medium surface colonization occurs when the evaporation flux is sufficiently high no matter what the type of efflorescence is on the fine medium surface. In addition, the analysis suggests that the colonization will always occur when the supersaturation is close to the solubility.

Abstract: Mixing conducting particles in cement present various applications in electromagnetic shielding and in-situ inspection of structures. In this study, graphite was incorporated in cement paste at varying concentrations which enhanced its EM shielding. The samples were characterized using Terahertz Time-Domain Spectroscopy (THz-TDS) to determine its optical properties and calculate for the conductivity. The Ultraviolet-Visible (UV-Vis) spectroscopy was also used to characterize the sample to confirm the variation of graphite content which showed small peaks at 258 nm caused by the excitation of π electrons in the graphitic structure. The refractive index, absorption coefficient and conductivities were determined from the amplitudes and phase difference obtained in the frequency domain. The spectral cut-off in the THz region decreases with increasing graphite content due to THz absorption of graphite. The THz refractive index appeared to be not frequency-independent while the absorption coefficient showed a power-law behavior. The THz conductivities were calculated and was found to be proportional to the graphite content. This is attributed to an increase in the conducting network of cement paste and increase in the charge carriers in the insulating cement matrix.

Abstract: The relevance of the use of nanosized particles in the structuring of cement pastes is due to the ability at the micro level to control the processes occurring during the hardening of cement. And it also provides the ability to purposefully control the composition of hydrated neoplasms. Plasticizer-stabilized aqueous suspensions of nanosized particles of non-hydraulically active materials in a cement matrix cause forced crystallization of hydrated neoplasms from a supersaturated liquid phase. And they act as seeds introduced from the outside, on the surface of which neoplasms are concentrated, that subsequently combine into conglomerates that make up the structure of a cement stone. The formation of the structure of the cement matrix occurs without the formation of large the portlandite fields, which are typical for no additive portland cement. The structure of a cement stone with nanosized particles is represented by poorly crystallized flaky calcium hydrosilicates. It was found that the degree of hydration of cements with suspensions of nanoparticles is 15-18% higher. The efficiency of using nanosized particles in structuring cement pastes is giving the cement paste mobility in the initial period of hydration and the possibility of plasticizing it without blocking the surface of cement particles with surfactants. On the other hand, it makes it possible to obtain dense and durable structures of a cement matrix with improved physical and mechanical characteristics. The porosity of cements with suspensions is lower on 15 - 20%, and after 28 days of hardening it is lower on 20 - 35% than that of non-additive cement. And the strength of the samples after 24 hours increased 1.5 - 1.8 times (from 19.5 to 27 - 34 MPa), at the brand age by 15 - 24% (from 64 to 75 - 80 MPa)

Abstract: Recent experimental investigations on the nanoscale of hardened cement paste revealed that the tensile strengths of the microstructural phases present amount to several hundreds of MPa. Confrontation with macroscopic tensile strength testing, by e.g. Brazilian splitting, shows a decrease over two orders of magnitude. A computational model based on a hierarchical representation of hardened cement paste microstructure is presented in this paper, attempting to shed light on the factors affecting the scaling of strength from the nanoscopic scale up to the macroscopic scale. The model is validated on a case study featuring a Portland-limestone cement paste subjected to an external sulfate attack. Such conditions compromise the nanoscopic integrity of the C-S-H gel as a consequence of the progressive decalcification and affect the overall load-bearing capacity of the macroscopic cement paste specimen.

Abstract: Based on the low-field NMR, this study inveitigated bleeding property of the fresh cement pastes mixed with various gypsum dosage, specific area of cement and water reducers. Results showed that the gypsum dosage between 3 % and 5 % will cause an decrease bleeding and a lower bleeding velocity, while a 1 % gypsum dosage will increase the bleeding as a function of time. The increase of the cement surface will lead to a less bleeding rate. This can be explained that the finer particle will contribute to the packing which will form a low permeability of the cement paste, as a result less bleeding water is observed. The PCEs-made sample has smaller hot zone area which indocated the PCEs has good bleeding stability when varing water cement ratio. Furthermore, bying comparing with the NPE, it was found the ACS type water reducer has higher bleeding sensitivity when high water cement ratios were used.

Abstract: The aim of this paper is to present results of an experimental analysis focused on the mechanical waves passing through fine-grained materials, prepared from the CEM I 52.5 R Portland cement and the water coefficient w/c = 0.40, during the setting and hardening phase. In the experiment differences in the wave propagation in cement pastes and mortars were investigated. After mixing, both materials were poured into a cylindrical mould with the diameter of 75.0 ± 0.3 mm. These test specimens were then used to observe, in particular, the development of the amplitude of mechanical waves using a measuring setup which was composed for this purpose from the equipment of the Brno University of Technology. The results show that the time of „critical changes" in the internal structure of the material can be determined. These changes are related to the quality of the bonds of the particles in the internal structure which are reflected in the propagation of the mechanical waves within the material. It is also expected that the experimental analysis will help extend the knowledge of mechanical waves propagation in cement-based composite materials during setting.

Abstract: The purpose of this study was to observe the effect of partially oxidized rubber powder on cement hydration. The experiment first pulverized the rubber and then put it into a reactor with a low oxygen concentration for partial oxidation. The surface functional groups of the rubber particles before and after the reaction were observed by Fourier Transform Infrared Spectroscopy (FT-IR). The area integral ratio of S-O to C-H3 in the FT-IR spectrum was used as the indicator of partial oxidation results. Finally, the rubber particle after the reaction was added to cement to prepare a cement paste, and its effect on the hydration reaction of the cement was observed by FT-IR, scanning electronic microscope (SEM), optical microscope (OM). SEM images also showed that such sample had very dense C-S-H structure. It confirmed that this composite had the hydrophobic property by observing the contact angle of water droplets on this composite surface.

Abstract: The paper deals with experimental analysis, which is focused on the use of acoustic measurement during the solidification process. As a material for monitoring was chosen fine-grained cementitious composites in the laboratory environment. For this purpose, a measuring device working on the principle of mechanical waves passing through the material was designed, assembled and verified. The experiment was conducted on cement pastes prepared from CEM I 42.5 R Portland cement with two different water coefficients (w/c = 0.40 and w/c = 0.33). The differences in the wave propagation in cement pastes were investigated. Simultaneously with this experiment, the monitoring and the saving records of the internal temperature was conducted. The results show the time of „critical changes" in the internal structure of the material can be determined. These changes are probably related to the quality of the particle’s bonds in the inner material structure, which is reflected in the propagation of mechanical waves. Overall, it is shown these experiments could be used to expand the understanding of the various processes occurring during early hydration of cement, and the application of these results to field situations (in the future) could lead to the other development of, non-destructive (and nonintrusive) monitoring techniques.

Abstract: The article deals with the temperature of the fresh cement pastes (CP) prepared from various type of cement in connection to its consistency. The aim of the experiment was to simulate the real condition at a concrete mixing plant with the comparison of laboratory method of cement testing – test of water need for achieving the normal consistency. In the experiment, cement types CEM I 42.5 R, CEM II/A-LL 42.5 R, CEM II/B-S 42.5 N, CEM III/A 32.5 R and CEM III/A 32.5 N were used. Results from the previous experiment with CEM II/A-S 42.5 R were adopted. Particular water-cement ratios were determined within each of cement type and stayed the same for each of CP temperatures. Increasing of temperature of the CP was achieved by mixing water with various temperatures. Different behavior of consistency change with increasing of temperature within different cement composition was observed. Approach to methods of cement testing at the concrete plant as well as using different cement type in different season of the year in context of durability and sustainability were also discussed.