Belite Crystals Research Articles

Aptness of nano alumina (Al2O3) with high range water reducer-incorporated concrete

PurposeThe purpose of this paper is to study the effect of nano alumina (Al2O3) on the properties of fresh concrete, hardened concrete and microstructure of concrete incorporated with high range water reducer (HRWR). This initiative was taken to improve characteristic properties of concrete using nano alumina because nano alumina can be easily be manufactured from a scrap of industrial aluminum products, so its incorporation in concrete will not only reduce industrial aluminum waste but will also change the morphology of concrete at the microstructural level.Design/methodology/approachTo accomplish the objectives of the research, four different concrete mixes with the constant water–cement ratio (W/C) and superplasticizer (SP) content 0.4 and 0.6% by weight of cement, respectively, were prepared, whereas nano alumina content was altered by 0.3% and 0.4% by weight of cement. Fresh property of concrete was analyzed by using slump cone test, whereas hardened properties of concrete were analyzed through compression test and flexural strength test. The interaction of nano alumina with concrete composite was evaluated using an X-ray diffraction test.FindingsIt was observed that 0.6% superplasticizer by weight of cement increased workability by 22% but with the addition of 0.3%, nano alumina by weight of cement workability decreased by 31%. Compressive strength increased by 4.88% with the addition of 0.6% superplasticizer but with the addition of 0.3% nano alumina by weight of cement compressive strength increased by 18.60%. Also, flexural strength increased by 1.21% with the addition of 0.6% superplasticizer by weight of cement but with the addition of 0.3% nano alumina by weight of cement flexural strength increased by 8.76%. With the addition of superplasticizer, alite and belite phases remained un-hydrated but with the addition of nano alumina alite phase was hydrated while belite phase was un-hydrated. The size of belite crystals in mixes having nano alumina was less than that of mix having 0.6% superplasticizer. Also with the addition of nano alumina, a calcium aluminum silicate phase was formed which was responsible for the increment of strength in mixes having nano alumina.Originality/valueIncorporation nano alumina (Al2O3) in concrete will not only reduce industrial aluminum waste but will also reduce CO2 emission. Nano alumina (Al2O3) also changes morphology of concrete at micro structural level.

Effect of clinkering conditions on phase evolution and microstructure of Belite Calcium-Sulpho-Aluminate cement clinker

This paper studies the effect of clinkering temperature and duration on the production, phase evolution and microstructure of low energy Belite Calcium-Sulpho-Aluminate (BCSA) clinkers. The mineralogy and microstructure of BCSA clinker, produced in the temperature range of 1270–1340 °C and clinkering duration periods of 20, 30 and 60 min, were studied by XRD/QXRD and SEM/EDS. In all clinkering conditions belite was detected in both αH′- and β-polymorphs while ye'elimite was mainly formed in the orthorhombic phase, accompanied by small amounts of cubic polymorph. Extensive formation of perovskite was also detected. It was shown that elevated concentrations of Ti and S affect belite's polymorphism; at low clinkering conditions their incorporation in the belite crystal led to the stabilization of αH′-polymorph, while at elevated temperatures their partial release from belite crystals favours the stabilization of β-polymorph. The results highlight the importance of experimental simulation in combination with theoretical clinker design.

Experimental study on recycled cement prepared from waste concrete

In this paper, waste concrete of different strength grades is used as a substitute for calcareous and siliceous materials. The effects of waste concrete with different strength grades, various raw material compositions, distinct calcination temperatures, burnability of cement clinker, lithofacies characteristics and microscopic morphology hydration products are studied systematically. The results demonstrate that waste concrete is conducive to the low temperature formation of clinker mineral phase, which promotes the firing of clinker and the incorporation of low-strength grade waste concrete benefits the calcination of raw material more obviously. Under the same conditions, the content of f-CaO in recycled cement clinker shows an upward trend with the increase of the strength grade of KH and waste concrete. Meanwhile, if the amount of waste concrete exceeds 40%, the stability of recycled cement would be poor. Alite and Belite crystals are formed with obvious characteristic peaks in the waste concrete calcined regenerated cement clinker of different strength grades. However, each sample group has different degrees of over-burning phenomenon, it is difficult to identify the adhesion between A and B minerals in clinker. The regenerated cement hydration products have different degrees of C-S-H gel and ettringite formation, the hydration products of recycled cement prepared by waste concrete with low strength grade are more compact.

