Portland Cement
Introduction
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Portland cement was invented in 1824 by Joseph Aspdin, an English mason
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Joseph Aspdin named his patented product as “Portland cement” because it
produced a concrete that resembled the color of the natural limestone quarried at Portland in
England
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Portland cements are hydraulic cements primarily composed of hydraulic calcium
silicates
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Hydraulic cements set and harden by reacting chemically with water
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Most of the hydration and strength development take place within the first month
of mixing, but they continue, though more slowly, for a long time
Oxide Composition of Portland Cement
Manufacturing of Cement:
Cement is manufactured by burning two raw materials; namely calcareous and
argillaceous in nature.
Calcium carbonate aluminum silicate Such as limestone
or chalk such as clays or shales
The calcareous material constitutes about 65 to 75% to provide lime, while the latter
forms 35 to 25% to produce silica; alumina and ferric oxides.
The lime and silica (CAO + SiO2) system provides the cementing properties to the
cement and called the primary oxides. While the alumina and ferric (Al2O3 + Fe2O3) system
does not provide the cementitious properties and provide only the flux (the medium) to reduce
the temperature of burning of CaO + SiO2 and hence called the secondary oxides.
Abbreviations:
When dealing with the chemistry of cements, some abbreviations are commonly used
and almost standardized:
CaO ------ C SiO2 -------S
Al2O3
A
Fe2O3 --------F
_
H2O -------H
SO3 -------- S
MgO --------Mg
_
CO2 -------- C
Table 1: Approximate Oxide Composition Limits of Portland Cements So, there are four major
oxides which constitute 90% of the cement weight:
Oxide
CaO
SiO2
Al2O3
Fe2O3
MgO
Na2O + K2O
TiO2
P2O5
SO3
C
90%
Composition
(% by weight of cement)
60-67
17-25
3-8
0.5-6.0
0.1-5.5
0.5-1.3
0.1-0.4
0.1-0.2
1-3
Primary
oxides
Major oxides
S
A
F
of these major oxides, only CaO is basic (pH > 7) while the other three are acidic The minor
oxides constitute 10% and form many oxides, some of them exist in marginal
quantities.
These are:
MgO
Na2O
10% K2O
TiO2
P2O5
minor oxides in addition to gypsum
Mineralogical Composition of Cement
During the manufacturing of cement clinker, and at the high temperature of the kiln,
calcium oxide combines with the other acidic components of the raw mix to form four
principal compounds (called phases) making 90% of the clinker weight:
C3S (Tri-calcium Silicate) = 3CaO.SiO2 C2S
(Di-calcium Silicate = 2CaO.SiO2 C3A
(Tri-calcium Aluminate) = 3 CaO. Al2O3 C4AF (Tetra-calcium Alumino-ferrite) =
4CaO. Al2O3.Fe2O3 The behavior of cement during all times (i.e., in the fresh and
hardened states)
depends very largely on these phases, rather than on the oxides.
Only in a few cases (i.e., such as unsoundness of cement and ASR), the oxide
composition plays an important role.
Characteristics of Major Constituents of Cements
.(i) C3S Phase
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Its content varies from 35 to 55%, with an average of 45%. It is not at all
uncommon to find C3S with 70%
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It is responsible for the early strength contribution of cements
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It is second phase to react with H2O.
(the fastest phase to react is C3A)
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It is the second highest heat-generating substance in cement (120 cal/g).
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It contains some impurities like MgO, Al2O3, Na2O, Fe2O3 , K2O.
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It generates on hydration twice as much Ca(OH) 2 and 75% of C-S-H as those
generated by C2S.
.(ii) C2S Phase
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Its quantity varies from 15 to 35% with an average of 25%.
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Its contribution to early strength is very little but its contribution to ultimate
strength is very high and probably more than that contributed by C3S.
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It is the slowest reacting substance in cement.
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It is the lowest heat-generating substance (60 cal/gm).
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It contains impurities (almost similar to C3S).
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It generates half of the Ca(OH) 2 and about 133% C-S-H as those generated by
C3S.
(iii)
C3A Phase
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Its quantity varies from 2 to 14% depending on the type of cement.
It does not contribute to strength (some controversy exists).
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It accelerates the hydration of C3S.
