Chemical Admixtures
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Admixtures
Reasons
(1) Improve or modify some or several properties of portland concrete.
(2) Compensate for some deficiency
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Admixtures
Classification
Surfactants (0.05-0.5%; new ones 2%)
Chemical Admixtures (1-4% by weight of cement)
Mineral Admixtures (> 15% by weight of cement)
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
A short (but important detour into surface chemistry)
The presence of a surface breaks the molecular symmetry that exists inside a material.
The molecules at the surface have different energy than the molecules inside the bulk material.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Surface energy
Surface energy, Usurf, is the difference between the energy of the molecules at the surface and the energy that they would have within the body.
Usurf = α S
Where S is the area of the interface and a is the surface tension.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Surface Energy
Nature brings a body to its minimum energy.
Small spherical drops of liquid and gas bubbles are good examples of surface minimization for a given volume.
The decrease in surface induces a contraction of the drop, increasing its internal pressure and making it higher than the external pressure.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
A balance of energy requires that the reduction in surface energy (αdS) be equal to the work done by the surface forces in reducing the surface.
The work done can be expressed as where is the volume change.
α dS = psurf dV
Surface Energy
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Applications
psurf = 2α r
Cylinder of radius r and height h S = 2π r h V = π r2 h
psurf = α r
as the size of the sphere or the cylinder decreases, the magnitude of the surface pressure increases greatly
S = 4 π r2 V = 4π r3 3Sphere of radius r,
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Consider a thin layer of liquid between two parallel plates
psurf =αr=
2 α cosθd
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Ice Formation in Concrete
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Air Voids
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Ice Forming in Air Voids
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Air-Entraining Surfactants
Salts of wood resins, protainaceous materials and petroleum acids, and some synthetic detergents.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Air-Entraining Surfactants
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Advantages of Using Air-Entrained Surfactants
Freezing and thawing cycles
Improve workability
Reduce tendency for segregation and bleeding
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Disadvantages of Using Air-Entrained Surfactants
Loss in strength ( for each 1% of air causes 5% loss in strength)
Increase permeability
In case of overdoses, they cause delay in setting and hardening
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Surface - Active Chemicals (Surfactants)
long-chain organic molecules, one end of each is hydrophilic (water-attracting) and the other hydrophobic(water-repelling).
hydrophilic end contains one or more polar groups, such as -COO- -SO3-, or -NH3+.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
ASTM Categories (C494) : Water Reducers
Low range: water reduction of 5% (minimum)WR (e.g. lignosulfonate)– Type A : normal– Type D : WR and retarding– Type E : WR and accelerating
High range: water reduction of 12% (minimum)HRWR, Superplasticizer (synthetic polymers: naphthalene-, melamine- or acrylate- based) – Type F : normal– Type G : HRWR and retarding
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Water Reducing Agents
Salts and derivatives of lignosulfonic acids, hydroxylatedcarnoxylic acids, and polysaccharides.
The anionic polar group is joined to hydrocarbon chain which itself is polar or hydrophilic.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Examples
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism
The polar chain is adsorbed alongside the cement particle; instead of directing a nonpolar end toward water, in this case the surfactant directs a polar end, lowering the surface tension of the water and making the cement particle hydrophilic.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Consequence
As a result of layers of water dipoles surrounding the hydrophilic cement particles, their flocculation is prevented and a well-dispersed system is obtained.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Advantages of Using Water-Reducing Admixtures
Increase the consistency
Achieve higher compressive strength
Cement saving
Important: not all three benefits can be obtained at the same time
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Lignosulfonate as a Water Reducer
O
H 3CO
O
OH
H 3COSO 3Na
HO
n
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Superplasticizers
Consist of sulfonated slats of melamine or napththaleneformaldehyde condensates.
Also called high range water-reducing admixtures because they are able to reduce 3 to 4 times water compared to normal water-reducing admixtures.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Superplasticizers
Long-chain, high-molecular mass anionic surfactants with a large number of polar groups in the hydrocarbon chain.
Normal dosage: 1-2% by weight of cement.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Superplasticizers
25 to 30% of water reduction for a given consistency (normal plasticizer: 5 to 10% of water reduction).
