Papers by Keyword: Cement Paste

Abstract: This paper presents the theoretical analysis and experimental results concerning the rheological behavior and mix design of a 3d printable cement paste as matrix for printing concrete. From the point of view of classical rheology of disperse systems, the application of 3D printing technology in construction has been analyzed. The general scientific concept for optimization of admixtures for 3D printable materials has been developed in terms of viscosity, consistency, and parameters of flocculation and structural built-up. The technological tools to control rheological behavior of visco-plastic admixtures are identified in all stages of 3D printing such as mixing, pumping, extrusion, multilayer casting and structural built-up in the printing layers. The relevant considerations include the concentration, size, morphology, chemical and mineralogical composition, the physical and chemical activity of the solid phase’s surface, and the ionic composition, viscosity, and density of the liquid phase. The squeezing test is used in this paper as a rheological behaviour identification tool of cement-based materials in order to evaluate the extrudability and buildability. It is shown that these properties are significantly influenced by the mix proportions such as W/C-ratio, concentration of plasticizer additives and viscosity modifier additives as factors of changes in the concentration of the dispersed phase and properties of the dispersion liquid.

Abstract: Viability of further research of variatropy is justified, as well as its extended use with the purpose of increasing quality of ferroconcrete building constructions. Raw components and equipment used for manufacturing of centrifuged products and variatropic objects are described. Laboratory centrifuge equipped with a direct current engine and thyristor units has been developed and used. Method of applying pressure to a unit mass during centrifuging has been studied. It has been identified that in order for mix particles rising to their most superior position to stick to the mold walls, it is necessary that gravitational force of mg particle and centrifugal force are equal. Effect of hydrodynamic pressure on particles constituting concrete mix has been studied using several formulas and provisions. It has been revealed that centrifugal molding may be used to seal low-viscosity plastic mixes; however, its use causes disintegration of concrete mix. In order to slow down this process, it is necessary to increase viscosity of the entire system and restrict the upper limit of aggregate fineness.

Abstract: The paper deals with the experimental investigation aimed at the continual monitoring of the process of setting and early hardening in cement pastes and mortars using the measurement equipment Vikasonic. The measurement principle consists in measuring the time of ultrasonic pulses transit through a test specimen placed between two ultrasonic transducers. This innovative method of measurement could in the future suitably complement or, in some cases, completely substitute the measurement of setting in cement composites using the Vicat apparatus. The pilot measurement results performed on the cement pastes and mortars are presented in the paper.

Abstract: The influence of concentration of cement particles φ_c on rheological behavior (flow behavior, dynamic yield stress, plastic viscosity) and thixotropy of the Portland cement CEM I 42.5 R paste was investigated in this research. Also, thixotropy was evaluated by index of reversible structure of Portland cement paste. Concentration of cement particles φ_c ranged from 0.434 to 0.518. Investigation was carried out using rotational rheometer Rheotest RN4.1 with coaxial cylinders. The tests revealed that as φ_c decreases from 0.518 to 0.434, rheological behavior of Portland cement paste is improved: yield stress decreases from 12.7 Pa to 7.1 Pa as well as plastic viscosity – from 4.3 Pa·s to 0.88 Pa·s. The exponential correlation between φ_c and index of reversible structure of Portland cement paste was found. As φ_c decreases from 0.518 to 0.434, index of reversible structure of Portland cement paste, which describes a thixotropy, decreases from 5291 Pa·min to 977 Pa·min.

Abstract: The gamma ray shielding properties of cement paste containing ilmenite and barite (BaSO₄) at different concentration were investigated using beam transmission method for 59.5 keV gamma ray energy of ²⁴¹Am radioactive isotopes by using NaI(Tl) scintillation detector. The mass attenuation coefficients (μ_m) of these shielding materials were calculated. It was found that the mass attenuation coefficients increase with increasing of ilmenite and barite content up to a value of 60% by weight. The highest value of μ_m was 0.425 ± 0.003 cm²/g for cement paste containing ilmenite and 1.598 ± 0.039 cm²/g for the cement paste containing barite. The mass attenuation coefficient values were compared between cement pasted containing ilmenite and barite. The result show that cement paste containing barite showed good gamma ray shielding performances than that for the cement paste containing ilmenite.

