Structure of concrete

Concrete is a heterogeneous composite at the microscopic level, treated as a three-phase system:

   +-----------------------------+
   |  hcp (binding medium)       |
   |      +---------+            |
   |      | AGGREG- |<- transition
   |      |   ATE   |   zone (ITZ)
   |      +---------+            |
   |  ( ) pores / voids          |
   +-----------------------------+

1. Aggregate phase

Occupies 70-75% of the volume. It governs unit weight, modulus of elasticity, dimensional stability and, for normal concrete, is stronger than the paste so it does not govern strength. Its size, shape, grading, texture and porosity influence workability and bond.

2. Hydrated cement paste (hcp, binding medium)

Contains C-S-H gel (50-60%), calcium hydroxide (20-25%), calcium sulphoaluminates (15-20%), unhydrated cement grains and pores (gel pores, capillary pores, air voids). It binds the aggregates and gives strength and durability.

3. Transition zone (interfacial transition zone, ITZ)

A thin layer, 10-50 µm, around the aggregate particles. It is the weakest link in the system.

• Formation: near the aggregate there is a "wall effect", so cement grains pack loosely, and bleed water collects under the aggregates. The zone has a higher w/c, so large crystals of $Ca(OH)_2$ and ettringite grow in preferred orientation, with more porosity and microcracks.

Effect of the transition zone on the properties of concrete

• Strength: concrete fails at a stress well below the strength of both paste and aggregate, since microcracks begin in the ITZ (at 30-50% of ultimate load they bond-crack, then join up with mortar cracks). Concrete is weaker in tension than compression due to ITZ.
• Stress-strain: nonlinear behaviour of concrete in spite of linear elastic aggregate and paste, because of microcrack growth.
• Modulus of elasticity: lowered (a composite of two stiff phases is less stiff than either).
• Permeability and durability: the porous ITZ forms a pathway for water and ions, so reduces durability (chloride, sulphate, frost attack).
• Bond of aggregate with paste and fracture toughness decrease. Remedy: use low w/c, silica fume/fly ash (which fill the ITZ and consume Ca(OH)$_2$), superplasticiser, good compaction and curing, and clean, rough aggregate.

Concrete as a three-phase system

At the microscopic level, concrete consists of:

1. Aggregate phase (70-75% of volume): gravel/crushed stone and sand; provides volume, stiffness and dimensional stability.
2. Hydrated cement paste (hcp): the binder phase.
3. Transition zone (ITZ): thin (10-50 µm) layer of paste near aggregate surfaces; weakest part.

 +---------------------------+
 | paste  ___ITZ___  paste   |
 |       / AGGREG- \         |
 |       \  ATE    /         |
 |        ---------          |
 | ( ) air voids    . . pores|
 +---------------------------+

Structure of hydrated cement paste

Solid phases

• Calcium silicate hydrate (C-S-H): 50-60% of solids, poorly crystalline gel with a huge surface area (100-700 m$^2$/g); the main source of strength through van der Waals forces.
• Calcium hydroxide (CH, portlandite): 20-25%, large hexagonal crystals; low strength and surface area, soluble, gives alkalinity (pH 12.5) but reduces chemical resistance.
• Calcium sulphoaluminates: 15-20%; ettringite (needle-like) and monosulphate; play a minor role in strength but attack by sulphate.
• Unhydrated cement grains: remain in the paste when the w/c is low.

Voids in hcp

• Interlayer space in C-S-H: about 0.5-2.5 nm; does not affect strength.
• Capillary pores: 10 nm to 50 µm, spaces left by water not filled by products; their volume depends on w/c and degree of hydration; they control strength and permeability.
• Entrapped/entrained air voids: 50 µm to 1 mm.

Water in hcp: capillary water, adsorbed water, interlayer water and chemically combined water.

Effect on concrete properties

• Strength is inversely related to the capillary porosity (Powers: $f = f_0 x^3$, where $x$ is the gel-space ratio).
• Lower w/c and longer curing reduce capillary pores, so strength and durability rise.
• Drying shrinkage and creep come from loss of adsorbed and interlayer water of C-S-H.
• Permeability and durability depend on capillary pore continuity; CH is vulnerable to acid and leaching; ettringite to sulphate.

Concrete is a three-phase composite (microstructure view): aggregate, hydrated cement paste and the interfacial transition zone.

  +----------------------------+
  | HCP (matrix)               |
  |    ,-------.               |
  |   / AGGREG- \  <-- ITZ ring|
  |   \  ATE    /              |
  |    `-------'    o pores    |
  +----------------------------+

1. Aggregate phase

About 70-75% by volume. Normally stronger and stiffer than the paste, so in ordinary concrete it does not govern strength; but it controls elastic modulus, density, shrinkage, thermal properties, wear resistance. In lightweight or weak aggregate concrete, aggregate strength limits concrete strength. Shape, size, texture and grading affect workability and bond.

