Optimization of Cement Manufacturing Process by Means of Clinker Micro Structure
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Transcript of Optimization of Cement Manufacturing Process by Means of Clinker Micro Structure
OPTIMIZATION OF CEMENT MANUFACTURING PROCESS BY
MEANS OF CLINKER MICROSTRUCTURE
Arnaldo Forti Battagin
Vagner Maringolo
Associação Brasileira de Cimento Portland Av. Torres de Oliveira, 76 - Jaguaré
CEP 05347-902 SÃO PAULO / BRASIL e-mail: [email protected]
ABSTRACT
This paper shows that important technological benefits such as monitoring of cement
manufacturing and prediction of the finished product behavior can be assessed by analysis of
portland cement clinker microstructure. In fact, raw meal travelling through the rotary kiln
gradually transforms into portland clinker whose structural features keep most of the past-
history of the whole production process.
Seven clinker samples from different manufacturing processes were chosen for analyses by
reflected light microscopy in order to provide different microstructures. Data on crystal size,
shape and distribution provided information on raw meal fineness and homogeneity, burning
conditions, cooling rate as well as prediction of the rheological behavior and mechanical
performance of cement during and after its hydration.
KEY WORDS: microstructure ,portland cement, clinker, reflected light microscopy INTRODUCTION
Throughout its history the cement industry has accumulated a broad experimental knowledge
that nowadays represents the basis for manufacturing quality control. By controlling
composition of raw materials, grain size distribution and homogeneity as well as operational
conditions of rotary kilns, the manufacturer is able to make costs and cement quality meet.
Today rotary kilns are mainly controlled by on-line electronic panels and expert systems. As
data however are sometimes insufficient to assess what is really going on inside the kilns,
some authors1 have established an analogy between the cement clinker to the airplane black
box. Along its way through the rotary kiln, the progressively sintered mass record all steps of
production in its microstructure. This way the clinkerization process differs substantially from
the smelting of pig iron in blast furnace or melting up to the liquid state of glass in glass pot,
which erase virtually all and any characteristics that may trace back to the original shape,
physical nature or mineralogical constitution of the raw materials.
The clinker features give clues to raw meal fineness and homogeneity after grinding, burning
conditions in the kiln, influence of waste fuels and cooling rate Also the rheological behavior
and mechanical performance of cement during and after its hydration can be predicted.
EXPERIMENTAL
Sample preparation
The study of clinker microstructure was developed by means of polished sections. The
method of preparation was based on the classic metallographic and petrographic techniques,
involving embedding of some fractions of grains in a mold containing plastic resins. After
hardening, test specimen were demolded and polished. CAMPBELL2 describes a detailed
procedure of sample preparation and emphasizes the importance of getting a well-prepared
surface in order to avoid incorrect interpretations.
Because clinker compounds present approximately the same reflectance under reflected light
microscope, polished sections had to be etched with some chemical reagents. Aqueous
solutions of NHCL (0,1%) as well as alcoholic solution of HNO3 (0,1%) were used to
distinguish the different clinker phases.
Microscopic Examination
Seven different clinker samples, representative of cement production in Brazil were selected.
Samples were chosen in order to cover different qualifications of manufacturing processes
through various microstructures. Magnifications used varied from 100x to 400x, sometimes
600x, in order to get a general view of textures and to observe detailed features of a
determined crystal.
The main clinker compounds are calcium silicates (alite and belite), calcium aluminate (C3A)
and ferrite (C4AF). Secondary compounds are free lime and periclase. Sulfate phases are
rarely identified by reflected light microscopy.
For each sample the following features related to both raw meal characteristics and different
steps of cement manufacturing were detailed under microscope:
� Size and morphology, presence of exsolution, decomposition on borders of alite crystals,
� Morphology (round or ragged), distribution in nests or as dispersed grains and size of
belite nests,
� Well-differentiated or vitreous matrix, prismatic C3A,
� Free lime distribution,
� Presence and distribution of periclase crystals and
� Presence of metallic phases.
RESULTS AND DISCUSSIONS
Raw meal grinding
Raw meal grain size distribution plays an important role during clinkerization as the higher
the material fineness, the higher is its surface area, yielding burnability. However, coarse
grains in the meal are not completely assimilated and typical relict microstructures are
generated that can be reliable guides for grinding problems.
Most samples presented well-defined, isolated, 300-to-400µm wide belite nests .According to
Fundal 3 this is a microstructural feature from remnant coarse quartz grains (>44µm) in the
meal. Because silicium ions have a high ionic diffusion, there is a radial migration from the
quartz grain thus originating a central pore, surrounded by poligonals belite crystals, with no
matrix in-between (Figure 1). One sample exhibited nests of C2S crystals within an abundant
matrix (alkali aluminate), and this feature is due probably to original coarse feldspar grains in
the meal (Figure2).
