Effect of prolonged heating at elevated temperatures on the phase composition and textures of portland cement clinker

Two kinds of portland cement clinker with widely different MgO and SO₃ content were reheated for a long time at elevated temperature. With the clinker rich in MgO and SO₃, alite increased while belite decreased in quantity after reheating. The alite crystals, overgrown with new precipitates, gave zonal structures. Thin platy hexagonal crystals were occasionally nucleated and grown separately in the bulk liquid. High degree of supercooling produced dismembered dendritic crystals of belite. The C/S ratio of the interstitial phase decreased with reheating. The above changes occur in association with the process in which the interstitial liquid, initially variable in basicity, is transformed to the uniform and most acidic one. This process is controlled by the counterdiffusion of CaO and SiO₂, the rate of which is such that, in normal clinker processing, the clinkering reaction terminates before reaching this stage. Rise in firing temperature increases the concentration of SiO₂ in the interstitial liquid and leads to higher solid C₃S/C₂S ratio in clinker. The presence of MgO and SO₃ in abundance lowers the viscosity of the liquid and hence accelerates the changes. No appreciable change could be recognized for the clinker low in MgO and SO₃ content.     

A comparative study of ordinary and mineralised Portland cement clinker from two different production units Part II: Characteristics of the calcium silicates

Portland cement clinkers from two production units were investigated in order to determine the effects of mineralisation on alite and belite; Plant 1: ordinary clinker (P1) and clinker mineralised with CaF₂₊CaSO₄ (P1m); Plant 2: ordinary clinker (P2) and two clinkers mineralised with CaF₂₊CaSO₄ (P2m, P2m′).The polymorphism of alite was studied using synchrotron X-ray powder diffraction (XRD), wavelength 1.5227 Å, and electron diffraction (ED) in a transmission electron microscope. The substitutions of minor elements in alite and belite were determined using electron microprobe analysis. Clinkers P1 and P1m both contained apparent rhombohedral alite (XRD) with an incommensurately modulated structure (ED), while clinkers P2, P2m, and P2m′ all contained monoclinic alite (XRD). The addition of mineralisers in the process caused increased content of fluoride in alite and increased substitution of Si(4+) by Al(3+) and S(6+) in both calcium silicates. The latter effect was most pronounced in clinker P1m due to its high molar SO₃ to alkali oxide ratio (R=2.18).The improved hydraulic activity of P1m compared to P1 was caused by substitutions rather than a change in symmetry. The decreased hydraulic activity of P2m and P2m′ compared to P2 was explained by the high levels of fluorine, which had a retarding effect on the hydration.     

Burning and nodulization process of clinker in the rotary kiln as viewed from the fine textures of the constituent minerals

Portland cement clinkers from the rotary kiln vary with their grain size in both chemical compositions and microscopic textures, according to the wide variation in the burning and nodulization environment in the kiln. The dense interior of large clinker nodules, first formed at high heating rates on and near the surface of the moving raw mix mass, is enriched with K₂O and SO₃ and made up from coarse-grained components of the raw mix. The alite crystals consist mostly of the M₃ phase with inclusions in the core. By contrast, the porous exterior, formed inside the mass at lower heating rates and firing temperatures, is less in K₂O and SO₃ content and made up from fine-grained components of the raw mix. The alite crystals are generally zoned with M₁ occurring in the core. Clinker nodules of medium size, similar in both the chemical composition and the fine textures of alite, are formed concurrently with the exterior of large nodules. Fine clinker nodules come from the core of the mass where the radial motion is stagnant and are formed, due to the large temperature gradient in the mass, at low heating rates and firing temperatures. Dust components comprise, besides small fragments of clinkers, separate alite and belite grains in quantity, indicating that they are separated mostly in the quenching cooler from the porous surface layers of the large clinker nodules. K₂O and SO₃, as well as the fine textures of alite, are useful as an indicator of the progress of firing and nodulization in the kiln.     

Very High Volume Fly Ash Cements. Early Age Hydration Study Using Na2SO4 as an Activator

The early age ambient temperature hydration of a hybrid cement formulation containing very high volumes of coal fly ash (~80% by dry mass) and activated by Na₂SO₄ is presented. The Na₂SO₄ salt acts as a safe and convenient in situ source of alkali to activate fly ash glassy phases without undesirable effects on cement clinker hydration. Comparison to a reference paste with gypsum instead of sodium sulfate revealed that Na₂SO₄ reduced setting times, shortened the induction period, and increased early alite hydration and compressive strength development, but also restricted ettringite formation. When replacing the active fly ash component for milled sand of a similar particle size, the Na₂SO₄‐activated pastes set even quicker, no ettringite was observed, and early strengths were considerably reduced. Possible reaction mechanisms in the hybrid pastes are discussed.     