Utilization of Blast Furnace Slag, Steel Slag in the Production of Clinker

The study on the use of blast furnace slag, steel slag and fly ash as raw materials to produce cement clinker has been carried out in the present investigation. The burnability of raw meal was determined by the linear shrinkage and free lime content of clinkers sintered at different temperatures. The microstructrue and properties of raw meal and clinker were investigated by the means of chemical analysis, X-ray diffraction and petrographic analysis. The physical and mechanical performance of cement was tested according to the standards. The results showed that the burnability was good as the free lime content was lower than 0.5w% in the clinker sintered at 1350. It is believed that the nucleation of alite only needs to grow bigger and rearrange with the rising temperature. The micrograph of clinker sintered at 1450 showed that there were some garlands of belite crystals around alite crystals because of high temperature or the local reducing atmosphere caused by the compact raw meal. High-quality cement clinker with compressive strength of 60.4MPa was produced by utilizing limestone, blast furnace slag, steel slag and fly ash as raw materials after curing 28 days.

Morphology of α′H-Ca2SiO4Solid Solution Crystals

Single crystals of α′L-(Sr0.3Ca1.7)SiO4 with well-developed smooth faces were grown by heating at 1473 K in a SrCl2 flux. A euhedral prismatic crystal ∼200 μm ×∼300 μm ×∼1000 μm in size was selected and examined by the combined use of the precession method and scanning electron microscope. Because the crystal developed within the stable temperature region of the α′H-phase, the faces were successfully identified as {111}, {011}, and {021} on the basis of the former α′H-phase lattice. The faces {011} and {021} constitute a zone with the zone axis [100], which was parallel to the elongation direction of the crystal. A similar morphology was confirmed for the belite crystals in a belite–gehlenite clinker, which were crystallized out of a melt within the stable temperature region of the α′H-phase.

The influence of mineralogical, chemical and physical properties on grindability of commercial clinkers with high MgO level

This research investigates various methods able to identify possible mineralogical, physical and chemical influences on the grindability of commercial clinkers with high MgO level. The aim of the study is to evaluate the hardness and elastic modulus of the clinker mineral phases and their fracture strength during the comminution processes, comparing samples from clinkers with low MgO level (0.5%) and clinkers with elevated MgO levels (> 5.0%). The study of the influence of mineralogical, chemical and physical properties was carried out using several analytical techniques, such as: optical microscopy, X-ray diffraction with Rietveld refinement (XRD) and X-ray fluorescence (XRF). These techniques were useful in qualifying the different clinker samples. The drop weight test (DWT) and the Bond ball mill grindability test were performed to characterize the mechanical properties of clinkers. Nanoindentation tests were also carried out. Results from the Bond ball mill grindability test were found to be related to the hardness of the mineral phase and to mineralogical characteristics, such as type and amount of inclusions in silicates, belite and alite crystals shape, or microcracked alites. In contrast, the results obtained by the DWT were associated to the macro characteristics of clinkers, such as porosity, as well as to the hardness and mineralogical characteristics of belite crystals in clusters. Hardness instrumented tests helped to determine the Vickers hardness and elastic modulus from the mineral phases in commercial clinkers and produced different values for the pure phases compared to previous publications.

Trace element analysis of belite in hardened cement bonded materials using electron microprobe analysis

This study presents an optimised analytical procedure for quantitative trace element analysis of belite crystals in hardened cement bonded materials by electron microprobe and its application. Within this project, trace element concentrations in belite crystals of 22 cement bonded materials were determined. In each sample 7 to 14 belite crystals were analysed to determine the concentrations of the elements Ca, Si (matrix elements), Na, Al, K, Ti, Fe, Mg, P, S, Zn, Sr, Pb, V, Cr, Mn, Ni, and Ba (potential trace elements) with a beam current of 15 and 120 nA and a total counting time of 30 minutes. Trace elements show total concentrations between 4 and 8 wt% of the oxides. Na, Al, K, Ti, Fe, Mg, S and Sr show high concentrations generally above 1000 ppm. P, Mn and Ba are generally within the 100–500 ppm level while Zn, V and Cr are below 100 ppm. Pb and Ni are below the limits of detection. The results show that trace element distribution of belite in each of the used cement materials has its characteristic trend. This opens up interesting perspectives for testing institutions, e.g. to evaluate, whether two cement bonded specimens are of the same origin or not.