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It is the first reacting substance in cement. Its reaction with water is
instantaneous (flash set). Gypsum is added to regulate the time of setting.
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It is the highest heat-generating substance in cement.
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It has a significant impact on the durability of reinforced concrete
structures from both the corrosion of reinforcing steel and sulfate attack of concrete.
(iv)
C4AF Phase
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Its quantity varies from 7 to 10% with 8% as average.
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Its heat of heat is 100 cal/gm.
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Its characteristics do not differ structurally very much from C3A. However, it is
generally less reactive than C A and does not affect significantly the durability of concrete,
hence; it does not control the behavior of cement.
Characteristics of Minor Constituents
The term “minor” does not mean less important but less in quantity or proportion.
Gypsum (CaSO4.2H2O):
Due to the affinity of C A to sulfate gypsum is added to the clinker to regulate the time of
setting of cement by forming layers of ettringite on C3A grains.
The addition of gypsum is intended to prohibit the reaction of C3A with water (to
prevent flash or quick setting).
Gypsum has a favorable effect on strength, as it accelerates the hydration of C3S.
Its content is affected by:
i. maximum strength;
ii. minimum shrinkage; and
iii. C3A content
Specifications (both BS and ASTM) indicate that the content of gypsum = 2.5 to 3.0% by
cement weight as SO3. Free (Uncombined) Lime (CaO):
All lime should be combined with the acidic oxides. If there is extra lime, it may react
(after the hardening of cement) with water to form Ca(OH)2.
Ca(OH) 2 has more volume than CaO, therefore, expansion occurs thereby resulting
in damage to concrete. As such, the content of free lime (uncombined with acidic oxides)
should be limited.
Magnesium (MgO): Unlike lime, magnesia does not combine
with acidic oxides at all. Some can be taken by the clinker
minerals as impurities, the remaining crystallizes as
periclase (MgO). Its reaction with water is delayed and
much slower than that for lime. Hence, the associated
expansion due to periclase is more dangerous. Most
standards limit the magnesia content to 4 to 5% by cement
weight. Alkalis (Na2O + K2O):
They exist in the clinker as impurities (from the clay). Their content varies from
0.4 to 1.3% by weight of cement. Its content is usually less than that in the raw materials
because:
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• They are volatile and removed by ventilation.
• They tend to be concentrated in the kiln dust which is also wasted.
If the aggregates contain reactive silica, then the concrete will be susceptible to
alkali-silica reaction (ASR)
The content of alkalis is calculated as “Na2O equivalent” and the total alkali
content should be less than 0.60% to prevent ASR.
Potential Compound Composition of Cements
As mentioned previously, chemical analysis is used to determine the content of major
and minor oxides.
From the oxide composition, the determination of the content of the four phases has to
be established to predict the behavior of cement.
This is based on the work of R.H. Bogue
Case A: A/F ≥ 0.64
_ C3S = 4.071C - 7.600S - 6.718A
- 1.430F - 2.852S C2S = 2.867S - 0.7544C3S C3A = 2.650A -1.692F C4AF = 3.043F
Case B:* A/F < 0.64
_ C3S = 4.071C - 7.600S –
4.479A – 2.859F – 2.852S C2S = 2.867S - 0.7544C3S C3A = 0 C4AF = 2.100A
+ 1.702F
*Note: In case B, C4AF has a different composition from case A, C2F may also be present
and is included in the quantity calculated.
Bogue’s equations involve certain assumptions that are not actually satisfied, however,
they are the most widely used because of their simplicity (there are more accurate techniques
i.e., XRD).
Caution:
As shown in the Table, small changes in the oxide compositions will lead to significant
changes in the phase composition of cement. Therefore, extreme care has to be practiced when
determining the phases of cement.
Table 1.4 Influence of Change in Oxide Composition on the Compound Composition
(A M Neville Text)
Percentage in Cement
1
2
3
Oxide
CaO
SiO2
Al2O3
Fe2O3
Others
Compound
C3 S
C2 S
C3 A
C4AF
66.0
20.0
7.0
3.0
4.0
63.0
22.0
7.7
3.3
4.0
66.0
20.0
5.5
4.5
4.0
65
8
14
9
33
38
15
10
73
2
7
14
For the Manufacturing of Cement See Figure 1.1 (AM Neville p.5)
Properties of Portland Cement
The quality of a Portland cement is assessed in terms of the following physical properties
determined through the lab. Tests on the cement samples collected in accordance with ASTM C
183:
1. Fineness
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seven days.
