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1.
The heating rate effect on the thermal behavior of clays from Arumetsa and Kunda deposits (Estonia) and an illitic clay from Füzérradvány (Hungary) was studied. Experiments were carried out under dynamic heating condition up to 1050 °C at the heating rates of 1.25, 2.5, 5 and 10 °C min ?1 in a stream of gas mixture containing 79 % of Ar and 21 % of O 2 with Setaram Labsys 1600 analyzer. Two different ashes were used as additives: the electrostatic precipitator ash from the first field and the cyclone ash formed, respectively, at circulating fluidized bed combustion (temperatures 750–830 °C) and pulverized firing (temperatures 1200–1400 °C) of Estonian oil shale at Estonian Power Plant. For calculation of kinetic parameters, the TG data were processed by the differential isoconversional Friedman method. The results of thermal analysis and the variation of the value of activation energy E along the reaction progress α indicated the complex character of decomposition of clays and their blends with Estonian oil shale ashes, and the certain differences in thermal behavior of different clays depending on their origin. 相似文献
2.
The use of active mineral additions is an important alternative in concrete
design. Such use is not always appropriate, however, because the heat released
during hydration reactions may on occasion affect the quality of the resulting
concrete and, ultimately, structural durability. The effect of adding up to
20% silica fume on two ordinary Portland cements with very different mineralogical
compositions is analyzed in the present paper. Excess gypsum was added in
amounts such that its percentage by mass of SO 3 came
to 7.0%.
The chief techniques used in this study were heat conduction
calorimetry and the Frattini test, supplemented with the determination of
setting times and X-ray diffraction. The results obtained showed that replacing
up to 20% of Portland cement with silica fume affected the rheology of the
cement paste, measured in terms of water demand for normal consistency and
setting times; the magnitude and direction of these effects depended on the
mineralogical composition of the clinker. Hydration reactions were also observed
be stimulated by silica fume, both directly and indirectly – the latter
as a result of the early and very substantial pozzolanic activity of the addition
and the former because of its morphology (tiny spheres) and large BET specific
surface. This translated into such a significant rise in the amounts of total
heat of hydration released per gram of Portland cement at early ages, that
silica fume may be regarded in some cases to cause a synergistic calorific
effect with the concomitant risk of hairline cracking. The addition of excess
gypsum, in turn, while prompting and attenuation of the calorimetric pattern
of the resulting pastes in all cases, caused the Portland cement to generate
greater heat of hydration per gram, particularly in the case of Portland cement
with a high C 3A content. 相似文献
3.
This new study must be regarded to be a direct outcome of two previous studies published by these same authors, which were
conducted to respond to interesting questions brought out about the effect of silica fume, SF and metakaolins, M and MQ, on
the heat of hydration of portland cements, PC, with very different C 3A and C 3S contents. The answer to these so interesting questions has been the primary objective of the present research. For this
purpose, the same PC, PC1 (14% C 3A) and PC2 (≈0% C 3A), metakaolins, silica fume and blended cements were once again used more 60/40 for sulphate attack, and the same analytical
techniques (CC, pozzolanicity and XRD analysis) and parameters determined as well. In this new research, the sulphate attack
was determined by two accelerated methods: Le Chatelier-Ansttet and ASTM C 452-68.
The experimental results of sulphate attack mainly, have demonstrated definitively that the high, rapid and early pozzolanic
activity exhibited by SF also is, as in the case of the two metakaolins, more specific than generic, for it indirectly stimulated
greater C 3A than C 3S hydration, but only in the first 16 h monitored in this study. Thereafter it is the contrary, i.e., anti- or contra-specific
for the same purpose. And the longer the hydration time, the more anti- or contra-specific it became, since, when exposed
to sulphate attack, SF blended cements resisted or even prevented the aggressive attack against PC1 which, with a higher C 3A content than PC2, was the more vulnerable of the two. By contrast, metakaolin MQ not only failed to hinder or prevent the
attack, but heightened its effects, rendering it more intense, aggressive and rapid, leading to what could be called a rapid
gypsum attack. 相似文献
4.
The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal
calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the
different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica
fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed
to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated
with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared
spectroscopy (FTIR) have been used to assess the microstructure of these formulations. 相似文献
5.
