Cement: The added components

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Cement: The added components

What’s cement?

How does cement react?

Added components

What’s cement?

Cement definition EN 197-1:2000

Hydraulic binder i.e. a finely ground inorganic material, which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability even under water.

Portland cement

Prepared by grinding together

clinker made in the kiln with  calcium sulfate &  other minerals

proportions and nature of the other minerals depend on the type of cement

Clinker and calcium sulfate

The clinker

Silicates  “alite”    C3S  50  to  70%

“belite”  C2S  10  to  20%

o Tricalcium aluminate  C3A    1  to  15%

o Aluminoferrite  C4AF    1  to  15%

o Free-lime  CaO      #  1%

For Setting and Placing

o Calcium sulfate          3  to  6%

How does cement react?

What happens when mixing water and cement?

dissolution in the water.

nucleation period

chemical reaction with the H2O molecules to build hydrates (new crystals contain water in their structure = crystallized “solid” water)

Cement hardening is NOT some kind of drying but a chemical reaction leading to a new structure with new bonds bringing strength to the material

Why do we add gypsum?

Calcium sulfate

Why?

for setting and placing

Where?

o natural origin

o gypsum  CaSO4 – 2 H2O

o anhydrite  CaSO4

o industrial product

o hemi-hydrate (plaster)  CaSO4 – 1/2 H2O

o other industrial products

o phosphogypsum  CaSO4 – 2 H2O

o sulfogypsum  CaSO4 – 2 H2O

o fluoanhydrite  CaSO4

Calcium sulfates by synthetic gypsum

Calcium sulfates solubility

Added components

What? What for?

either additional to cement or instead of part of the cement

effects :

o positive impact on the concrete price

o modified concrete properties (to have the right concrete for the right job)

can even be the result of a constraint, material is on site and has to be used

Where? When?

In the cement mill

In the separator

In a silo after cement milling operations

In the concrete

Fine materials added in cement

Mineral binders

Let’s see the table …

Slag

 

 

consequences on properties

reduces Ca(OH)2 % after hydration

o pH is lower, less protection of reinforcement

o better behaviour in corrosive water

hydration is slower

o setting time is longer

o strength at 1 day is weaker but long term can be higher

o heat of hydration is lower

Activity

o casting temperature

o mineralogy

o vitreous phases

o activators

Hydration

o without Portlandite Ca(OH)2

Slag composition

Slag: influence of fineness

Influence of slag / inert filler

 

Pozzolanas

= a lot of silica and alumina very reactive:

o pozzolanas + water + CaO              CSH + C4AHn

o No Ca (OH)2  pH – less protection of reinforcement

o CH combined with pozzolanas

o low hydratation energy

o lower initial strength

Natural pozzolanas

Natural pozzolanas (examples)

Influence of pozzolanas

Pozzolanas reactivity

Fly ashes

Definition

By-product of coal burning in power station, obtained in the gas de dusters

 

Properties

Spherical particles  (1-100 µm)

Pozzolanic activity

Fly ashes, chemical composition

Silico-aluminous fly ash

o contains little lime and behaves like pozzolanas

Sulfo-calcareous fly ash

o  contains silica and free lime

o hydrates on its own to give CSH but one has to be careful with the free lime which may create large expansion

o behaves like pozzolanas when mixed with Portland clinker

o because of their often spherical shape they improve the rheological properties of concrete

Influence of fly ashes

Concrete characteristics

o improved workability

o shrinkage and initial heat of hydration reduced

o delays cracking

o increased durability in aggressive and pure waters

o less concrete segregation (bleeding)

o better pumpability

Attention to their compatibility with admixtures (ex: air entrainers)

Fume silica

By-product of the fusion process used to make siliceous metals and ferro-siliceous alloys

Characteristics

o SiO2 = 85 – 98%

o particles size (average): 0,1 to 0,5 µm

o spherical particles, often agglomerated

o amorphous structure

They are used in concretes, they:

o lead to a higher compacity of concrete due to their fineness (better packing of the particles)

o improve the mechanical strength (pozzolanic reaction)

o improve chemical corrosion behaviour because of less Ca(OH)2 (in concrete)

improve freeze-thaw behavior and resistance to de-icing salt

Fume silica

Disavantages

o not easy to use:

o  mix proportions, mixing, curing…

oW/C ratio increase without fluidifyers

o difficult to handle

o  variable on quality (by-product)

o disponibility

o cost (if of good quality)

Principal uses

o pozzolanic filler (complements cement)

o additive to increase concrete density

o high and very high performance concretes

Fillers (calcareous ou siliceous)

Definition

o product obtained from fine milling or pulverisation of natural or other rocks, wich react mainly because of their granulometry, and their physical properties with certain types of cement

improved workability

lower permeability and capillarity

reduced cracking

Fillers: Hydration of cement paste

OPC – blended cement  Same clinker

OPC  SSB 2250 cm2/g

Blended cement  SSB clinker 3970 cm2/g – 27% limestone

Bound water: % bound water/ pure paste W/C = 0,50

Compared with a non blended cement, a cement  made with fillers having the same strength is characterised by:

o higher initial strengths

o mechanical strengths, hydration rates and porosities equivalent to the 28-day term

o the limestone filler is preferable to the siliceous filler

o the nature and the fineness of filler can affect the rheology

o  choice of a suitable limestone

nDurability

oOPC and blended cement (filler addition) have a similar behaviour

heology

sensitivity of strength to temperature

resistance to carbonation

behaviour depending on freeze-thaw cycles

diffusion of chloride ions

resistance to sea water

o because of a similar texture and structure of the hydrated pastes

o conditions

choice of fillers

manufacturing process (cogrinding, addition of concrete)

Limestone fillers

Finely ground limestone (cement fineness) improves workability. Concretes are easy to place and sedimentation (bleeding) is less

The limestone has to be very pure (avoid dolomite limestone, clay and organic material)

Limestone has to be hard, otherwise it is going to perturb the grinding of the clinker

During hydration a small amount of carbo-aluminate is formed with the C3A

Admixtures in concrete

Admixtures for surface treatment of concrete

Surface retarding agents for exposed aggregates

Surface hardening agents

Surface sealing agents

Surface curing agents

Demoulding agents

Fine materials

The fine materials can either be added to the concrete or to the cement depending on the country’s policy

The fine materials can either be added to the concrete or to the cement depending on the country’s policy

 

 

 

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