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Search Results: 1 - 10 of 4357 matches for " calcium aluminate cement "
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Calcium Aluminate Cement Hydration Model
Ukrainczyk, N.,Dabi?, P.,Matusinovi?, T.
Kemija u Industriji , 2011,
Abstract: Calcium aluminate cement (AC) is a very versatile special cement used for specific applications. As the hydration of AC is highly temperature dependent, yielding structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. The kinetics of AC hydration is a complex process and the use of single mechanisms models cannot describe the rate of hydration during the whole stage.This paper examines the influence of temperature ( =5–20 °C) and water-to-cement mass ratio (mH /mAC = 0.4; 0.5 and 1.0) on hydration of commercial iron-rich AC ISTRA 40 (producer: Istra Cement, Pula, Croatia, which is a part of CALUCEM group), Figs 1–3. The flow rate of heat generation of cement pastes as a result of the hydration reactions was measured with differential microcalorimeter. Chemically bonded water in the hydrated cement samples was determined by thermo-gravimetry.Far less heat is liberated when cement and water come in contact for the first time, Fig. 1, than in the case for portland cement (PC). Higher water-to-cement ratio increases the heat evolved at later ages (Fig. 3) due to higher quantity of water available for hydration. A significant effect of the water-to-cement ratio on the hydration rate and hydration degree showed the importance of water as being the limiting reactant that slows down the reaction early. A simplified stoichiometric model of early age AC hydration (eq. (8)) based on reaction schemes of principal minerals, nominally CA, C12A7 and C4AF (Table 1), was employed. Hydration kinetics after the induction period ( < 20 °C) had been successfully described (Fig. 4 and Table 2) by a proposed model (eq. (23)) which simultaneously comprised three main mechanisms: nucleation and growth, interaction at phase boundary, and mass transfer. In the proposed kinetic model the nucleation and growth is proportional to the amount of reacted minerals (eq. (18)), the interaction at phase boundary was described by a bimolecular consumption of both reactants (eq. (19)), cement and free water, while the mass transfer mechanism was described relative to the limiting reactant (eq. (21)).Increasing temperature from 5 to 20 °C decreases the rate of nucleation and growth (NR) (Fig. 6), increases the rate of interaction (I) according to Arrhenius law (E12 ≈43 kJ mol-1) (Fig. 7), and increases the rate of mass transfer (k) linearly (Fig. 8).
Bond Durability of Carbon-Microfiber-Reinforced Alkali-Activated High-Temperature Cement Adhering to Carbon Steel  [PDF]
Toshifumi Sugama, Tatiana Pyatina
Engineering (ENG) , 2017, DOI: 10.4236/eng.2017.92007
Abstract: The study aims at evaluating the bond durability of a carbon microfiber (CMF)-reinforced alkali-activating calcium aluminate cement (CAC)/fly ash F (FAF) blend cementitious material adhering to carbon steel (CS) under stresses induced by a 350 heat-25 water cooling cycle. This cementitious material/CS joint sample was originally prepared in an autoclave at 300 under a pressure of 8.3 MPa. For comparison, two reference geothermal well cements, Class G modified with silica (G) and calciumaluminum phosphate (CaP), were employed as well reinforced with CMF. In the CAC/FAF blending cement systems, the CAC-derived cementitious reaction products preferentially adhered to CS surfaces, rather than that of FAF-related reaction products. CMF played a pivotal role in creating tough interfacial bond structure of cement layer adhering to CS. The bond toughness also was supported by the crystalline cementitious reaction products including sodalite, brownmillerite, and hedenbergite as major phases, and aragonite, boehmite, and garronite as minor ones. The brownmillerite as an interfacial reaction product between cement and CS promoted the chemical bonding of the cement to CS, while the other phases served in providing the attractive bonding of the cement to CS. The post-stress-test joint samples revealed the formation of additional brown-millerite, aragonite, and garronite, in particular brownmillerite as the major one. The combination of chemical bonding and self-advancing adherence behavior of the cement was essential for creating a better interfacial bond structure. A similar interfacial bond structure was observed with CaP. The crystalline phase composition of the autoclaved cement revealed apatite, zeolite, and ferrowyllieite as major reaction products, and aragonite and al-katoite as the minor ones. Ferrowyllieite was identified as cement/CS interfacial reaction product contributing to the chemical bond of cement, while the other phases aided in providing the attractive bond of cement. After a stress test, two phases, ferrowyllieite and aragonite, promoted the self-advancing adherence of cement to CS. However, the effectiveness of these phases in improving adherence performance of cement was less than that of CAC/FAF blend cement, reflecting the fact that the bond durability of CAC/FAF blend cement was far better than that of the CaP. In contrast, the autoclaved silica-modified G cement consisting
Cimento aluminoso e seus efeitos em concretos refratários magnesianos espinelizados in situ
Milanez, D. H;Sako, E. Y;Maia, M. F;Braulio, M. A. L;Bittencourt, L. R. M;Pandolfelli, V. C;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000100015
Abstract: calcium aluminate cement (cac) bonded alumina-magnesia refractory castables present great advantages for steel ladle applications as a result of in-situ expansive formation of spinel and ca<6, which leads to high basic slag and thermal shock resistance. the cac content in those castables strongly influences its expansive behavior mainly due to ca6 formation. in the present work, the effects of the cac content in magnesia-alumina castables were analyzed. the results showed that calcium aluminate cement affects the volumetric stability of mgo-spinel system: the lower the cac content, the lower the shrinkage. these effects on the sintering and in the mechanical properties after sintering at high temperatures are also presented and discussed.