The influence of the alite polymorphism on the strength of the Portland cement

The influence of the alite polymorphism on the strength of cement was monitored in a set of laboratory-prepared clinkers with equal quantitative phase composition and different ratio of modifications. The alite polymorphism in clinkers was influenced by the change of the MgO and SO₃ side oxides in clinker, raw meal reactivity change, raw meal preheating, burning temperature or by the adding of crystallisation nuclei. The differences in the hydraulic properties of the M₁ and M₃ modifications were determined. In the case of all the hydration periods monitored, the strength of cements with the M₁ modification was 10% higher than the strength of cements with the M₃ modification.     

Factors influencing the phase constitution of alite in portland cement clinker

The phase constitution of alite in portland cement clinker highly depends not only in the chemical composition of a raw mix but also on the kinetics of crystallization from the interstitial melt. From the melt of high supersaturation alite can be crystallized so quickly as to take up foreign ions in quantities in favor of the occurrence of M₃ at ambient temperature. The reverse is the case for the crystallization from the melt low in supersaturation, where M₁ tends to be formed. The formation of zoned alite can be explained by the change in the degree of supersaturation during crystallization. The phase constitution of alite can be correlated with its crystal size in terms of the supersaturation of the melt. The SO₃ in clinker considerably lowers the viscosity of the interstitial melt and thus encourages the formation of M₁ along with the grain growth of alite. The relation between the phase constitution of alite and the quality of cements has been remarked in conjunction with the Ono's method.     

Influence of character of clinker on the early hydration process and rheological property of cement paste

Influence of characteristics of alite and interstitial phase in five Japanese clinkers on early hydration of cement and rheological properties of fresh cement paste was investigated. The yield value of fresh cement paste increased rapidly at first due to the formation of ettringite, later due to the formation of C-S-H until it reached setting. The setting time was strongly influenced by alite intense hydration, which started when Ca(OH)₂ saturation ratio in the liquid phase approached a maximum value. That position mainly was dependent upon the free CaO content in clinker and was partially dependent upon water soluble alkali content in clinker and hydraulic reactivity of alite itself. Changing the burning degree of clinker and the hydraulic modulus of raw mix, and intergrinding of clinker with quick lime are practical ways to reduce slump loss and to set an optimum setting time for concrete. All of these methods are related to free lime content for which optimum amounts are varied with burning degree of clinker and quick lime.     

预分解窑生产中碱对熟料质量的影响

报告了国内某大型现代干法水泥厂预分解窑生产中碱对熟料质量的影响,这种影响导致熟料安定性不良,后期强度降低,对高碱熟料的岩相分析发现,碱对熟料矿物,特别是阿利特开貌具有明显影响,从保证熟料质量的角度提出了熟料中碱的限量标准。     

Influence of cement and superplasticizers type and dosage on the fluidity of cement mortars—Part I

Concrete quality is controlled by the flow behavior of cement paste, which is related to the dispersion of cement particles. Superplasticizers (SPs) provide the possibility of a better dispersion of cement particles, thereby producing paste of higher fluidity. With the development of high strength, high performance concrete, SPs are becoming indispensable. SPs are adsorbed on the cement particles. This adsorption is uneven and depends upon the clinker composition of cement and the type of SP used. This work is focused on the study of the influence of lignosulfonic acid (LS)- and melamine sulfonic acid (SMF)-based SPs on the fluidity of mortars made with ordinary Portland (OPC), low alkali (LAC) and white cement (WC) at different water to cement ratio. It is shown that LS are more effective than SMF in providing better fluidity. Further WC has given the highest fluidity among the cements used. It is attributed to the lower C₃A+C₄AF and alkali content, and higher SO₃ content.     