On the other hand, the observed 100-400µm-wide free lime clusters, bearing limestone crystal
shape, indicate the presence of coarse calcic carbonatic grains in the meal. These free lime
clusters, in association with periclase crystals indicate the origin from dolomitic or magnesian
limestone. That was a quite common feature in all samples studied. According to Fundal3
during clinkerization, the core of limestone grains >125µm do not react due to little ionic Ca++
mobility that remains in the form of free lime (Figure 3).
Raw meal homogeneity
Efficient homogeneity is essential to guarantee an adequate mix of siliceous and lime
components of raw meal, in such a way as to produce a clinker with well-distributed C3S and
C2S. An inefficient homogeneity, resulting in silica-enriched and lime-depleted regions, and
vice-versa, corresponds to clinkers with local dosage problems and determine the appearance
of C2S nests and C3S and free lime clusters. Thus, wide and interconnected C2S zones,
generally >800µm, in contrast to generally irregular portions of C3S and free lime, correspond
to a deficient degree of meal homogeneity. On the other hand, a random dispersion of C3S and
C2S characterize normal homogeneity conditions. Figures 4 and 5 show these microstructural
features, the former rarely observed in samples selected for the study.
With the advent of coal as a substitute for fuel oil, a new microscopic feature was suddenly
perceived in clinkers. Elongated C2S zones resulted from incomplete assimilation of siliceous-
aluminous coal ashes. Although it does not relate to any homogenization problems, the
incorporation of these ashes making up 35% of coal, yields problems in meal dosage,
generating locally silica and alumina-rich regions. When completely assimilated thanks to
adequacies in dosage ashes are not perceptible in clinker structure.
Burning conditions
Clinker burning conditions are related to factors such as residence time ,maximum
temperature and burning rate. According to the variation in these parameters, there will be
variations concerning formation and growth of clinker silicates, mainly alite crystals
According to Centurione4 C3S crystals >60µm characterize a long-term clinkerization, in
other words, over burnt clinkers. Figure 6 presents this feature exhibited by one of the
samples On the other hand, 15 to 20µm alite crystals result from a poor burning and generally
are associated to free lime-rich clinker (Figure 7). Normal burnt clinkers present 30 to 40 µm
alite crystals(Figure8) which was the feature shown by most of the samples.
Finally, free lime distribution and porosity constitute complementary parameters to evaluate
burning conditions, besides others such as metallic phases and C3S exsolution, the latter
specifically indicating reducing burning conditions(Figure 9).
Cooling rate
It is well established that cooling at the end of kiln (1st cooling) after clinker reach maximum
temperature in clinkering zone and immediately before the kiln outlet is responsible for the
stability of silicates (C3S and C2S).
C3S crystals are formed from reaction of C2S with CaO and makes up a stable high-
temperature phase. However, under adequate cooling conditions, these crystals can be
preserved in room temperature thus making them reactive. Under slow cooling conditions,
there is the reversible reaction C3S → C2S + CaO, determining decomposition of C3S along
the borders and forming free lime and secondary C2S. Normal 1st cooling conditions
characterize idiomorphic and pristine C3S crystals and rounded C2S (Figure 10). On the
contrary , slow 1st cooling will determine decomposed and xenomorphic C3S crystals, with
ragged borders, and formation of secondary C2S, followed by digit C2S crystals (Figure 11).
Industrial cooling conditions (2nd cooling) respond for the crystallization degree of matrix
and, partly, also of periclase. When 2nd cooling is slow, there is perfect crystallization of
matrix allowing perfect distinction of C3A and C4AF, followed by crystallization of periclase
(Figure 12). On the other hand, under fast cooling, vitrification of the liquid phase occurs and
distinction between C3A and C4AF is not easily seen (Figure 13). Periclase crystals under
these conditions are xenomorphic and ill-crystallized. Between both ends there is the so-called
semicrystalline matrix and subidiomorphic periclase crystals, which are typical features of
normal 2nd cooling conditions (Figure 14).
Raw mix evaluation
Another possibility offered by microscopy refers to the evaluation of dosage of raw materials
that will provide clinker formation. The presence of random free lime crystals together with
C3S crystals, allied to low frequency of C2S crystals, are indicative of possible utilization of
high Lime Saturation Factor. Similarly, high frequency of silicates (C3S and C2S) and low
frequency of matrix occur when Silica Modulus is high. A great predominance of C3A over
C4AF, easily obtained by visual assessment, occur when Alumina Modulus is high.
Other complementary information on raw meal composition are, for example, the presence of
periclase and of alkali aluminate that indicate use of magnesian limestone and incorporation
of alkali elements in raw materials, respectively (Figure 15).
