The retarding action of sugars on cement hydration

Sugars retard the hydration of Portland cement. The effectiveness of different sugars is compared from studies of solution analysis, calorimetry, calcium binding ability and alkaline stability. The best retarders, sucrose and raffinose, have a remarkable ability to solubilize cement constituents and in particular give rise to dramatic increases in the amount of silica in solution. However, ¹³C and ²⁹Si N.M.R. do not reveal the existence of sucrose-silicate complexes. The retarding action of sugars is explained in terms of adsorption onto and poisoning of hydrate surfaces.     

Early hydration of cement constituents with organic admixtures

The hydration of C₂S, C₃S, C₃A, C₄AF and type 1 portlant cement in the presence of calcium lignosulfonate and salicylic acid was studied at a high(20/1) water-cement ratio. The effect of these admixtures on the development, microstructure and surface area of the hydration products was investigated.     

A study of the iron phase and its hydration behaviour in high alumina cement

The Mössbauer spectroscopic investigations of dry sample of Indian high alumina cement (HAC) show that Fe₂O₃ occurs in the CF, CA and C₂F_1?xA_x phases. The presence of CF component in this cement is being reported for the first time and its unambiguous detection has been made possible by the selectivity of Mössbauer spectroscopy. The spectra of hydrated HAC have been discussed in terms of hydration of the above mentioned compounds. The X-ray diffraction studies of dry as well as hydrated HAC are also included.     

Impact of admixtures on the hydration kinetics of Portland cement

Most concrete produced today includes either chemical additions to the cement, chemical admixtures in the concrete, or both. These chemicals alter a number of properties of cementitious systems, including hydration behavior, and it has been long understood by practitioners that these systems can differ widely in response to such chemicals.In this paper the impact on hydration of several classes of chemicals is reviewed with an emphasis on the current understanding of interactions with cement chemistry. These include setting retarders, accelerators, and water reducing dispersants. The ability of the chemicals to alter the aluminate–sulfate balance of cementitious systems is discussed with a focus on the impact on silicate hydration. As a key example of this complex interaction, unusual behavior sometimes observed in systems containing high calcium fly ash is highlighted.     

A viscometric method of monitoring the effect of retarding admixtures on the setting of cement pastes

The effect of retarders on the rheological behaviour of fresh cement pastes has been studied, using rotational viscometry. Cement pastes subjected to continuous shear undergo reaggregation, indicated by the initial build up section of the torque-time curve. It is followed by a dormant period where the paste is at equilibrium which ends when setting starts and the final build up phase begins. The delay time and the kinetics of the final phase depend on the concentration of admixture and nature of the cement. The rates of build up in the initial and final build up sections of the torque-time curve are independent of retarder concentration but vary with the cement. The correlation between these results and those obtained with the Vicat test for the initial setting time of cement paste is discussed.     

高铝水泥及含高铝水泥耐火材料中Al_2O_3的化学分析方法

提出了测定高铝水泥及含高铝水泥的耐火材料中Al2O3的化学分析方法,并把该方法与“粘土、高铝质耐火材料化学分析方法”及“高铝水泥化学分析方法”做了对比。采用本方法分析高铝水泥及含高铝水泥的耐火材料中Al2O3含量,具有准确、简便等特点。     

The composition of pleochroite in high alumina cement clinker

The chemical analyses of pleochroite crystals in four high alumina cement clinkers have been carried out by electron probe micro analysis. It has been shown that individual crystals within one clinker have different compositions and that the average composition of the pleochroite from clinker specimens vary from clinker to clinker. The results indicate that there is a considerable variation in composition of pleochroite, but they all fit the general formula (Ca Na K Fe″)_A (Fe″″ Al)_B(Al₂O₄)₈(Al O₄)_{6 ? x}(SiTi O₄)_x where A and B vary to keep the ionic balance. Typical values are A = 28 and B = 13.     

