MATERIAIS CIMENTÍCIOS (MCM)
PORTLAND CEMENT: COMPOSITION AND
HYDRATION REACTIONS
Dr. Sivaldo Leite Correia
PORTLAND CEMENT COMPOSITION
 Portland cement (often referred to as OPC, from Ordinary
Portland Cement) is the most common type of cement in
general use around the world because it is a basic ingredient of
concrete and mortar
 It is a fine powder produced by grinding Portland cement
clinker (more than 90 %), a limited amount of calcium sulphate
(which controls the set time) and up to 5 % minor constituents
as allowed by various standards
 Table 1 presents typical composition of Portland cement
PORTLAND CEMENT COMPOSITION
 Table 1. Typical constituents of Portland cement
Constituent
Chemical
structure
Notation
Composition
(wt. %)
Tricalcium silicate
(CaO)3.SiO2
C3S
45-75
Dicalcium silicate
(CaO)2.SiO2
C2S
7-32
Tricalcium aluminate
(CaO)3.Al2O3
C3A
0-13
(CaO)4.Al2O3.Fe2O3
C4AF
0-18
Gypsum
CaO.SO3.(H2O)2
CSH2
2-10
Potassium, sodium, magnesium
K2O, Na2O, MgO
Tetracalcium aluminoferrite
traces
INTRODUCTION TO HYDRATION REACTIONS
 The understanding of the phenomena between the phase
development and microstructures of cement-based materials,
as well as their relationship with the physic-chemical and
mechanical properties is an important step to allow the design
and fabrication of concrete and mortar products with improved
performance
 The characteristics of the cement hydration have attracted
both academic and practical interests
 From an academic point of view, the chemical and
microstructural phenomena that characterize cement hydration
are quite complex and interdependent, making it difficult to
resolve the individual mechanisms or the parameters that
determine their rates
INTRODUCTION TO HYDRATION REACTIONS
 Fundamental study of hydration therefore offers significant
scientific challenges in experimental techniques and multiscale theoretical modelling methods
 From a more practical standpoint, the path to produce more
sustainable concrete materials is leading to more complex mix
designs that include increased amounts of secondary mineral
additions, often originating as by-products of other industrial
processes, and a wide variety of chemical admixtures that can
enhance concrete performance
 More complete knowledge of basic concepts of hydration is
needed to provide a rational basis for mixture proportioning as
well as the design and selection of chemical admixtures for
producing optimised cement-based materials
HYDRATION REACTIONS USING PORTLAND CEMENT
 The formation of cementitious material by the reaction of free
lime (CaO) with pozzolan admixture in the presence of water is
known as hydration
 The hydration reaction forms hydrated calcium silicate gel
(CSH) or calcium aluminate gel together with crystalline
products, which include calcium aluminate hydrates and
alumina-silicate hydrates
 This involves many different reactions, often occurring at the
same time
 As the reactions proceed, the products of the hydration
process gradually bond together the individual sand and gravel
particles, and other components of the concrete or mortar, to
form a solid mass
HYDRATION REACTIONS USING PORTLAND CEMENT
 The reactions which occur are mostly exothermic
 The products of the reaction between cement mixture and
water are termed hydration products
HYDRATION REACTIONS USING PORTLAND CEMENT
 In concrete (or mortar or other cementitious materials) there
are mainly the following reactions
 (1) Silicate reactions
 Tricalcium and dicalcium silicate reactions
(CaO)3.SiO2
+
C3S
5.3 H2O
5.3 H
(CaO)1.7.SiO2. (H2O)4
+ 1.3 CaO.H2O
(C1.7SH4 gel)
1.3 CH
(calcium silicate hydrate)
(CaO)2.SiO2 +
C2S
4.3 H2O
4.3 H
(CaO)1.7.SiO2. (H2O)4
(C1.7SH4 gel)
(calcium silicate hydrate)
(1)
+
(calcium hydroxide)
0.3 CaO.H2O
0.3 CH
(calcium hydroxide)
(2)
HYDRATION REACTIONS USING PORTLAND CEMENT
 The hydration reaction of calcium silicates provide the main
reaction products as well as become the main source of
Portland cement concrete strength
 The calcium silicate hydrate phase presents the following
characteristics:
 (a) The structure ranges from poorly crystalline to amorphous
 (b) The C-S-H phase occupies 50-60 % of the solid volume of
the hydrated cement paste
 (c) Ratio C/S varies between 1.1 to 1.9 and the ratio ~1.5 is
typical
 (d) The amount of water H is even more variable
 (e) The C-S-H phase presents huge surface area (100-700 m2/g)
HYDRATION REACTIONS USING PORTLAND CEMENT
 The calcium hydroxide
characteristics
phase
presents
the
following
 The structure presents morphology from large, hexagonal
prisms to thin, elongated crystals
 Size of the crystals depends on the amount of space available
 Much lower surface area than C-S-H
 The CH phase occupies 20-25 % of the solid volume of the
hydrated cement paste and does not contribute much with the
strength
 The CH phase keeps the pore solution alkaline (pH 12.4-13.5)
HYDRATION REACTIONS USING PORTLAND CEMENT
 (2) Tricalcium aluminate reactions
 (a) Hydrogarnet phase
(CaO)3.Al2O3
C3A
+
6 H2 O
(CaO)3.Al2O3.(H2O)6
6H
C3AH6
(hydrogarnet)
(3)
 Hydrogarnet is found in small amounts and can be seen as
“near perfect” octahedral morphology
 (b) Ettringite phase
(CaO)3.Al2O3
C3A
+ 3 CaO.SO3.(H2O)2 + 26 H2O
3 CSH2
(gypsum)
26 H
(CaO)6.Al2O3.(SO3)3.(H2O)32
(4)
C6AS3H32
(ettringite)
 Ettringite is present as needle-like crystal morphology,
contributes to stiffening of mixture and provides some early
strength
HYDRATION REACTIONS USING PORTLAND CEMENT
 (2) Tricalcium aluminate reactions
 (c) Monosulfate phase
(CaO)6.Al2O3.(SO3)3.(H2O)32 + 2 (CaO)3.Al2O3 + 4 H2O
2 (CaO)4.Al2O3.SO3.(H2O)12
C6AS3H32
2 C3A
3 C4ASH12
(ettringite)
(tricalcium aluminate)
(monosulfate)
(5)
 Monosulfate presents a plate morphology which can contain
impurities and tends to occur in the later stages of hydration, a
day or two after mixing
HYDRATION REACTIONS USING PORTLAND CEMENT