Processo de hidrata??o e os mecanismos de atua??o dos aditivos aceleradores e retardadores de pega do cimento de aluminato de cálcio
Garcia, J. R.;Oliveira, I. R. de;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000100007
Abstract: one of the main aspects for the development of refractories castables is to master the knowledge regarding calcium aluminate cement (cac), as this binder is the most applied in these products. the objective of this work was to analyze the available information in the literature in order to explain the understanding regarding the actions of retarder and accelerator additives in the setting mechanisms of cacs. the analysis of the compiled information pointed out that the retarder additives, in a general way, act in two ways: 1) inhibiting the process of cement dissolution, by the build up of insoluble barriers around the cement particles, and 2) enhancing the formation of more soluble hydrates, which increase the time necessary for the hardening beginning. on the other hand, the accelerators can induce the formation of less soluble hydrates, diminishing the time necessary for the setting or speeding up the beginning of the hydrate nucleation. the overall analysis pointed out that the action of some retarder and accelerators additives takes place in different periods of the setting time of the cacs. this aspect allows the combination of additives confering a good workability associated to a short demolding time.
Adi??o de cimento de aluminato de cálcio e seus efeitos na hidrata??o do óxido de magnésio
Salom?o, R;Amaral, L. F;Pandolfelli, V. C;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000200007
Abstract: calcium aluminate cement (cac) and magnesium oxide (mgo) are two of the most important raw materials for refractory castables industry and both present a high driving force for hydration. the effects of this reaction for each compound are well known: whereas the hydrated cac behaves as a binder, hardening the castable, mgo hydration can cause the total disintegration of the material. due to the technological interests involved, it is important to study the peculiarities in these processes and their potential interactions. in the present work, the effects of the addition of different cac contents on mgo hydration were investigated in aqueous suspensions by means of apparent volumetric expansion, ph measurements and qualitative x-ray diffraction. it was found out that the deleterious effects of mgo hydration can be significantly reduced with a proper control of the cac content for the formulations.
Propriedades e bioatividade de um cimento endod?ntico à base de aluminato de cálcio
Oliveira, I. R.;Pandolfelli, V. C.;
Ceramica , 2011, DOI: 10.1590/S0366-69132011000300017
Abstract: the mineral trioxide aggregate (mta), a material primarily developed as a root-end filling has been extensively investigated as an innovative product for endodontic applications. however, changes in its formulation/composition involving its mineral aggregates and the development of alternatives of materials have been proposed in an attempt to overcome its negative physical-chemical characteristics. in this work, the influence of additives addition on the development of a novel endodontic cement based on calcium aluminate, has been evaluated. in addition, the properties of endodontic calcium aluminate cement (ecac) were compared with the gold standard mineral-trioxide-aggregate in contact with simulated body fluid (sbf). manipulation tests and measurements of compressive strength, apparent porosity, setting time, ph and ionic conductivity were carried out on plain mta and calcium aluminate cement with and without various additives: a polymeric dispersant, cacl2 as plasticizer, zno as radiopacifier and accelerator additives. based on the ecac properties and likely bioactivity in contact with sbf solution, this alternative material can be indicated as a potential compound for multipurpose use in endodontics.
