Cement Portland Microscopy

PCA SP030

Table 7-1. Microscopical Interpretation of Clinkers General Features of Clinkers Observations

Interpretations

Evenly distributed phases; idiomorphic alite; rounded belite; finely differentiated matrix; scarce, small, free-lime crystals

Optimized manufacturing conditions: “correct” chemical composition of raw feed, well mixed, no particles too coarse, satisfactory maintenance of sintering and cooling temperatures (Hofmänner, 1973)

Even distribution of silicates

Ideal clinker structure, good production conditions (Fundal, 1980)

Increase in alite content and crystal size, increasing difficulty in burning, reduction in cement strength

Relative increase in lime saturation factor (Long, 1982b)

Increase in silicate abundance, decrease in liquid, higher temperature required for combination of feed ingredients, decrease in alite size

Relatively higher silica ratio (Long, 1982b)

Very heterogeneous clinkers as seen in degree of burning, size, and distribution of minerals

Wet process, introduction of precipitator dust after chain system (Hawthorne, Richey, and Demoulian, 1981)

Clinker shape: (a) single grains, (b) lumpy

(a) rotary kiln, (b) shaft kiln (Gille and others, 1965)

Edges of alite damaged, notched and pitted belite, dark intermediate material with low reflectivity

Typical clinker from large kilns (5x185m) (Kolenova, 1974)

Prismatic alite; round belite; light-colored, highly reflective intermediate material; dark prismatic aluminate

Typical clinker from small kiln (4.5x170m) (Kolenova, 1974)

Nodule size greater than 25-mm diameter

Requires longer burning time or higher burning temperature (Heilmann, 1952)

Increasing clinker size

Higher burning temperatures and larger amount of liquid phase; more time for nodulation (Eby, 1985)

Chains of silicates

Bridging reaction between constituents, and between burning zone and cooler inlet; easily eroded, producing dust (Fundal, 1980)

Large pores, wide bridges, and large solid areas

Dry process (Krämer, 1960)

Numerous small pores, narrow bridges, crescent-shaped voids

Pelletized raw mix (Krämer, 1960)

Pores with wide range of sizes, shape, and distribution; abrupt size changes in adjacent pores; small bridges

Wet process (Krämer, 1960)

Increasing roundness of pores, decrease in porosity; higher ferrite and belite content at expense of aluminate and alite; decrease in melt viscosity, increase in grinding time

Increasing MnO content (Knöfel, Strunge, and Bambauer, 1983)

Extremely dense structure, large alite

Kiln wall (Fundal, 1980)

Dense clinker structure, closed pore system

Densification at temperature below melt formation (Fundal, 1980)

High clinker porosity

Low degree of burning (Gille and others, 1965)

Highly porous clinker, open pores, 50-90 m alite

Sandy raw meal (Fundal, 1980)

Compact clinker with spherical, closed pores; 10-20 m alite

Marl-type raw mix (Fundal, 1980)

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Microscopical Examination and Interpretation of Portland Cement and Clinker

Table 7-1. Microscopical Interpretation of Clinkers (continued) General Features of Clinkers Observations

Interpretations

Large pores, nontwinned belite, large alites

High SO3 (Tsuboi and Ogawa, 1972)

Rough pore surfaces

Low temperature burning (Tsuboi and Ogawa, 1972)

Very porous clinker consisting of small bright grains with rounded edges

Underburned clinker (Törnebohm, 1897)

Porous, alite-rich, lumpy clinker, some with dense centers; dusty clinker, ring and “stalagmite” formation in kiln and cooler; large alite crystals; sparkling luster of clinker; lowered cement strength

Excessive hard burning (Long, 1982a)

Friable clinkers surrounded by a deposit of alite crystals alone

Very hard burning of high silica ratio materials (Pollitt, 1980)

Clinker nodules of varying composition

Inadequate blending of feed or segregation in kiln (Long, 1982a)

Clinker inhomogeneity

Lower reactivity of ash, too short retention time at clinkering temperature, high ash content, lump coal for precalcining carbon-rich fly ash as a raw material, waste-derived fuels containing graphite (Sprung, 1985)

Nests

Excessive particle size in raw mix (greater than 0.1 mm) (Gille and others, 1965)