The durability and thermal stability of hardened Portland cement pastes containing vermiculite (V) and expanded vermiculite (EV) exposed to high temperatures were studied. Different mixtures were prepared using 2.5, 5, and 10 wt% of both types of V. Each mixture, after 28?days of hydration, was heated at 300, 600, and 800?°C for 3?h. Two modes of cooling were used; gradual cooling in air and rapid cooling in cold water. The percentage of residual strength, chemically combined water content, change in phase composition, and the thermal stability of the heated specimens were studied. The specimens cooled in water showed greater loss in strength than the air-cooled specimens. The presence of V improved the heat resistance of ordinary type I Portland cement (OPC) pastes. 5 wt% replacement revealed the best performance at all heating temperatures. The EV showed better thermal resistance than the nonexpanded one. Addition of silica fume (SF) with V in OPC pastes lead to superior performance. This can be explained as result of the combined effects of insulation properties of V and pozzolanic reactivity of SF which accounts for the notable increase in the residual strength for these mixes. 相似文献
6.
This study focuses on the thermal and mineralogical transformations of clay ceramic pastes. The pastes contain different amounts of sugarcane bagasse ash waste. Thermal and mineralogical changes occurring during firing were characterized by differential thermal analysis, thermogravimetry analysis (TG), X-ray diffraction (XRD), and scanning electron microscopy. On heating three endothermic events within the 73.5–75.7, 276.9–283.5, and 567.1–573.5 °C temperature ranges were identified. The endothermic valleys could be mainly interpreted as the release of physically adsorbed water, dehydration of hydroxides, and dehydroxylation of kaolinite, respectively. Two exothermic events within the 618.9–690.1 and 948 °C temperature ranges were identified. The exothermic peaks are associated with the decomposition of organic compounds and crystallization of mullite from metakaolinite, respectively. TG results indicate that the clay ceramic pastes had a total mass loss in the 13.1–13.6 % range, and are dependent on the sugarcane bagasse ash waste amount added. It was found that the replacement of natural clay with sugarcane bagasse ash waste, in the range up to 20 wt%, influenced the thermal behavior and technological properties of the clay ceramic pastes. In addition, the thermal analysis results agree well with the XRD. 相似文献
7.
In order
to evaluate the pozzolanic activity of metakaolin, several pastes were prepared,
by mixing metakaolin with hydrated lime, in different ratios. The pastes were
stored in standard conditions ( RH=99±1%, T=25±1°C) and evaluated using thermal
analysis (DTA/TG), X-ray diffraction (XRD), compressive strength tests and
mercury intrusion porosimetry (MIP), in time. The obtained results revealed
that the compounds formed are CSH, C 2ASH 8
and C 4
11
while C 4AH 13 was not detected
up to 270 days of curing. The calcium hydroxide consumption increases as the
initial amount of the metakaolin in the paste augments. The maximum strength
development is obtained for metakaolin/lime ratio:1. 相似文献
8.
Journal of Thermal Analysis and Calorimetry - The effect of fired drinking water sludge (FDWS) as a mineral admixture on the physico-mechanical properties and the fire resistance of pozzolanic... 相似文献
10.
Ceramic powder has been used as an artificial pozzolanic addition, in preparing pozzolanic mortars for the historic/traditional
structures’ construction. In order to evaluate the pozzolanic activity of ceramic powder, several pastes were prepared, by
mixing it with hydrated lime, in different ratios. The pastes were stored in standard conditions (RH=99±1%, T=25±1°C) and evaluated using thermal analysis (DTA/TG), X-ray diffraction (XRD), compressive strength tests and mercury intrusion
porosimetry (MIP), in time. The obtained results revealed that the compounds formed were CSH and C 4ACH 11 (monocarboaluminate) after 270 days of curing. The calcium hydroxide consumption increases as the initial amount of the ceramic
powder in the paste augments. The maximum strength development is obtained for ceramic powder/hydrated lime ratio 3:1. 相似文献
11.
Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique
to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate
hydrate (C–S–H), ettringite, gehlenite hydrate (C 2ASH 8), calcium hydroxide (Ca(OH) 2) and calcium carbonate (CaCO 3) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH) 2 with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding
increase in calcium silicate hydrate (C–S–H). 相似文献
12.