A??o sonoquímica e influência das condi??es de tratamento térmico na prepara??o de cimentos do sistema binário CaO-Al2O3
Louren?o, R. R;Exposito, C. C. D;Angélica, R. S;Rodrigues, J. A;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000100006
Abstract: cements of the cao-al2o3 binary system were prepared through a sonochemical process at room temperature followed by heat treatment. the conventional production consists of a reaction in which a stoichiometric mixture of lime and bauxite or alumina is melted or sintered. high temperatures and energy consumption is an inconvenience associated to this type of synthesis. in the sonochemical process, the initial reagents, calcia and alumina, are put in an aqueous suspension under the action an ultrasonic bath during some time. after that, the material is heat treated, after the evaporation of water. ultrasonic waves can induce changes in the surface morphology of the particles, including size reduction of them. consequently, the reagent particles become more reactive and this facilitates the final synthesis of the calcium aluminates during the heat treatment. the action of ultrasonic waves and the influence of thermal treatment conditions were studied on two initials molar compositions calcia:alumina of 1:1 and 1:2. temperatures of 1000, 1200 and 1300 oc for 1 and 6 h were employed. sem and x-ray diffraction were used to characterize the obtained materials and the phases were semi-quantified through rietveld method. in addition, mechanical strength of the products was evaluated through splitting tensile tests. pastes consisting of cement, alumina and water were prepared using the calcium aluminates prepared through the sonochemical process and commercial cement as reference.
Influência de aditivos dispersantes e acelerador na hidrata??o de cimento e cimento-matriz
Oliveira, I. R.;Garcia, J. R.;Pandolfelli, V. C.;
Ceramica , 2006, DOI: 10.1590/S0366-69132006000300012
Abstract: a growing demand for refractory castables with specific behaviors has been promoting a continuous technological evolution, in which, one of the most important aspects concerns in a deep knowledge of hydraulic binders. these materials present a great influence on the rheological properties and mechanical strength development of castables, defining their workability periods and demoulding times, respectively. the hydration process of calcium aluminate cement is influenced by the presence of additives, which affects the setting and demoulding time of the shaped body. besides that, the alumina addition to cement also influences the hydration behaviour, as well as, the length of the induction period, the composition of the phases and of the hydration products. therefore, in this work it was studied the influence of the dispersant and/or accelerator additives on the hydration process of calcium aluminate cement and cement-matrix. independent of the system, the dispersant additives acted as retarders on hydration process, mainly the citric acid and diammonium citrate. these additives were the most efficient to combine with the accelerator (li2co3) resulting an intermediate setting time, good workability and a short demolding time.
Cinética de hidrata??o de ligantes à base de alumina hidratável ou aluminato de cálcio
Oliveira, I. R.;Garcia, J. R.;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000100004
Abstract: the dispersion of refractory castables matrix presents a great influence on their rheological behavior, which defines the most appropriate methods for placing these materials. the growing demand for automatically transported refractory castables has promoted the use of pumpable castables, usually specified as self flow compositions. nevertheless, castables with higher fluidity present longer workability, leading to extended demoulding times. because the strength development is intimately linked to the hydration process of calcium aluminate cement or hydratable alumina, it needs to be controlled in order to reach the minimum time for demoulding, contributing to reducing overall costs. the control of cement hydration depends on the knowledge of the variables that determine the kinetics of the involved reactions. in this context, the objective of this work was to evaluate the influence of the sources of the hydraulic binder, the temperature and the presence of matrix or inorganic additives on the hydration process, carried out through temperature measurements and oscillatory rheological tests, as a function of the time.
Functional Acrylic Polymer as Corrosion Inhibitor of Carbon Steel in Autoclaved Air-Foamed Sodium Silicate-Activated Calcium Aluminate/Class F Fly Ash Cement  [PDF]
Toshifumi Sugama, Tatiana Pyatina
Engineering (ENG) , 2013, DOI: 10.4236/eng.2013.511109

The study focused on investigating the effectiveness of functional acrylic polymer (AP) in improving the ability of airfoamed sodium silicate-activated calcium aluminate/Class F fly ash cement (slurry density of 1.3 g/cm3) to mitigate the corrosion of carbon steel (CS) after exposure to hydrothermal environment at 200?C or 300?C. Hydrothermally-initiated interactions between the AP and cement generated the formation of Ca-, Al-, or Na-complexed carboxylate derivatives that improved the AP’s hydrothermal stability. A porous microstructure comprising numerous defect-free, evenly distributed, discrete voids formed in the presence of this hydrothermally stable AP, resulting in the increase in compresive strength of cement. The foamed cement with advanced properties conferred by AP greatly protected the CS against brine-caused corrosion. Four major factors governed this protection by AP-incorporated foamed cements: 1) Reducing the extents of infiltration and transportation of corrosive electrolytes through the cement layer deposited on the underlying CS surface; 2) Inhibiting the cathodic reactions at the corrosion site of CS; 3) Extending the coverage of CS by the cement; and 4) Improving the adherence of the cement to CS surface.

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