Effect of CaF 2 and MgO on sintering of cement clinker

CaF 2 (0.25, 0.50, and 1%) and MgO (1, 2, and 3%) was added to a mixture composed from limestone, sand, and loam, then burned at 1410°C for 112 min. Clinker samples were cooled to room temperature and the amount of free lime (CaO f) was determined by titration while the relative amounts of alite and belite were determined by X-ray diffraction (XRD). Microstructure was investigated by scanning electron microscopy (SEM). The amount of alite decreased in CaF 2-added samples and the amount of belite increased. The CaO f content of the sample containing 1% MgO was less than that of the pure sample. For the other MgO-added samples, due to the increase in lime saturation factor value no evident change was detected in the amount of CaO f. Both XRD and SEM analyses showed a decrease of the amount of alite and an increase of the amount of belite in MgO-added samples when compared to pure samples. The reason for the decrease in amount of alite in MgO-added samples is believed to be coarse belite crystals. Due to the prolonged holding time at the alite formation temperature, coarse belite crystals were formed. Because of the high diffusion rate at this period, belite crystals lose their surface activity and consequently alite formation was hindered.

Influence of heating rate on the burning of cement clinker

The effects of heating rate on the burning of cement raw mix are analyzed in view experiments. Raw mix is produced from limestone, loam, and sand, then granulated and burnt in the gradient furnace to produce clinker. To investigate the dependence of burning of raw mix on heating rate, two different heating regimes were applied. The first heating regime was final temperature 1450 °C, burning duration 145 min, and rates of heating 29.5 and 3.85 K/min. The second heating regime was final temperature 1410 °C, duration 112 min, and rates of heating 36.4 and 5.3 K/min. As a result of microscopic investigations, it was noted that there were no significant differences between two clinker samples in terms of the sizes, forms, and distributions of free CaO, alite, and belite crystals. The amount of free CaO is the most important criterion for the burning of raw mix using a definite heating regime. Although there is a lower temperature and shorter burning duration, 2.09% free CaO is found in the clinker sample because of the high burning rate. It has been concluded that the clinker can be burnt at a lower temperature and for a shorter time with a high heating rate.

Thermoelasticity of belite in Portland cement clinker

Belite crystals, separated from an industrial cement clinker, were examined by high-temperature powder X-ray diffractometry. We determined the starting and finishing temperatures of the α′ L-to-β martensitic transformation and those of the reverse (β-to-α′ L) transformation. The thermal hysteresis was negative (−65°C), indicating the thermoelasticity of the transformation. Each of the transformation temperatures was lower than that of pure Ca 2SiO 4. Because the finishing temperature of the α′ L-to-β transformation was higher than ambient temperature, the α′ L-phase was entirely inverted to the β-phase during cooling.

X-ray mapping of interstitial phases in sulphate resisting cement clinker

Industrial sulphate-resisting cement clinker has been investigated by elemental mapping and X-ray microanalysis (EDS) in a scanning electron microscope; powder X-ray diffraction (XRD) and X-ray fluorescence analysis. The interstitial material mainly consists of ferrite Ca2·01(07)Al0·71( 7)Fe0·89(6)Mg0·08(2)Mn0·05(2)Si0·16(4)Ti0·03(1)P0·009(3) O5 (EDS), plus minor amounts of an orthorhombic aluminate (XRD) and potassium sulphate (EDS). In the elemental maps (Ca, Si, Al, Fe, K. S), recorded using the characteristic Ka X-rays; alite, belite, ferrite, potassium sulphate and pores are readily identified, while aluminates are only rarely distinguishable. Free lime, periclase, unreacted silica, and secondary alite should be easy to recognize, but were not observed. Chlorine or sodium were not detected in the clinker. More potassium sulphate was associated with belite, than with a lite, and the texture of the interstitial potassium sulphate suggests that at high temperature, molten potassium sulphate wets the belite crystals, but not the alite crystals.

Formation of belite clusters from quartz grains in portland cement clinker

The conversion process of quartz grains into belite clusters in portland cement clinker has been followed under isothermal heating conditions at 1400 °C. On heating, quartz grains are surrounded by layers consisting of belite and liquid, which continue to grow thick with the progress of conversion (outer layers of belite). The conversion process has been divided into three stages considering the chemical composition of the liquid phase formed in each stage. In the first stage are formed two kinds of liquids the compositions of which lie across the primary phase field of wollastonite from each other. The belite crystals precipitated constitute thin, dense layers (inner layers of belite). At the beginning of the second stage, the inside of the belite layers is completely converted into a liquid without quartz and wollastonite. The inner layers extend inward and grow thick; the impurity concentration is lowest for this part of the belite layers. With further progress of diffusion, belite crystals nucleate and grow independently inside the layers. The composition of the coexisting liquid, rich in alkalies, moves on the primary phase field of C 2S toward that of C 3A. In the third stage the conversion is completed and the liquid phase, enriched with Al 2O 3 and Fe 2O 3, is similar in composition to the interstitial liquid in portland cement clinker. The belite crystals, especially those of the inner layers, undergo grain growth with an increase in impurity content with duration of heating. Three kinds of belite crystals different in origin, texture and composition are thus distinguishable in the belite clusters.