The effect of pressure on tricalcium silicate hydration at different temperatures and in the presence of retarding additives

The hydration of tricalcium silicate (C₃S) is accelerated by pressure. However, the extent to which temperature and/or cement additives modify this effect is largely unknown. Time-resolved synchrotron powder diffraction has been used to study cement hydration as a function of pressure at different temperatures in the absence of additives, and at selected temperatures in the presence of retarding agents. The magnitudes of the apparent activation volumes for C₃S hydration increased with the addition of the retarders sucrose, maltodextrin, aminotri(methylenephosphonic acid) and an AMPS copolymer. Pressure was found to retard the formation of Jaffeite relative to the degree of C₃S hydration in high temperature experiments. For one cement slurry studied without additives, the apparent activation volume for C₃S hydration remained close to ~ ? 28 cm³ mol^? 1 over the range 25 to 60 °C. For another slurry, there were possible signs of a decrease in magnitude at the lowest temperature examined.     

Thermal behavior of cement matrix with high-volume mineral admixtures at early hydration age

Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinder and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition have been examined at early hydration age. Results show that with addition of fly ash, coal gangue and blast furnace slag the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases.There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.     

Cement hydration in the presence of high filler contents

To realise self-compacting concrete, high filler contents are often used, and in order to avoid problems with excessive heat development during hardening, inert filler materials can be used. In this research two different filler types, limestone and quartzite, are considered in combination with different Portland cements. Although the filler material has often been considered to be inert, experimental results show that it does influence the hydration processes. On the one hand the reaction rate is influenced due to a modified nucleation possibility, and on the other hand, in some cases, the reaction mechanisms are altered, with a new hydration peak occurring. Based on isothermal conduction calorimetry on different cement-filler systems, an existing hydration model for blended cement is modified for these systems. Within the degree of hydration based hydration model, the cement/powder ratio seems to be an important parameter for the cement-filler systems. The model accurately predicts the heat of hydration during the hardening process.     

Early hydration of white Portland cement in the presence of bismuth oxide

Abstract

Abstract

Mineral trioxide aggregate (MTA) is a clinical product comprising a mixture of 80 wt-% Portland cement and 20 wt-% bismuth oxide, which is used as a root-filling material in dentistry. The influence of bismuth oxide on the hydration reactions of Portland cements is not well understood. In this study, the impact of 20 wt-% replacement of bismuth oxide on the hydration of white Portland cement was monitored by powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and transmission electron microscopy (TEM). The findings of this research have confirmed that bismuth oxide is an inert additive in white Portland cement, which does not participate in the hydration reactions.     

The hydration products of Portland cement in the presence of tin(II) chloride

The hydration products of Portland cement pastes cured using water containing tin(II) chloride have been compared with those using distilled water. In the latter case, the expected products—portlandite, ettringite and calcite—were observed. The X-ray diffraction patterns of the cement pastes cured in the presence of tin(II) chloride showed several additional peaks that have been attributed to the formation of calcium hydroxo-stannate, CaSn(OH)₆, and Friedel's salt (tetracalcium aluminate dichloride-10-hydrate), Ca₃Al₂O₆·CaCl₂·10H₂O. The amount of portlandite formed was reduced in the presence of tin(II) chloride. Calcium hydroxo-stannate contains tin in the +IV oxidation state and equations are presented to account for the oxidation of Sn(II) to Sn(IV) preceding the formation of CaSn(OH)₆ and Friedel's salt.     

利用机械力化学原理提高水泥混合材掺量的研究

从充分发挥水泥熟料矿物潜在活性的观点出发，通过对水泥熟料的细磨，提高水泥中小于20μm的熟料颗粒含量，以达到提高水泥中混合材掺量的目的。本研究采用62．5MPa熟料，掺入50%粉煤灰，其水泥胶砂强度可达42．5MPa掺入45％烧粘土，其水泥胶砂强度可达43.4MPa。     

Ultrasound monitoring of the influence of different accelerating admixtures and cement types for shotcrete on setting and hardening behaviour