Cement performance prediction
Some clinker microstructure features can be of great utility in prediction of respective cement
performance.
A few features and their relation to the cement properties follow:
(a) Free lime or periclase-rich clinkers may present problems of volume stability;
(b) C3S-rich clinkers show high initial strength and
(c) Similarly, C3S-rich cements have high hydration heat;
(d) Keeping the other properties constant, a C3A-richer clinker show more rapid setting.
Vitreous matrices are however less reactive than crystalline ones and characterize
cements with longer setting times (SYLLA5);
(e) Clinker showing developed C3S crystals (>60µm) are less reactive than those with
crystals between 25 and 30µm and will probably make up cements with lower
mechanical strengths (BUTT & TIMASHEV6) ;
(f) C3A-poorer clinkers produce cements highly resistant to sulphate attack and
(g) Alkali-rich clinkers showing frequent elongated alkali C3A make up more vulnerable
cements to alkali-aggregate reactions, if reactive aggregates are used in concrete.
CONCLUSIONS
In general it is possible to conclude that besides traditional chemical methods and physical-
mechanical tests applied to quality control of cement, the use of microscopy can constitute a
valuable tool for prevention and elucidation of problems. Effectively, the microscopy
technique by reflected light can provide information concerning conditions of preparation and
dosage of raw materials, burning and cooling conditions, besides providing predictions related
to cement performance after hydration, based solely on microstructural clinker features.
Evolution on technological knowledge of cement manufacturing through the use of
supplementary techniques allows fuel savings, improvement of quality and minimization of
industrial costs, factors that will be beneficial to all producers and consumers.
PHOTOMICROGRAPHS
Picture 1 - Belite clusters (B) originated from coarse quartz grains in the raw meal.
Intersticial phase is almost absent in the clusters.P(pores) and C(free lime)
Picture 2 - Belite cluster (B) probably originated from feldspar grain in the raw meal.
Intersticial phase is almost totally formed by prismatic alkali- C3A (D)
Picture 3 -Free lime nest (C) originated from coarse calcareous grain
Picture 4 - Area of uniform crystal distribution indicating efficient homogeneity of meal.
Picture 5 - Heterogeneous crystal distribution resulting from a defficient degree of meal
homogeneity. P(pores),B(belite),A(alite).
Picture 6 - Large alite crystals (A) which indicate over burnt clinker.F(matrix).
Picture 7 - 15 to 20µm alite crystals (A) originated from a poorly burnt clinker. P(pores)
Picture 8 - Normally burnt clinker exhibiting 30-40 µm alite crystals (A). F(matrix)
Picture 9 – Alite(A) with belite striations indicating reducing burning conditions. B(belite),
F(matrix).
Picture 10 - Rounded belite crystals (B) formed under normal 1st cooling. F(matrix).
Picture 11 - Slowly cooled clinker determining decomposition of C3S xenomorphic crystals
(A), with ragged borders, and formation of secondary C2S (B).
Picture 12 - Coarsely crystalline aluminate (D) and dull-reflecting ferrite (E) in slowly cooled
clinker (2nd cooling).
Picture 13 - Not well-differentiated matrix (F) indicating fast cooling rate (2nd cooling).
Picture 14 - Normally cooled clinker presenting semi-crystalline matrix
Picture 15 - Prismatic alkali-rich aluminate crystal (D) indicating insufficient sulfate to
balance alkalis/sulphur.E(ferrite),C(free lime)
REFERENCES
1.Ono, Y., O controle de qualidade do cimento por microscopia de clínquer (Método Ono), In:
REUNIÃO DE TÉCNICOS DA INDÚSTRIA DO CIMENTO, 31, São Paulo 17 e 18 julho de
1980. Anais... São Paulo : ABCP, 1980. v. 1. (Anexo n.7).
2.Campbell, D.H., Microscopical Examination and Interpretation of Portland Cement and
Clinker, Portland Cement Association, second edition, 1999.
3.Fundal, E., The burnability of cement raw mixes, World Cement Technology, 10 (6), 195,
Jul./Aug., 1979.
4.Centurione,S.L., The infuence of burning conditions on alite crystal characteristics,
Proceedings of Seventeenth International Conference on Cement Microscopy, Albeeta,
Canada, 1995.
5.Sylla, H.M., Influência do resfriamento do clínquer na pega e na resistência do cimento,
Serrana S.A. de Mineração, São Paulo, s.d. (Inf. Técnica 120)
6.Butt, Y.M. and Timashev, V.V., Effect of phase composition of portland cement clinkers on
the binding properties of cement, apud Chaterjee, A K., Phase composition, microstructure,
quality and burning of portland cement clinkers: a review of phenomenological interrelations.
World Cement Techology, 10(4), 124, May, 1979.