Streaks and spots in clinker

Nonhomogeneity of raw mix, segregation of dust in air ducts during transfer to silo or in kiln (Gille and others, 1965)

Spots in clinker

Local increase of individual components of raw material; low burning degree (temperature too low or burning time too short); low lime content (Gille and others, 1965)

Overall crystal size

Function of temperature, time, and chemical composition of surrounding material; absorbed nests and prior nonhomogeneities (Gille and others, 1965)

Large crystals

High temperature, long burning time, surrounding material is low lime (belite) (Gille and others, 1965)

Large crystal cements

Unsatisfied charges on broken crystal surfaces resulting in agglomerations, abnormal setting characteristics, and increased grinding time (Hansen, 1977)

Large segregated silicates, belite nests, poor matrix distribution, high free lime

Long burning zone, maximum temperature below 1500o C, preheater kiln (DeHayes, Grady, and Vidergar, 1986)

Pronounced segregation of silicates and matrix phases, wide alite size range, large crystals, some > 100 11m, high porosity, relic coarse quartz grains

Short residence time, high production rates, roller-mill raw grind, coarsely ground coal precalciner kiln (DeHayes, Grady, and Vidergar, 1986)

Large crystal size and narrow crystal size range

Unfavorable grindability (Tachihata, Kotani, and Jyo, 1981)

Overall increase in crystal size; alite more than belite, which tends toward yellow color

Lowering of feed to speed ratio, thinning clinker bed depth as burning zone moves uphill (Rader, 1985)

Coarsely crystalline clinker

High early strengths, abnormal setting problems (Hansen, 1980)

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PCA SP030

Table 7-1. Microscopical Interpretation of Clinkers (continued) General Features of Clinkers Observations

Interpretations

Coarsely crystalline clinker with a high degree of hydration and per unit specific surface

High compressive strengths (Entin, Nekhoroshev, Sorochkin, 1980)

Hard, dense clinker, with large alite, plus soft, porous clinker with low alite content

Variable burning, flushing (passage of charge too rapidly through kiln), or excessive feed (Long, 1982a)

Small, poorly formed phases; very high porosity; uneven phase distribution; alite surrounded by large amounts of liquid phase; belite in clusters

Flushing or sintering temperature is too low (Hofmänner, 1973)

Increase in specific crystal surface (surface area of solid particles/volume of solid particles) and reduction in melt volume

Results in an increase in specific pore surface and reduction of power requirement for grinding (Petersen, 1980)

Three zones in clinker: (a) Dense core (b) Intermediate porosity (c) Cokelike

(a) Primary nodules (kiln ring fragments or nodules formed before burning zone) (b) Forms in burning zone (c) Forms between burning zone and cooler due to heavy dust load (Fundal, 1980)

Dense core, high-porosity shell

Agglomeration of dust (less than 1.0 mm) on nodule between burning zone and cooler inlet (Fundal, 1980)

Peripheral zones in clinkers

Differentiation or segregation of melt during liquid stage of groundmass (Gille and others, 1965)

Fine clinker

Soft burning, insufficient liquid phase; extreme hard burning and abrasion of porous shells on clinkers; Mn mineraliser (large alite crystals) (Long, 1984b)

Dusty clinker: high porosity, “breaded” nodules, agglomerated fine particles, alite-rich, large alite crystals, and relatively scarce liquid phase

(a) Decomposition of outer clinker shell and concentration of liquid phase in clinker core (Allegre and Terrier, 1960) (b) Recycling of precipitator dust, reducing zones, lack of Al2O3 in raw slurry (Hofmänner, 1973)

Dusty clinker (poor nodulization) and snowmen

(a) (b) (c) (d)

Belite nest with dense, thick layer of alite with very porous outermost zones

Typical in dust formation (Fundal, 1980)

Clinker dust with abundant belite clusters

Inferior burnability (Fundal, 1980)

Dusty clinker, coarsely crystalline silicates, low-porosity nodules, poor grindability

Slow temperature rise, higher clinkering temperature, longer time in burning zone and transition zone (Wolter, 1985)

Coal ash shells on clinkers and nests at boundary zones

Nonuniform combustion of fuel and partial reaction on clinker surface; inadequate grinding of coal and distribution (Krämer, 1960)

