Contents
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