Effect of Al/Fe Ratio in Belite on the Microtexture Induced by the Remelting Reaction

During the remelting reaction, belite crystals with Al/Fe ratio <1 produced a liquid which formed droplets on the lamellar boundaries; the rate of remelting reaction was very low. Belite crystals with Al/Fe >1 produced a liquid which readily spread on the lamellae with a resulting high rate of reaction. At 1450°C the distribution coefficient of Al and Fe atoms between the belite and the interstitial liquid ((Al/Fe)Belite/(Al/Fe)Liquid) was about 0.8.

Texture and grindability of the dust component in portland cement clinker

The dust component of portland cement clinker from the rotary kiln is generally less than 250 μm in size and consists of clinker fragments of various sizes, inclusive of separated alite and belite crystals with some interstitial material. In view of the close similarity in chemical composition and microscopic texture, dust grains are supposed to be separated mechanically from the outermost part of clinker nodules weak in adhesion and further fractured into small pieces during percolating toward the core of the moving mass. This process should occur after the complete crystallization of the interstitial liquid that acts as a medium of clinker nodulization. With the dust particles flying in the atmosphere of higher temperature the interstitial material remelts and, on quenching, is transformed into the glassy state; the alite and belite crystals undergo a concomitant texture change on the surface, too. The generation of dust can be depressed if clinkers are fully nodulized and sintered prior to the crystallization of the interstitial liquid in the kiln. The dust component is inferior in grindability to the outer part of clinker nodules. This is due presumably to the difference in grinding hysteresis.

Microtextures Formed by the Remelting Reaction in Belite Crystals

When they are cooled below the stable temperature region of the α phase, crystals of belite (Ca2SiO4 solid solutions) inverted to the α′H phase without a change in chemical composition and gave six sets of lamellae within the crystals. After this transition remelting reaction occurred within the inverted α′H‐phase grains, the liquid phase became nucleated and grew heterogeneously on the lamellae boundaries, resulting in a decrease in impurity concentration in the host α′H phase. When the exsolved liquid wetted the lamellae, complete dissolution of the parent crystals occurred during slow cooling. A variety of microtextures resulted, depending on both quenching temperature and cooling rate.

Orientation of coherent interphase boundaries formed by the α to α' H phase transition in belite crystals

The orientation of the α' H-phase lamellae in the host α phase in belite (Ca 2SiO 4 solid solutions) crystals has been derived mathematically on the assumption that the interphase boundaries are completely coherent. The computation is based on the matrix algebra analysis of martensitic transformations. The lamellae intersect (0001) α at 26.82°, which is compatible with 27° obtained experimentally.

Remelting Reaction within Belite Crystals during Cooling

Belite crystals give six sets of twinned αH′l‐phase lamellae upon transition from α to αH′. During further cooling a remelting reaction occurs within the crystals with heterogeneous nucleation and growth of a liquid phase on the lamella boundaries. The liquid, spherical in shape (∼0.1 μm in diameter), is made up of 70 wt% Fe2O3, 20 wt% CaO, and 10 wt% SiO2 at 1100°C and vitrified on rapid cooling.

Magnesium sulfate attack on portland cement paste-I. Microstructural analysis

The mechanism of magnesium sulfate attack in portland cement and portland cement with silica fume paste was studied by investigating the microstructure and composition of specimens immersed for one year in magnesium sulfate solution. Results are presented in two parts. Part I, presents the microstructure of the pastes. Part II, presents the chemical and mineralogical variations across the specimen together with a proposed mechanism of magnesium sulfate attack. The magnesium sulfate attack resulted in the formation of the “surface double-layer” which was composed of brucite and gypsum layers. In addition, there was a sequential formation of inner gypsum layers resembling the occurence of lisegang bands found in rocks. The surface double-layer was composed of brucite, about 40 to 120 μm thick, and followed by a contiguous layer of gypsum, about 20 to 70 μm thick. Subsequently, up to four internal gypsum layers were deposited at depths of up to 1200 μm from the surface. These layers were parallel to the beam surfaces ranging in thicknesses from 15 to 60 μm. In portland cement with silica fume paste there were almost no inner gypsum layers, instead, massive deposition of dispersed gypsum crystals appeared up to at a distance of about 800 μm from the surface. The attack had apparently influenced the local degree of hydration of the cement constituents; near the surface only residual ferrite and belite crystals were observed, but in the middle, unreacted alite and belite were predominant.