2. Hydrated cement paste phase (matrix)

C-S-H gel (50-60%), $Ca(OH)_2$ (20-25%), sulphoaluminates and unreacted cement, with gel pores and capillary pores. It binds the particles and governs strength (lower w/c, fewer capillary pores, higher strength), shrinkage, creep and permeability.

3. Interfacial transition zone (ITZ)

A 10-50 µm thick shell around each aggregate particle with higher porosity, oriented $Ca(OH)_2$ crystals, and microcracks, because of bleed water films and wall effect. It is the weakest link: microcracking starts there, so it limits strength (especially tensile), modulus of elasticity and durability. It is improved by low w/c, pozzolanic admixtures (silica fume), and good curing.

Role of each phase in strength

• Strength of concrete $\approx$ f(strength of hcp, bond strength at ITZ, aggregate strength) - the weakest of the three fails first.
• In normal strength concrete: governed by hcp and ITZ; in high-strength concrete the ITZ is made denser and aggregate strength starts to matter; in light-weight concrete the aggregate governs.
• Under load the cracks begin at the ITZ, then spread into the paste, finally joining up to cause failure.

Solid phases in hydrated cement paste (hcp)

1. Calcium silicate hydrate (C-S-H): 50-60% of the volume of solids. A poorly crystalline gel (fibres, sheets), with a very high surface area; bonds by van der Waals forces and is responsible for the strength and other properties of the paste.
2. Calcium hydroxide (CH, portlandite): 20-25%; large hexagonal crystals; low surface area so contributes little to strength; makes the paste alkaline (protects steel), but is soluble and attacked by acids.
3. Calcium sulphoaluminates: 15-20%; ettringite (needles), later monosulphate; small contribution to strength, and a source of sulphate attack.
4. Unhydrated cement grains: especially at low w/c; they hydrate later if moisture is available (long-term strength). Also present: voids (gel pores, capillary pores, air voids) and water.

Effect of the transition zone on the properties of concrete

The ITZ is a thin layer (10-50 µm) of paste around aggregate surfaces which has higher w/c, larger oriented CH and ettringite crystals, more porosity and microcracks.

• It is the weakest link, so it limits the compressive and especially the tensile and flexural strengths.
• It causes the nonlinear stress-strain curve (bond microcracks form at about 30-40% of ultimate load).
• It lowers the elastic modulus below the value expected from the phases.
• Its porosity gives paths for water, chlorides and sulphates, reducing durability and increasing permeability.
• Improved by silica fume, fly ash, low w/c, superplasticiser and proper curing.

The transition phase (transition zone, ITZ) is the thin shell of cement paste, about 10-50 µm thick, immediately surrounding the aggregate particles. Its structure differs from that of the bulk paste.

How it forms

1. Because of the "wall effect", cement grains cannot pack closely against the large aggregate surface, so the zone is richer in water and has a higher w/c.
2. A film of bleed water accumulates around the aggregate (especially under flat, large particles) and gets trapped.
3. In this water-rich space, larger crystals of calcium hydroxide (oriented with the c-axis perpendicular to the aggregate) and ettringite grow, and less C-S-H forms. The result is a zone with high porosity, a low density and microcracks (from shrinkage and thermal movement).

 aggregate | ITZ (porous,  | bulk paste
           | CH crystals)  | (dense C-S-H)

Effect on concrete properties

• Strength: concrete is weaker than either aggregate or paste; microcracks start in the ITZ at about 30-40% of ultimate load. Tensile and flexural strengths are very sensitive to it.
• Stress-strain relationship: non-linear, due to progressive bond cracking.
• Elastic modulus: lower than predicted from the aggregate and paste.
• Durability and permeability: the porous, interconnected zone allows ingress of water and aggressive ions; affects frost, chloride and sulphate resistance.
• Fracture: governs crack paths and brittleness.

It can be improved by a low w/c ratio, silica fume and fly ash, a superplasticiser, rough clean aggregate, and good compaction and curing.

Concrete is a three-phase composite consisting of (i) the aggregate phase, (ii) the hydrated cement paste (binding medium) and (iii) the interfacial transition zone (ITZ) between them.

  +--------------------------+
  | paste  ,---------.       |
  |       / aggregate \      |
  |       `---------'  ITZ   |
  +--------------------------+

• Aggregate phase: 70-75% by volume; particles of crushed stone/gravel and sand.
• Paste (matrix): C-S-H, calcium hydroxide, sulphoaluminates, pores; binds aggregate and gives strength.
• ITZ: 10-50 µm thick porous shell around aggregate; the weakest link and origin of microcracking.