(d) Additional information about ettringite and monosulfate
phases
 In a concrete or mortar made from cement containing just
clinker and gypsum, ettringite forms early on in the hydration
process, but gradually replaced by monosulfate
 This is because the ratio of available alumina to sulfate
increases with continued cement hydration
 On first contact with water, most of the sulfate is readily
available to dissolve, but much of the C3A is contained inside
cement grains with no initial access to water
 Continued hydration gradually releases alumina and the
proportion of ettringite decreases as that of monosulfate
increases
HYDRATION REACTIONS USING PORTLAND CEMENT

(d) Additional information about ettringite and monosulfate
phases
 The aluminium can be partly-replaced by iron in both
ettringite and monosulfate phases
 The sulfate ion in monosulfate phase can be replaced by
other anions; a one-for-one substitution if the anion is doublycharged (eg: carbonate, CO22-) or one-for-two if the
substituent anion is singly-charged (eg: hydroxyl, OH- or
chloride, Cl-)
HYDRATION REACTIONS USING PORTLAND CEMENT

(d) Additional information about ettringite and monosulfate
phases
 The sulfate in ettringite can be replaced by carbonate or,
probably, partly replaced by two hydroxyl ions, although in
practice neither of these is often observed
 If fine limestone is present, carbonate ions become available
as some of the limestone reacts
 The carbonate displaces sulfate or hydroxyl in monosulfate.
The proportion of monosulfate or hydroxy-monosulfate
therefore decreases as the proportion of monocarbonate
increases
HYDRATION REACTIONS USING PORTLAND CEMENT

(d) Additional information about ettringite and monosulfate
phases
 The displaced sulfate typically combines with remaining
monosulfate to form ettringite, but if any hydroxymonosulfate is present, the sulfate will displace the hydroxyl
ions to form more monosulfate
 The key here is the balance between available alumina on the
one hand, and carbonate and sulfate on the other
HYDRATION REACTIONS USING PORTLAND CEMENT

(3) Tetracalcium aluminoferrite reactions
 (a) Ettringite, calcium hydroxide and iron hydroxide phases
(CaO)4.Al2O3.F2O3
+
3 CaO.SO3.(H2O)2
C4AF
(CaO)6.Al2O3.(SO3)3.(H2O)32
C6AS3H32
(ettringite)
+
3 CSH2
+
CaO.H2O
CH
(calcium hydroxide)
30 H2O
30 H
+
F2O3.(H2O)3
FH3
(iron hydroxide)
(6)
HYDRATION REACTIONS USING PORTLAND CEMENT

(3) Tetracalcium aluminoferrite reactions
 (b) Monosulfate, calcium hydroxide and iron hydroxide
phases
2 (CaO)4.Al2O3.F2O3
+
(CaO)6.Al2O3.(SO3)3.(H2O)32
C4AF
3 (CaO)4.Al2O3.SO3.(H2O)12
3 C4ASH12
(monosulfate)
C6AS3H32
+
2 CaO.H2O
2 CH
(calcium hydroxide)
+
12 H2O
12 H
+
2 F2O3.(H2O)3
2 FH3
(iron hydroxide)
(7)
HYDRATION REACTIONS USING PORTLAND CEMENT

(3) Tetracalcium aluminoferrite reactions
 (c) Hydrogarnet, calcium hydroxide and iron hydroxide
phases
(CaO)4.Al2O3.F2O3 + 10 H2O
C4AF
10 H
(CaO)3.Al2O3.(H2O)6 + CaO.H2O + F2O3.(H2O)3
C3AH6
(hydrogarnet)
CH
FH3
(calcium hydroxide (iron hydroxide)
 (d) Calcium aluminoferrite hydrates
(CaO)4.Al2O3.F2O3
+
C4AF
(CaO)4.(Al2O3, F2O3).(H2O)13
C4 (A, F)H13
(calcium aluninoferrite hydrate)
2 CaO.(H2O)
2 CH
+
(8)
+
14 H2O
14 H
(Al2O3, F2O3).(H2O)3
(A, F)H3
(iron or aluminium hydroxide)
(9)