少熟料水泥中大掺量矿渣激发条件的研究

研究在粉碎活化、碱性激发和硫酸盐复合激发的作用下,大掺量矿渣的活性激发与强度的关系。研究结果表明,对矿渣进行活化激发可显著提高大掺量矿渣水泥的早期和后期强度。将各种激发条件复合并在工业生产中应用可生产高标号少熟料和无熟料砌筑水泥。     

A comparative study of ordinary and mineralised Portland cement clinker from two different production units: Part I: Composition and hydration of the clinkers

Portland cement clinkers from two production units were investigated; Plant 1: ordinary clinker (P1) and clinker mineralised with CaF₂+CaSO₄ (P1m); Plant 2: ordinary clinker (P2) and two clinkers mineralised with CaF₂+CaSO₄ (P2m, low SO₃ and P2m′, high SO₃). The chemical composition of the clinkers was determined by X-ray fluorescence, ICP analysis, titration (free lime) and ion selective electrode measurements (F). Observed clinker parameters (LSF, SR, AR, R, wt.% MgO, F, SO₃, free lime): P1 (0.96, 2.72, 1.27, 1.04, 0.78, 0.06, 0.64, 0.71); P1m (1.03, 2.21, 1.58, 2.18, 0.87, 0.23, 1.95, 0.69); P2 (1.00, 2.66, 1.72, 0.75, 4.06, 0.20, 1.38, 1.51); P2m (1.01, 2.91, 1.96, 0.90, 3.21, 0.39, 1.72, 2.06); P2m′ (0.97, 2.70, 1.84, 1.15, 3.86, 0.42, 2.48, 0.89). The qualitative and quantitative phase compositions were characterised using X-ray powder diffraction, backscattered electron imaging, X-ray microanalysis and elemental mapping, plus optical reflection microscopy. Phases observed in all clinkers were: alite, β-belite, cubic aluminate, ferrite and free lime. Additional phases observed were: aphthitalite (P1, P2, P2m, P2m′), calcium langbeinite (P1m) and periclase (P2, P2m, P2m′). The clinker composition and texture differ more between the two plants, than between ordinary and mineralised clinker from the same production unit. Laboratory cements were prepared by mixing ground clinker with CaSO₄·2H₂O. The cements were hydrated in an isothermal calorimeter at 20 °C (water/cement weight ratio=0.5) during 33 h. After 12 h, the laboratory cement based on P1m reached a higher level of reaction than the one based on P1. The P2m and P2m′ laboratory cements had a slower reaction than the P2 cement.     

掺杂锶对高阿利特水泥熟料制备及性能的影响

采用正交试验方法研究了煅烧温度、保温时间及掺杂氧化锶和硫酸锶对高阿利特水泥熟料煅烧与性能的影响.借助于XRD和SEM-EDS等测试方法研究了水泥熟料的组成、结构及水泥性能.结果表明,高阿利特水泥熟料的最佳制备条件是煅烧温度为1450℃,保温时间为60min,氧化锶和硫酸锶的适宜掺量分别为1.5％、0.5％.在熟料中掺入氧化锶和硫酸锶可以促进阿利特矿物的形成,有利于水泥强度的提高.掺加适量氧化锶时,水泥的早期强度明显提高;而掺加硫酸锶时,有利于该水泥后期强度的提高.     

碱对阿利特－硫铝酸盐水泥熟料矿物形成及水泥性能的影响

以石灰石、粉煤灰、粘土、石膏等为原料,研究了 K2O、 Na2O对阿利特－硫铝酸盐水泥熟料矿物形成及性能的影响。结果表明,少量碱能改善水泥生料的易烧性,促进 fCaO的吸收,过多的碱使 C4A3矿物难以形成;当碱掺加量约 1 2％时, Na2O有利于 C3S矿物的形成,并提高水泥的早期强度,而 K2O则使 C3S的形成量减少,水泥的强度降低;掺加碱使水泥的凝结时间延长。     

Phase identification of alite in portland cement clinker

Unequivocal phase identification of alite in portland cement clinker has been made by combined use of microscopy during heating, X-ray powder diffraction, and differential thermal analysis. Alite thus far assigned to M_1b by X-ray powder diffractometry, in reality, gives every indication of being M₃. Only M_1a should therefore be denoted simply as M₁. The M₁ form of alite in clinker has long been mistake for R because of an apparent trigonal character of its cell. It is M₃ that gives the monoclinic cell as first described by Jeffery.     

Role of delayed release of sulphates from clinker in DEF

Two clinkers rich in sulphate burned in the pilot plant rotary kiln and cements prepared from them were investigated. Clinker richer in sulphate (SO₃=3.6%) contained independent anhydrite grains as well as inclusions of anhydrite in belite. The mortar from it expanded after heat treatment at 90 °C and the addition of Na₂SO₄ or NaOH accelerated and increased this expansion. The expansion occurred irrespective of the fact that the clinker contained only 3% of C₃A, although the C₄AF content was 13%. The second clinker with 2.6% SO₃ contained mainly calcium langbeinite and expanded only when 2% of Na₂SO₄ was added. The SEM examination of the mortars revealed the presence of numerous bands of massive ettringite around sand grains. Agglomerates of cracked ettringite in cement gel were also present. In addition, microcracks were seen inside the darker C-S-H gel. The conclusion is that anhydrite forming inclusions in belite gives an expanding mortar after heat treatment at 90 °C independently of the tricalcium aluminate content. Such clinkers are not typical of industrial conditions. The expansion is caused by the bands of massive ettringite as well as its agglomerates present in the cement gel and nanometric ettringite in the C-S-H phase.     