Role of the aggregate phase

1. Volume filler: cheaper than cement; occupies most of the volume, so it reduces the cost and the heat of hydration.
2. Strength and stiffness: gives the concrete its high compressive strength and elastic modulus (E of aggregate 50-100 GPa against 20-30 GPa for paste); in normal concrete the aggregate is stronger than the paste, but in lightweight concrete it can limit strength.
3. Dimensional stability: restrains shrinkage and creep of the paste, thereby reducing cracking.
4. Durability and wear: abrasion resistance, soundness, fire and weather resistance.
5. Unit weight of the concrete is mostly decided by aggregate density (normal, lightweight or heavy concrete).
6. Workability: the grading, shape and texture control water demand and bond with the paste.

Concrete is a three-phase composite: (1) the aggregate phase, (2) the binding medium (hydrated cement paste, hcp) and (3) the transition zone between them.

 +----------------------------+
 | hcp  .------------.  ITZ   |
 |     / aggregate    \       |
 |     '--------------'       |
 +----------------------------+

The aggregate (70-75% of volume) gives bulk and stiffness; the ITZ (10-50 µm thick) is the weak, porous layer around the aggregate.

Binding medium phase (hydrated cement paste)

Cement reacts with water to produce the paste, which hardens into a stone-like binder.

Solid products of hydration

• C-S-H gel (50-60% of solids): very fine, high-surface-area, gives strength through van der Waals forces; stable.
• Calcium hydroxide (20-25%): crystalline; little strength; alkalinity; weak against acid.
• Calcium sulphoaluminates (15-20%): ettringite and monosulphate; lower role in strength; sulphate attack.
• Unhydrated cement remaining.

Voids

• Interlayer (gel) space in C-S-H (below 2.5 nm); capillary pores (10 nm-50 µm), depend on w/c and degree of hydration; air voids (entrapped 1 mm, entrained 50-200 µm).

Water: capillary water, adsorbed water, interlayer water and chemically combined water.

Significance

• Strength depends on the volume of capillary pores (gel-space ratio); a low w/c and good curing give a strong, impermeable paste.
• It governs shrinkage and creep (loss of adsorbed water from C-S-H) and permeability.
• It binds aggregates by adhesion, holds the steel and protects it by its high pH.

Series and parallel models

Concrete is idealised as a composite of mortar (hcp + sand), the interface (ITZ) and coarse aggregate. Their arrangement under load decides how the stress is transferred.

 Parallel model        Series model
 load ↓ ↓ ↓            load ↓
 +----+---+----+       +---------+
 | M  | A | M  |       | mortar  |
 |    |   |    |       +---------+
 +----+---+----+       |aggregate|
 same strain           +---------+
 each phase            same stress,
 shares load           strains add

• Parallel (Voigt) model: the phases act side by side; each has the same strain and the load is shared in proportion to stiffness. Upper bound of modulus:

$$E_c = E_m V_m + E_a V_a$$

• Series (Reuss) model: the phases are in layers one after the other; each carries the same stress and strains add. Lower bound:

$$\frac{1}{E_c} = \frac{V_m}{E_m} + \frac{V_a}{E_a}$$

where $V$ is the volume fraction (subscripts $m$ = mortar, $a$ = aggregate). The actual modulus of concrete lies between the two bounds (a combination of both models, as in Hirsch's model); including the weak ITZ in series reduces it. Load transfer is by bond/shear at the interface, so the weak interface causes microcracks and nonlinearity.

Solids in hydrated cement paste

• C-S-H gel (50-60%): main binding product.
• Calcium hydroxide (20-25%): hexagonal crystals.
• Calcium sulphoaluminates (15-20%): ettringite, monosulphate.
• Unhydrated cement grains.

The microstructure of ordinary Portland cement paste refers to the structure of the cement paste after hydration, as seen under an electron microscope.

Unhydrated cement

Cement is a fine powder (particle size 1-100 µm) of the clinker compounds C$_3$S, C$_2$S, C$_3$A and C$_4$AF, with gypsum. The grains have an irregular shape.

Hydrated structure

On mixing with water, the grains dissolve, and products precipitate in the water-filled space between grains:

1. C-S-H gel (50-60% by volume of solids): initially fibrous or honeycomb-like, later massive, with enormous surface area (about 100-700 m$^2$/g). It forms by hydration of C$_3$S and C$_2$S and gives the strength.
2. Calcium hydroxide (portlandite) (20-25%): large hexagonal plate crystals filling spaces.
3. Ettringite (needles, early) changing into monosulphate plates (15-20%) from C$_3$A + gypsum.
4. Unhydrated cores of cement grains, especially large ones.
5. Pores: gel pores (below 3 nm), capillary pores (up to some µm), and air voids.

 Early (hours)   ->   Later (days)
  o   o   o           oooo C-S-H
 grains in water      fills spaces,
  + needles           CH hexagons

The strength depends on the amount of solids and how well they fill the space (gel/space ratio), so it rises with age as hydration products grow and capillary pores shrink. The w/c ratio (higher w/c, more capillary pores) and curing decide the porosity and thus strength, permeability and durability.