Clinker color: earth brown to light brown to dark brown to black with greenish brown hue

In order of increasing degree of burning (Gille and others, 1965)

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Unfavorable temperature distribution Too little melt Too much coarse quartz, lime, slag Heavy alkali circulation resulting in early crystal growth of belite and free lime and large silicate crystal size (Miller, 1980)

Microscopical Examination and Interpretation of Portland Cement and Clinker

Table 7-1. Microscopical Interpretation of Clinkers (continued) General Features of Clinkers Observations

Interpretations

(a) Reddish brown (b) Dark gray with reddish hue (c) Slate gray

(a) Abundance of Fe++ (b) Fe partly replaced by Mn (c) Normal clinker with Mg in greenish-brown ferrite (Gille and others, 1965)

Gray-black clinker color

Overall oxidizing environment (Long, 1982b)

Yellow-brown clinker colors

Rapid cooling (Long, 1982b)

Gray color of clinker

Burning under oxidizing or neutral conditions; MgO in lattice of aluminoferrite, presence of trivalent iron (Sylla, 1981)

Gray color of clinker

Burned under reducing conditions, cooled in air after removal from kiln at temperature greater than 1250o C (Sylla, 1981)

Brown clinker

Burned under reducing conditions, cooled at approximately 1250o C under reducing conditions, further cooled in air (Sylla, 1981)

Bleached region in clinker

Localized extreme reduction due to partly burned coal deposition (Long, 1982b)

Well-defined yellow band separating gray-black periphery from brownish black core

Reducing environment with reoxidation (Long, 1982b)

Black, oxidized parts of same sample show usual features but with three sets of belite lamellae strictly oriented crystallographically with host alite

Moderate reducing conditions (Woermann, 1960)

Clinker is densely burnt, light brown; ferrite has distinctly lower reflectivity than in normal clinker; calcium and iron sulfides

Reducing conditions (Woermann, 1960)

Disappearance of ferrite phase, iron transformed to metallic state, clinker color changes to white, alite decomposition structures vanish

Extreme reducing conditions (Woermann, 1960)

Brown-centered clinkers, larger alite, lower birefringence

Reducing conditions; longer, cooler flame; reductions in set control and strength (Brugan, 1979)

Spotty, banded coloration

Chemical differences or varying burning conditions (Gille and others, 1965)

Green clinker nodules, chromium-rich green belite, gehlenite matrix

Consumption of refractory brick during production at less than optimum kiln capacity or excessive flame length for rated capacity conditions (Brugan, 1979)

Greenish brown—greenish yellow

Entrapped magnesium (Gille and others, 1965)

Clinker weight (liter)

Varies as a function of total porosity (Gille and others, 1965)

High liter weight

High alite content (Brown, 1948); increased time of burning at high temperature (Hawkins, 1979)

Low liter weight

MgO slightly high, CaO distinctly high; aggregated clinkers (Brown, 1948)

Hydration shells on clinker

Air-exposed piles of clinker with “roofs” of partial hydration (Krämer, 1960).

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PCA SP030

Table 7-1. Microscopical Interpretation of Clinkers (continued) General Features of Clinkers Observations

Interpretations

Green, yellow, and brown clinkers

Addition of Cr+3, Ti+4, and Fe+3/Mn+2, respectively (Laxmi, Ahluwalia, and Chopra, 1984)

Weathered clinker and low Blaine fineness of cement

Problems in thickening time and free water in oil-well cement (Reeves, Bailey, and McNabb, 1984)

PHOTOMICROGRAPHS OF GENERAL FEATURES OF CLINKERS

Photograph 7-1 Porous outer zone of clinker, surrounding relatively dense clinker core. Evenly distributed, round, clear belite crystals and angular alite. Pores filled with epoxy. Moderately high maximum temperature, long burning time, slow heating rate, quickly cooled. Dry-process kiln with flash calciner, 5000 tons/day. (S#A6621) Thin section Transmitted, plane-polarized light Field dimensions = 0.53x0.53 mm

Photograph 7-2 Polished section of cement in epoxy, illustrating large multiphase (composite) particles and small, single phase particles, a typical relationship for most portland cements. (S#A6622) Nital etch Field dimensions = 0.38x0.38 mm

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