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Fineness of cement affects heat released and the rate of hydration
More is the fineness of cement more will be the rate of hydration
Thus the fineness accelerates strength development principally during the first
Fineness is measured by the following tests:
-Wagner turbidimeter test (ASTM C 115)
-Blaine air-permeability test (ASTM C 204)
-Sieving using No.325 (45 µm) sieve (ASTM C 430)
2. Soundness
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Soundness is the ability of a hardened paste to retain its volume after setting
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A cement is said to be unsound (i.e. having lack of soundness) if it is subjected to
delayed destructive expansion
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Unsoundness of cement is due to presence of excessive amount of hard-burned
free lime or magnesia
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Unsoundness of a cement is determined by the following tests:
-Le-Chatelier Accelerated test (BS 4550: Part 3)
-Autoclave-expansion test (ASTM C 151)
.3. Consistency
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Consistency refers to the relative mobility of a freshly mixed cement paste or
mortar or its ability to flow
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“Normal or Standard Consistency” value of a cement sample is used for preparing
the pastes for the determination of the setting time, unsoundness, compressive and tensile
strength of the cement
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Normal or Standard consistency of cement is determined using the Vicat’s
Apparatus. It is defined as that percentage of water added to form the past which allows a
penetration of 10 ± 1mm of the Vicat plunger
.4. Setting Time
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This is the term used to describe the stiffening of the cement paste
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Setting time is to determine if a cement sets according to the time limits specified
in ASTM C 150
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Setting time is determined using either the Vicat apparatus (ASTM C 191) or a
Gillmore needle (ASTM C 266)
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“Initial setting time” is the time from the instant at which water is added to the
cement to the instant at which the Vicat’s initial set needle penetrate to a point 5 mm from the
bottom of a special mould. ASTM C 150 prescribes a minimum initial setting time of 60 minutes
for Portland cements
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“Final setting time” corresponds to the time at which the Viact’s final set needle
makes an impression on the paste surface but the cutting edge fails to do so ASTM C 150
prescribes a maximum final setting time of 10 hours for Portland cements
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Gypsum in the cement regulates setting time. Setting time is also affected by:
cement fineness, w/c ratio, and admixtures
.5. False Set
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Initial and final sets should be distinguished from “false set”
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False set is evidenced by a significant loss of plasticity, i.e. stiffening, without the
evolution of much heat shortly after mixing
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False set cause no difficulty in placing and handling of concrete if the concrete is
mixed for a longer time than usual or if it is remixed without additional water before it is
transported or placed
.6. Compressive Strength
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Compressive strength of cement is the most important property
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It is determined conducting compression tests on standard 2-inch mortar cubes in
accordance with ASTM C 109
.7. Heat of Hydration
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It is the quantity of heat (in joules) per gram of un-hydrated cement evolved upon
complete hydration at a given temperature
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The temperature at which hydration occurs greatly affects the rate of heat
development
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Fineness of cement also affects the rate of heat development but not the total
amount of heat librated
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The amount of heat generated depends upon the chemical composition of cement.