Synthesis of belite cement clinker of high hydraulic reactivity

This study is concerned with the increase of the cooling rate of belite clinker, by using the water quenching for the chemical stabilization of reactive belite, which improves the hydraulic properties of this clinker. The addition of adequate mineralizers, as NaF and Fe₂O₃, contributes to the improvement of the clinker properties obtained at low burning temperature. X-ray fluorescence spectroscopy, X-ray diffraction analysis and optical microscopy were used to determine the chemical and mineralogical compositions of this clinker. The samples were analyzed by means of a scanning electronic microscope connected with an energy-dispersive X-ray spectrometer to detect the composition of the belite phase and its morphology. Physical and mechanical properties of this clinker cement were determined. The results show that the belite clinker obtained at 1150 °C, with lime saturation factor 0.67, is characterized by a great hydraulic reactivity, similar to that of the ordinary alite clinker. The addition of 2% of NaF and the water quenching improved the chemical, mineralogical and structural properties, while improving the cement hydraulic properties.     

Active low-energy belite cement

The term ‘low-energy cement’ is used for cements that can be in some applications used instead of OPC, and which are produced with less energy. A more extensive utilization of these cements would lead to the lowering of expenses on production of binders as well as a reduction of undesirable emissions. The belite-rich cement belongs to this group. Pure belite clinkers with interstitial matter consisting of C₃A and C₄AF have not been produced, as they have insufficient strength. This work describes the results of hydraulic activation of belite-rich clinkers with sulfate anions. The principle of activation is used for the preparation of belite-rich clinkers with an increased Ca:Si ratio in the structure of dicalcium silicate and partial substitution of SiO₄⁴⁻  by SO₄²⁻. Cements, prepared from these belite-rich clinkers, containing up to 20% of alite, which are burned at 1350 °C, have the same technological properties, including early strengths, as OPC.     

The identity of the sulfur‐containing phases present in cement clinker manufactured using a high sulfur petroleum coke fuel

Cement clinker produced using a high sulfur petroleum coke fuel has been analysed to determine the identity of the sulfate‐containing phases. Quantitative X‐ray diffraction methods were used in conjunction with extraction procedures to concentrate or extract the sulfate phases. The minerals of interest were anhydrite (CaSO₄), aphthitalite (3K₂SO₄·Na₂SO₄), arcanite (K₂SO₄), calcium langbeinite (K₂SO₄·2CaSO₄) and thenardite (Na₂SO₄). Overall sulfur content of the clinker increased in proportion with the amount of sulfur in the fuel. The clinker produced using the high sulfur fuel was found to contain a significantly increased concentration of aphthitalite but a reduced amount of thenardite. Comparison of the experimental results with theoretical predictions based on the Bogue formulae shows differences in respect of calcium langbeinite, which is not detected in the clinker, and thenardite, which is detected. Copyright © 2004 Society of Chemical Industry     

分光光度法快速测定粉煤灰水泥中粉煤灰含量

通过分光光度法,对粉煤灰水泥中Fe2O3和SO3总含量进行检测,后分别检测粉煤灰、熟料、脱硫石膏中的Fe2O3和SO3含量,最后通过混合材掺量计算公式,计算粉煤灰水泥中的粉煤灰含量。该检测方法准确性较高,并且检测快捷方便,检测结果绝对误差范围小于5%。     

The influence of SO3 content of portland cement on the creep and other physical properties of concrete

It is shown that the creep of concrete is very sensitive to the SO₃ content of cement. Examples are given in which creep doubles when SO₃ is reduced from 3.7 to 1.6%. The SO₃ requirement for minimum creep is high, and can exceed 4.0% even when the C₃A¹ and alkali contents of the cement are low to medium and fineness is moderate. It is also demonstrated that the dependence of creep on the SO₃ contents of the cement does not vary with the duration of the creep test. Compressive strength to 1 year, modulus of elasticity, drying shrinkage and 3-year expansion in water were measured and the relationships between these properties of concrete and the SO₃ content of cement were determined.