On the combined effect of water soluble lignosulfonates and carbonates on portland cement and clinker pastes — II. Mode of action and structure of the hydration products

The combined effect of Na-lignosulfonate and Na₂CO₃ upon hydration of pure portland clinker and of portland cement has been studied. Both substances reacted with the clinker constituents at the clinker particle surface thus controlling the setting process. The observed liquefying effect of the additives seems to be related to their strong dispersing capacity upon clinker particles in water suspensions. The initial hydration was retarded by the additives. By SEM no noticeable changes in the morphology of the hydration products were detected.     

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.     

Physical,chemical and electrical analysis of dust generated from cement plants for dust removal with an electrostatic precipitator

The physical, chemical and electrical characteristics of cement dust generated from a cement plant have been investigated by using a dust analyzer and a high voltage conductivity cell based on JIS B 9915. Major constituents of raw material cement dust generated from the first grinding process are CaO (41.77%), SiO₂ (11.72%), Al₂O₃ (3.45%), and Fe₂O₃ (1.47%), while the cement clinker dust generated from the second grinding process consists of mainly CaO (48.09-65.50%), SiO₂ (14.02-21.56%), Al₂O₃ (2.86-3.76%), and Fe₂O₃ (1.77-2.66%). Size distribution of the raw material cement dust is bi-modal in shape and the mass median diameter (MMD) is 3.68 μm, whereas the cement clinker dust also displays bi-modal distribution and the MMD of the cement clinker dust is in the range of 7.89-58.78 μm. The resistivity of raw material cement dust is so high as 10¹⁴ ohm·cm at 300 °C, that cement dust would not precipitate well by the electrostatic precipitator.     

On the role of free calcium oxide in expansive cements

Results of research, tending to elucidate the effect of free CaO content in portland cement clinker upon the expansion of specimens of expansive cements, prepared in semi-commercial scale by intergrinding of portland cement clinker, C₄A₃$\text{S}$-phase containing special clinker and gypsum, by means of examination of liquid phase composition and porosity of mortars, have been presented. Special clinker was obtained by burning a mix of limestone, fly ash with high Al₂O₃ content and gypsum. Obtained results confirm the advantageous effect of free lime upon the hydration process and properties of expansive cements. The concentration of CaO in the liquid phase seems to influence rather the rate of ettringite formation than the size of its crystals.     

Effect of CuO on the formation of clinker minerals and the hydration properties

The purposes of this study are to explore the mechanisms of Cu element in clinker burning and hydration processes and to make effective use of waste containing copper in cement production. The effect of CuO on clinker mineral composition, C₃S polymorph and size, Cu element distribution and state, compressive strengths, hydration products, non-evaporable water quantity and hydration heat release rate was analyzed by XRD, SEM, DTA, isothermal heat-conduction calorimetry, etc. Results show that as the amount of CuO increases the formation and growth of C₃S grain are accelerated, R C₃S is gradually transformed into M₃ and the content of C₄AF increases; a small quantity of CuO increases the 3-day and 28-day strengths and the hydration degree of clinker, but excessive CuO has adverse effects. Those effects of CuO on clinker burning process are attributed to the formation of low-melting Cu₂O and the dissolution of CuO in C₄AF which decrease the formation temperature of liquid phase and increase its quantity. The effects on hydration process result from the combined action of the following factors: the induction period is prolonged; the hydration reactions in the initial and acceleration periods are accelerated.     

Corrosion of spinel clinker by CaO–Al2O3–SiO2 ladle slag

Three different grades of sintered spinel clinker were used containing 47, 69 and 94 wt.% Al₂O₃, respectively, i.e. MgO-rich, stoichiometric and Al₂O₃-rich. Based on these clinkers, the corrosion mechanism of each spinel clinker by CaO–Al₂O₃–SiO₂ slag was investigated and the corrosion and penetration behavior of castables containing powdered spinel clinker examined. A layer of MgO·(Al, Fe)₂O₃ complex spinel formed at the slag-refractory interface was proportional to the MgO content of the spinel clinkers, and it depressed the slag corrosion. The free MgO and spinel minerals in each spinel clinker mainly trapped Fe₂O₃ from the slag. CaO–Al₂O₃ compounds were formed at the slag-clinker interface by the reaction between free Al₂O₃ in the Al₂O₃-spinel clinker and CaO from slag. Slag penetration into the spinel clinkers was retarded by these compounds. As a result of adding fine spinel powder to the matrix of Al₂O₃-based castables, it was observed that higher content of MgO in spinel clinker showed better resistance to slag corrosion but lower resistance to slag penetration.     

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.     

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.     

Microstructural control and hydration of novel micro-dendritic clinkers with CaO/SiO2 = 1.4

In this paper we describe the production of CaO/SiO₂-based hydraulic binders formed of dendritic belite embedded in an amorphous phase. These hydraulic binders are produced by a process involving heating the raw materials with a specified CaO/SiO₂ ratio to a temperature bellow the liquidus surface, followed by a two-step cooling ramp, in order to obtain during solidification a dendritic morphology of the crystalline phase. After milling the clinker obtained by this process, and by adding up to 25% of water, the paste sets showing mechanical resistances that went up to 4 times higher the values obtained for a reference round shape belite clinker. This result opens the possibility of developing a novel belite-based clinker, that may lead to a relevant reduction of the CO₂ emissions, as compared to common alitic clinkers.     

