Concrete is a three-phase material consisting of aggregate, hydrated cement paste, and a transition zone between the aggregate and cement paste. The aggregate phase provides strength, weight, and thermal properties to the concrete. The hydrated cement paste phase is made up of products like calcium silicate hydrate and calcium hydroxide and binds the aggregate together. The transition zone between the aggregate and cement paste is weaker due to microcracking and limits the strength of the concrete.

1. Concrete as a three phase system
Binaya Prasad dhakal
acme ENGINEERING COLLEGE

2. introduction
• At macroscopic level, concrete may be considered two
phase material consisting of aggregate particles dispersed
in the matrix of cement paste
• In microscopic level, there exists an interface between
cement paste and aggregate which is known as transition
phase
• Thus concrete is heterogeneous material composed of
three phases, viz-aggregate phase, hydrated cement paste
phase and transition phase

3. Aggregate phase
• Aggregates in concrete are chemically stable, The properties of concrete are
influenced by the physical properties of aggregates
• This phase is responsible for unit weight, elastic modulus and thermal
properties of concrete
• More angular and rougher aggregates give better bonding with cement paste
• Larger the size of aggregates in concrete greater will be the internal bleeding
due to less surface area
• This phase is stronger than other two phases
• Failure through aggregates occur only when other two zones are stronger than
it in high strength concrete

4. Hydrated cement phase (HCP)
1. Solids in HCP
a. Calcium Silicate Hydrate (C-S-H)
• Comprises 50-60% of solids in HCP
• Appears as very fine small fibrous crystals which fills the space
formerly occupied by water
b. Calcium Hydroxide
• Comprises 20-25% oh HCP
• Tends to form large hexagonal crystals
c. Ettringite
• Hexagonally shaped crystals longer than CH crystals
• Becomes unstable in absence of sulfate
d. Monosulfate hydrate
• formed by the decomposition of mineral ettringite
c. Un-hdyrated Clinker grains
• Clinker size 1-50μm
• Hydration of clinker particles result in the formation of very dense
hydration product

5. • C-S-H gel is the main binding element in hydrated cement
with enormous surface area due its geometry formed by
hydration of C3S and C2S
• C-S-H is thus main component for strength
• CH just remains as a precipitate in the shape of plate like
hexagonal crystals
• Ettringite is formed by reaction of gypsum and other
sulphate compound with calcium aluminate, responsible
for early setting
• Ettringite is needle shaped and long initially which
decreases on long run
• C4AF is for initial setting and formed within 24 hours

6. • The growth of C-S-H and
calcium hydroxide crystals
beside each other. The
dominating phase in forming
the shape in the contact
surface is C-S-H probably due
to its significantly higher
strength
• The C-S-H has water held
between the interlayer spaces

7. • Aft: Alumina, Ferricoxide,
TriSulphate : initial form called
Ettringite
• AFm : Alumina, Ferric oxide,
mono sulphate : formed by
decomposition of ettringite

8. 2. Voids in HCP
a) Interlayer space in C-S-H
• Void size is small and has no effect on strength and
permeability, about 2nm in diameter
b) Capillary voids
• These are the spaces not filled by solids of HCP
• Irregular in shape and has adverse effects on strength
and permeability
• 10-50 nm in low w/c and 3-5μ in high
c) Entrapped air voids
• Irregular in shape
• Produced due to improper mixing of concrete
• Larger than capillary voids
d) Entrained air voids
• Spherical in shape
• Produced due to addition of air entraining mixture
• 0.02-1mm

9. 2. Water in HCP
a) Capillary water
• Water present in voids larger than 2nm
• Water held by capillary action
• Affects strength and shrinkage
b) Adsorbed water
• Water held to solid surface due to attractive force
• Responsible for shrinkage of HCP on drying
c) Interlayer water
• Associated with C-S-H structure
• Lost only on drying
• Responsible for shrinkage of C-S-H on drying
d) Chemically combined water air voids
• Integral part of structure
• No loss on drying

11. • The hydrated cement paste or cement gel consists of solid products of
hydration and water held physically or adsorbed on the hydrated
surface
• This water is called gel water and is located in between the solid
products of hydration in so called gel pores. The size of gel pores is
about 2nm
• The volume of gel water is 28% of the mass of the dry cement
• Addition to gel water, water combined chemically or physically with the
products of hydration is held firmly
• This amount of water is the non-evaporable water and in fully hydrated
cement represents about 23 % of the mass of dry cement
• The solid products of hydration occupy a volume less than the sum of
the absolute volumes of original dry cement giving rise to residual
space within gross volume of paste which is about 18.5 % of original
volume of dry cement
• These residual space form of voids or capillary pores which can be full
or empty depending upon original mix water and curing conditions

13. INTERFACIAL Transition zone in
concrete(itz)
• Interfacial region between particles of core aggregate and hardened cement paste
• Transition zone is a plane of weakness and has greater influence on mechanical
behavior of concrete
• Although this zone has bulk cement paste the quality of paste is of poorer quality
• Due to internal bleeding water accumulate below elongated flaky and large pieces
of aggregates resulting in reduction of bond between paste and aggregate
• Due to drying shrinkage or temperature variation the transition zone develops
micro cracks even before the structure is loaded
• When the structure is loaded these micro cracks propagate and larger cracks are
formed causing failure of bond
• Considered as strength limiting phase of concrete
• The formation of micro cracks can be also due to differential volume change
between aggregates and cement paste