The possible use of ultrasound measurements for monitoring setting and hardening of mortar containing different accelerating admixtures for shotcrete was investigated. The sensitivity to accelerator type (alkaline aluminate or alkali-free) and dosage, and accelerator-cement compatibility were evaluated. Furthermore, a new automatic onset picking algorithm for ultrasound signals was tested. A stepwise increase of the accelerator dosage resulted in increasing values for the ultrasound pulse velocity at early ages. In the accelerated mortar no dormant period could be noticed before the pulse velocity started to increase sharply, indicating a quick change in solid phase connectivity. The alkaline accelerator had a larger effect than the alkali-free accelerator, especially at ages below 90 min. The effect of the alkali-free accelerator was at very early age more pronounced on mortar containing CEM I in comparison with CEM II, while the alkaline accelerator had a larger influence on mortar containing CEM II. The increase of ultrasound energy could be related to the setting phenomenon and the maximum energy was reached when the end of workability was approached. Only the alkaline accelerator caused a significant reduction in compressive strength and this for all the dosages tested.     

Class H cement hydration at 180 °C and high pressure in the presence of added silica

Under deep oil-well conditions of elevated temperature and pressure, crystalline calcium silicate hydrates are formed during Portland cement hydration. The use of silica rich mineral additives leads to the formation of crystalline hydrates with better mechanical properties than those formed without the additive. The effects of silica flour, silica fume (amorphous silica), and a natural zeolite mixture on the hydration of Class H cement slurries at 180 °C under externally applied pressures of 7 and 52 MPa are examined in real time using in-situ synchrotron X-ray diffraction. For some compositions examined, but not all, pressure was found to have a large effect on the kinetics of crystalline hydrate formation. The use of silica fume delayed both C₃S hydration and the formation of crystalline silicate hydrates compared to what was seen with other silica sources.     

The relationship between mineral composition and strength development of high alumina cement

The strength development curves of 1:3 mortars made from high alumina cement (HAC) containing varying proportions of monocalcium aluminate (CA) and pleochroite as determined by Quantitative X-Ray Diffraction (QXRD) have shown that the rapid strength development and high strength is due to CA. Pleochroite will give a high strength, not as high as is obtained from CA, but the strength development is much slower. No recommendation of HAC is implied in this paper dealing with the mineralogy of such cements and relating mineralogy to strength.     

Influence of precuring on high pressure steam hydration of tricalcium silicate in the presence of other constituents of portland cement

The influence of precuring at room temperature on the autoclave hydration of C₃S in the presence of other constituents of clinker and of gypsum was studied. C₃A, C₄AF and β-C₂S hampered the formation of C₃SH_1.5 and, especially for short precuring times, favored the formation of α-C₂SH. Gypsum hampered the formation of both the crystalline hydrated silicates. When the steam treatment took place after a long precuring, C-S-H was the prevailing hydrated silicate formed.     

The accelerating and retarding effects of hydrogen on carbon deposition on metal surfaces

Carbon deposition of benzene on iron was studied at 550–700°C with 0–1 atm hydrogen in the carrier gas. At least three types of carbon are formed: amorphous, graphitic and carbidic (Fe₃C). The surface of Fe₃C is essentially inactive for benzene decomposition. In the presence of H₂, a metallic surface is maintained resulting in a high activity and hence an accelerating effect by H₂. In the reaction system five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Based on the results in this study, the retarding effects of H₂ on carbon deposition reported in the literature can also be explained. The methanation reaction of surface carbon by H₂ becomes important under conditions when the surface is relatively inactive for hydrocarbon decomposition, and under such conditions, H₂ has a retarding effect on carbon deposition.     

高铝水泥生料球失水试验与生产

０引言我厂高铝水泥１号窑原规格为Φ３．３m／２．５m×７８m,热端扩大,成球盘的湿料球直接入窑,预烧能力差。在２００１年５月份采用国外先进技术———微晶种预成球窑外加热技术进行了工艺改造,窑型改为Φ３．３m／２．５m／３．３m×５７m,窑尾增加固定床料球预热器,实施微晶种成球、失水烘干贮存、料球预热等一系列工艺措施,料球预热能力得到显著增加,熟料质量上了一个新台阶。在该生产线中设计的一条湿料球烘干系统,将成球盘新成的湿料球烘去表面水分,避免在以后的流程中发生堵料粘结,同时提高料球强度,使料球在输送和预…