On the determination of belite in clinker by X-ray method

The results of β-C₂S analysis in clinker can be essentially distorted due to the overlap of potassium sulfate lines on analytical line 120. In case of significant quantities of K₂SO₄ in clinker β-C₂S determinations are suggested to be performed either on a sample washed by water or on the coarse fraction of clinker powder obtained by its separation on the sieve.     

X-ray photoelectron spectroscopy of the cement clinker phases tricalcium silicate and β-dicalcium silicate

We have used X-ray photoelectron spectroscopy (XPS) to investigate both tricalcium silicate (Ca₃SiO₅, C₃S) and β-dicalcium silicate (Ca₂SiO₄, β-C₂S), the principal components of cement clinkers. In addition to showing how the two phases may be characterised and differentiated, we show how the sensitivity of these phases to atmospheric carbon dioxide and moisture may, as a result of improper sample preparation, lead to erroneous results. The observed alteration processes of the clinker minerals shed light upon the aging process of cement clinker during storage.     

Incorporation of trace elements in Portland cement clinker: Thresholds limits for Cu, Ni, Sn or Zn

This paper aims at defining precisely, the threshold limits for several trace elements (Cu, Ni, Sn or Zn) which correspond to the maximum amount that could be incorporated into a standard clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). These threshold limits were investigated through laboratory synthesised clinkers that were mainly studied by X-ray Diffraction and Scanning Electron Microscopy. The reference clinker was close to a typical Portland clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF). The threshold limits for Cu, Ni, Zn and Sn are quite high with respect to the current contents in clinker and were respectively equal to 0.35, 0.5, 0.7 and 1 wt.%. It appeared that beyond the defined threshold limits, trace elements had different behaviours. Ni was associated with Mg as a magnesium nickel oxide (MgNiO₂) and Sn reacted with lime to form a calcium stannate (Ca₂SnO₄). Cu changed the crystallisation process and affected therefore the formation of C₃S. Indeed a high content of Cu in clinker led to the decomposition of C₃S into C₂S and of free lime. Zn, in turn, affected the formation of C₃A. Ca₆Zn₃Al₄O₁₅ was formed whilst a tremendous reduction of C₃A content was identified. The reactivity of cements made with the clinkers at the threshold limits was followed by calorimetry and compressive strength measurements on cement paste. The results revealed that the doped cements were at least as reactive as the reference cement.     

Evaluation of P2O5 distribution inside the main clinker minerals by the application of EPMA method

The formation of Portland clinker phases has taken place in thermodynamically non-equilibrium state between macro-oxides CaO, SiO₂, Al₂O₃, Fe₂O₃ and MgO from raw meal and P₂O₅ from bone meal. The paper deals with the study of clinker minerals as solid solutions with P₂O₅ during the clinkerization of raw mixture containing bone meal (BM). The ash of BM has contributed as a raw material to the formation of different clinker phases. Electron probe microanalysis (EPMA) method was used to determine the preferential distribution of P₂O₅ inside calcium silicate phases and its influence upon C₂S/C₃S ratio. Basing on these results, composition of solid solution of C₂S and C₃S was established.     

Role of mineral additions in reducing CO2 emission

The influence of the substitution of a part of clinker with the mineral additions, in the process of production of mixed cement, on mechanical characteristics of cement and the reduction of CO₂ emission is researched. Experiments are organized through the production of clinker, which has different phase structures, and cement, using different kinds and amount of mineral additions, with joint grinding and addition to the already ground cement. The most important of the effects analyzed, which include the reduction of thermal and electric energy, is the reduction of CO₂ emission.     

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.     

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     

Effect of the clinker composition on the threshold limits for Cu,Sn or Zn

The threshold limit corresponds to the maximum amount of trace element that could be incorporated into clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). For Cu, Zn or Sn, these threshold limits in a standard clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF) were equal to 0.35, 0.7 and 1 wt.% respectively (Gineys et al., in press). This paper presents the effect of the clinker composition on these threshold limits. Laboratory made clinkers having different mineralogical compositions was characterised by XRD and SEM. Results showed that the threshold limits for Cu, Zn or Sn were consistent. The threshold limit for Sn was affected by the Bogue content in interstitial phases. On the other hand, the threshold limit for Zn was affected by the Bogue content in C₃S of clinker while that of Cu was unaffected by any modifications of clinker composition.     

Influence of the clinker SO3 on the cement characteristics

This paper aims to clarify the influence of the clinker SO₃ on the cement characteristics. The impact on the strength development rate and the level of sulfate resistance were studied .The results show that increasing the amount of clinker SO₃ at low alkali level reduces the percentages of the tricalcium aluminate (C₃A) and alite as well as the alite/belite ratio, leading to a modification in the cement quality.For these reasons cements produced from a clinker containing high sulfate and low alkali, have slower strength development and higher sulfate resisting level than that produced with low sulfate clinker.     

Effect of clinker composition on grindability

Four commercial Portland clinker samples with different chemical compositions and grindability indices were examined. It was found that clinker with low percentage of Al₂O₃ and Fe₂O₃ is easier to grind. Clinker with higher percentage of liquid phase shows a higher work index, i.e., hard to be ground. SEM shows quite a variation in the microstructure of the four different samples. It is necessary to control the chemical composition of the kiln feed and the amount of the liquid phase in the clinker to obtain economical burning and grinding processes.     

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.