The kaolin sand containing 36 wt% of kaolinite was thermally transformed at 650 °C/L h to the burnt kaolin sand (BKS) with
relevant content of metakaolinite. Thermal behaviour of composites with substitution of Portland cement (PC) by the BKS containing
0, 5, 10 and 15 wt% of metakaolinite and water-to-solid ratio of 0.5 kept for 90 days in 20 ± 1 °C water was studied by thermal
analysis. TG/DTA/DTG studies concerned calciumsilicate hydrate and calciumaluminate hydrate formation, portlandite dehydroxylation
and calcite decarbonation. The influence of curing time and metakaolinite content were estimated. The reduction in portlandite
content was observed in PC–BKS composites opposite to that found in the reference PC system. Compressive strength uptakes
were observed in PC–BKS composites relative to that of reference PC system. BKS is characterized as effective pozzolanic material
giving cement composites of high performance. The above findings were confirmed by X-ray diffraction analysis (XRD) and scanning
electron microscopy (SEM) results. 相似文献
13.
This study aimed to utilize laboratory-prepared nano-silica (NS) and nano-alumina (NA) as low-cost nano-oxides additions for improving the mechanical properties and thermal resistance of hardened ordinary Portland cement (OPC) pastes. NS was synthesized from rice husk ash in the absence of any surfactant, while NA was synthesized from AlCl 3 in the presence of CTAB as a surfactant. The average particle sizes of synthesized NS and NA were 30 and 40 nm, respectively. Nano-silica or nano-alumina was added to OPC as a single phase with different ratios of 0.5, 1, 2 and 3 by mass % of OPC. The physico-chemical characteristics of different OPC-NS and OPC-NA hardened pastes were studied after 1, 3, 7, 14, 28 and 90 days of hydration. The resistance of the hardened composites for firing was evaluated for specimens cured for 28 days under tap water and then fired at 300, 600 and 800 °C for 3 h. The fired specimens were cooled by two methods: gradual cooling and rapid cooling. The compressive strength test was performed for all mixes at each firing temperature. The compressive strength results revealed that the optimum addition of NS is 1, whereas the optimum addition of NA is 0.5 by mass % of OPC. XRD, TG/DTG and SEM results indicated that ill-crystalline and nearly amorphous C–S–H, C–A–S–H and C–A–H were the main hydration products. 相似文献
14.
Journal of Radioanalytical and Nuclear Chemistry - This paper investigated the influence of silica fume on radioactive fluoride concentrate liquid waste cementation with magnesium potassium... 相似文献
16.
The objective of this work was to coat aluminosilicate clays on an inert silica support, and to characterize the properties and stability of the clay-silica coating. Two polymers, polyacrylamide (PAM) and polyvinyl alcohol (PVA), were used to bind kaolinite, illite, and smectite onto silica grains. The clay-polymer composites were studied by X-ray diffraction, FTIR, and electrophoretic mobility. Clay coatings on silica grains were characterized by mass coverage, scanning electron microscopy, specific surface area, and pH stability. Silica sand was successfully coated with clays by using the two polymers, but with PVA, the clay coating had a greater mass coverage and was more stable against pH variations. Less polymer was needed for the clay coating using PVA as compared to using PAM. Clay-polymer complexes and pure clay minerals had similar cation exchange capacities and electrophoretic mobilities, indicating that overall surface charge of the clays was little affected by the polymers. Some decrease in hydrophilicity was observed for illite and smectite when clays where coated with the polymers. The methodology reported here allows the generation of a clay-based porous matrix, with hydraulic properties that can be varied by adjusting the grain size of the inert silica support. 相似文献
18.
Journal of Thermal Analysis and Calorimetry - The mechanical properties of concrete based mainly on flexural and compressive bearing capacity. Generally, researchers have an interest in the... 相似文献
19.
Present study deals with the influence of metakaolin (MK), silica fume (SF) and ground granulated blast-furnace slag (BFS) on middle hydration of ordinary Portland cement replaced by 45 mass% of particular supplementary cementitious materials (SCMs). Acceleration of cement hydration by SF and MK was proved up to the first 12 h by isothermal calorimetry as well as by thermogravimetric analyses. From the beginning of deceleratory period, when SCMs stopped to act as accelerators, more evident influence of the dilution effect was observed. Nevertheless, the presence of pozzolanic reactions was demonstrated already after 15 h of curing and even when SF and MK were used in the amount equal to 5 mass%. Synergic effect of the used SCMs allowed to increase the quantity of BFS up to 35 mass% without significant changes in their positive action. 相似文献
20.
Journal of Thermal Analysis and Calorimetry - Two Portland cement (PC) replacement levels (35, 45 mass%) and three locally available supplementary cementitious materials (SCMs; metakaolin... 相似文献
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