Abstract: The hydration of cement is a very intricate process. A great amount of heat is generated during the reaction, which requires close monitoring especially in large concrete members. Modified cement pastes are simpler systems and can be easily used to observe the effect of plasticizing admixtures on the development of temperatures during cement hydration as well as its rheology and mechanical properties. Knowledge of the development of hydration temperatures can be of assistance in deliberate regulation of cement hydration and the generation of hydration heat. The paper describes what influence different amounts of different plasticizers have on the properties of cement pastes, with added focus on the development of their hydration temperatures, rheology and compressive strength.

Abstract: This paper shows a micromechanical study of interfacial transition zone (ITZ) around steel fiber in cement paste. It investigates microstructure and mechanical performance of the ITZ by a combination of nanoindentation and scanning electron microscopy (SEM). The investigated specimens were made from cement CEM I 42.5R paste with dispersed reinforcement in the form of steel fiber TriTreg 50 mm. The SEM demonstrated larger porosity and smaller portion of clinkers in the ITZ. Nanoindentation delivered values of elastic modulus, hardness and creep parameters around the fiber. An average value of elastic modulus in ITZ was at the level of 67% in comparison with cement bulk and the width of ITZ was about 40 µm. The value of hardness was found to be 60% of the average hardness of the bulk cement paste. The measured load-displacement curves were used for calculation of creep indentation parameter (CIT) and the creep compliance function. An average value of the creep compliance in the ITZ was found to be two times higher than in the cement bulk.

Abstract: This study deals with experimental determination of tensile properties of cement paste hydration products at micro-scale. Cantilever micro-beams with length of about 16 µm and pentagon cross section with micrometer dimensions were fabricated by focused ion beam milling on hydrated cement paste samples. Nanoindentation was used for evaluating elastic properties while tensile properties were derived from beam bending tests. Displacement controlled micro-scale tests give access to both tensile strength and estimates of fracture energy based on the load-displacement curves measured with the nanoindenter. The mean tensile strength and the fracture energy of inner hydration product were assessed as 791 MPa and 16.7 J/m², respectively. The huge difference of the micro-scale properties when compared to macroscopic values comes from the scaling properties of concrete.

Abstract: The article aims to present a research into the impact of the dosage and effectiveness of superplasticizers based on modified acrylic polymer and polycarboxylate ester (from 0 to 1.2 %) on rheological properties of the cement pastes (yield stresses and plastic viscosities), of different testing times after mixing (from 0 to 90 min). Materials used in the study: Portland cement CEM I 42.5 R, superplasticizers SP1 (modified acrylic polymer based), SP2 (polycarboxylate ester based), and water. Investigation was carried out using rotational rheometer Rheotest RN4.1 with coaxial cylinders. The tests revealed that superplasticizer SP2 is more effective than SP1 – cement paste (W/C = 0.30) exhibits better flowability and improved rheological qualities. Superplasticizers SP1 and SP2 exhibit different levels of plasticizing effectiveness and ability to retain the effect’s duration. Due to the increase in the dosage of superplasticizers SP1 and SP2 from 0 to 1.2 %, plasticizing effect increases. It is also observed that larger dosage of SP1 (0.6-1.2 %) results in slower increase in plasticizing effects until the 90 min margin. In conclusion, from the start of mixing until the 90 min margin, the best plasticizing effect and its retention achieved by superplasticizer SP2. Recommended SP2 dosage – from 0.6 to 0.8 %.

Abstract: This paper is focused on the cracks growth in mode II of specimens made of cement paste with fly ash. Three prescriptions were made for testing in mode II, during the making air bubbles arose at the bottom of the specimen that were lubricated with oil. These little caverns can have an influence on the growth of the cracks in the fracture process zone. On the other side, specimens that were lubricated with a wax, no caverns were observed. There are complex mechanisms of crack growth in the fracture process zone, cavern or inertial elements have a fundamental effect on the success of the real tests in mode II. Specimens lubricated with wax have a 50 % higher successful running of the test than those, whose forms were lubricated with the oil.