Following are the heat of hydration generated on hydration of the four compounds of cement
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• The approximate amount of heat generated, during the first 7 days are as follows:
.8. Loss on Ignition
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• The test for loss on ignition is performed in accordance with ASTM C 114
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A high weight loss on ignition of a cement sample is an indication of prehydration and carbonation, which may be cause by:
Compound
Heat of Hydration
Remarks
C3 S
502 j/g
-
C2 S
260 j/g
Minimum
C3 A
867 j/g
Maximum
C4AF
419 j/g
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Type
Name
H.O.H (% of H.O.H. of Type I
cement
II
Moderate
80 to 85
III
High early strength
IV
Low heat of hydration
40 to 60
V
Sulfate resistant
60 to 75
Up to 150
-Improper and prolonged storage -Adulteration
during transport and transfer
9. Specific Gravity
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The specific gravity of a cement is determined by ASTM C 188
It is used in concrete mixture proportioning calculations
On an average the sp. gravity of cement is 3
Types of Portland Cements
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Different types of Portland cement are manufactured to meet the requirements for
specific purposes
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The American Society for Testing and Materials (ASTM) Designation C 150
specifies the following eight types of Portland cement
Type
Name
Type I
Normal
Type IA
Normal, air entraining
Type II
Moderate sulfate resistance
Type IIA
Moderate sulfate resistance, air entraining
Type III
High early strength
Type IIIA
High early strength, air entraining
Type IV
Low heat of hydration
Type V
High sulfate resistance
Type 1 cement
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• (C3S + C2S) ≥ 67% (by mass)
• The ratio (CaO/SiO2) ≥ 2 (by mass)
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MgO ≤ 5% (by mass)
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Lime saturation factor:
1.0(Ca) – 0.7(SO3)
--------------------------------------------- = 0.66 to 1.02
2.8(SiO2) + 1.2(Al2O3) + 0.65(Fe2O3)
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SO3 ≤ 3.5% (by mass)
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CI ≤ 0.1% (by mass)
2
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Fineness = 300 to 400 m /kg
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Expansion (i.e. unsoundness) measured by Le Chatelier test ≤ 10 mm
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Setting time and minimum strength have been already mentioned earlier
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It is a general-purpose cement used in concrete for making pavements, floors,
reinforced concrete buildings, bridges, tanks, pipes, etc.
Type II cement
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It is used where precaution against moderate sulfate attach is important, as in
drainage structures, which may be subjected to a moderate sulfate concentration from ground
waters
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It usually generates less heat of hydration at a slower rate than Type I cement and
therefore can be used in mass structures such as large piers, heavy abutments, and retaining walls
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Due to less heat generation it can be preferred in hot weather
Type III cement
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It is chemically and physically similar to Type I cement, except that its particles
2
2
have been ground finer (higher fineness 450 to 600 m /kg, compared with 300 to 400 m /kg for
Type I cement)
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It provides high early strengths at an early period, usually a week or less
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It is used when forms need to be removed as soon as possible or when the
structure must be put into service quickly
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It is preferred in cold weather for reduction in the curing period
Type IV cement
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It is used where the rate and amount of heat generated from hydration must be
minimized
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It develops strength at a slower rate than other cement types
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It is most suitably used in massive concrete structures, such as large gravity dams,
where the temperature rise resulting from heat generated during hardening must be minimized to
control the concrete cracking
Type V cement
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It is used only in concrete exposed to severe sulfate action – principally where
soils or ground waters have a high sulfate content
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It high sulfate resistance is due to its low C3A content of about 4%
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It is not resistant to acids and other highly corrosive substances
Air-Entraining Portland Cements
(Types 1A, IIA, and IIIA)
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These cements have same composition as types 1, II, and III, respectively, except
that small quantities of air-entraining material are inter-ground with the clinker during
manufacture
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These cements produce concrete with improved resistance to freeze-thaw action
and to scaling caused by chemicals applied for snow and ice removal
White Portland Cement
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It has composition same as Type I or Type III cement, except that it has a white
color instead of gray color
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It is made of selected raw materials containing negligible amounts of iron and
magnesium oxides-the substances that give cement its gray colors
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It is used primarily for architectural purposes
Blended Hydraulic Cements
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These cements are produced by intimately and uniformly blending the
Portland cement and the by-product materials, such as blast-furnace slag,
fly ash, silica
fume and other pozzolans
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ASTM C 596 recognizes five classes of blended cements:
-Portland blast-furnace slag cement-Type IS
-Portland pozzolan cement-Type IP and Type P
-Pozzolan-modified Portland cement-Type I(PM)
-Slag cement-Type S
-Slag-modified Portland cement-Type I(SM)
Masonry Cements
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These cements are used in mortar for masonry construction
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ASTM C 91 classifies masonry cements as:
Type N, Type S, and Type M
Expansive Cements
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These cements are primarily used in concrete for shrinkage control
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ASTM C 845 classifies expansive cements as:
Type E-1(K), Type E-1(M), Type E-1(S)
Special Cements (Not covered by ASTM)
Type
Uses
1. Oil-well cements
For sealing oil wells
2. Waterproof Portland cements
For reducing capillary water transmission
3. Plastic cements
For making plaster and stucco