No problem with bleeding and segregation because of the colloidal size of the long-chain particles of the admixture which obstructs the path of the bleed water.
flocculated deflocculated dispersed in less water
"Physical" effects operative in any slurry or paste
Highfluidity
Intermediatefluidity
Lowfluidity
Mode of Action of Superplasticizers
"Physical" binding and dispersionCourtesy from Carmel JOLICOEUR
Illustration of Physical Dispersion Effect
Mineral Paste+10 % water +0.1 wt% PNS
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Characterization of Superplasticizers
BulkpH, conductivity% solidviscosityspecific gravitysurface tensionloss on ignition, TGA
Physico-chemicalelemental and ionic analysisacid-base titrationcharge densitymolar mass distributionNMR, IR, UV spectroscopy
Functionalrheology of pastes (inert or reactive minerals)zeta potential on reference minerals (dilute)adsorption on various minerals (dilute and pastes)influence on hydration reactionsspecific interactions
Courtesy from Carmel JOLICOEUR
500X
SF
SF
50X
SO4
Typical Scale of Components in SF-Cement Paste
SF
Cement
SF
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Influence of PNS on Ettringite Morphology
Without PNS With PNS
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mode of Action of Superplasticizers"Chemical" Effects: Changes in morphology
SEM micrographs of a high alkali cement paste
0% PNS; 30 min hydr. 4% PNS; 30 min hydr.
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
OPTIONS IN SP APPLICATIONS
30
40
50
60
120 140 160 180 200 220 240
Water content (kg/m3)
Flow
tabl
e sp
read
(cm
)
with SP
without SPIncreasedstrength
Increasedworkability
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism 2
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism 3
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism 4
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
M+n
HRWR or Superplasticizers:Synthetic Water-Soluble Polyelectrolytes
Type of monomer
Length (Mw)
Branching, cross-linking
Charge, counter-ions
- - - - - -
Co-polymers
(building blocks)
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Structure of PolyNaphthaleneSulfonate (PNS)
CH 2
SO 3Nan
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Structure of PolyMelamineSulfonate (PMS)
N N
N NHNH
HN
O
SO 3Na
n
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Structure of a Co-Polymer
CH
C
CH CH 2
COONa
NH
CH
N
SO 3Na
n
OO
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Structure of PolyAcrylic Esters (PAE)
CH 2 C
R1
COONa
CH 2 C
R1
CO
O
n m
R
CH 2 C
R1
X o
Courtesy from Carmel JOLICOEUR
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Set-Controlling Chemicals
Rheological changes in a fresh concrete mixture
Stiffening: loss of consistency by the plastic cement paste
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Set-Controlling Chemicals
Setting: beginning of solidification. At the initial set the paste becomes unworkable so the placement, compaction and finishing of concrete beyond this point becomes difficult (4 to 6 hrs. at 70 F). Final set is the time required for the paste to solidify completely.
Hardening: strength gain with time.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Classification
Accelerator: decreases the setting time.
Retarder: increases the setting time.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism of Action (I)
The action of set-controlling chemicals on portland cement can be attributed mainly to dissolving of the anhydrous constituents forming anions (silicate and aluminate) and cations (calcium), the solubility of each being dependent on the type and concentration of the acid and base ions present in the solution.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism of Action (II)
An accelerating admixture must promote the dissolution of the cations (calcium ions) and anions from the cement.
A retarding admixture must impede the dissolution of the cement cations (calcium ions) and anions.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism of Action (III)
The presence of monovalent cations in solution (i.e., K+ or Na+) reduces the solubility of Ca2+ ions but tends to promote the solubility of silicate and aluminate ions. In small concentrations, the former effect is dominant; in large concentrations, the latter effect becomes dominant.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Mechanism of Action (IV)
The presence of certain monovalent anions in solution (i.e., Cl, N03-, or S042-) reduces the solubility of silicates and aluminates but tends to promote the solubility of calcium ions. In small concentrations, the former effect is dominant; in largeconcentrations, the latter effect becomes dominant.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Accelerating admixtures
Useful for modifying the properties of concrete, particularly incold weather, to:
(a) expedite the start of finishing operations and, when necessary, the application of insulation for protection;
(b) reduce the time required for proper curing and protection;
(c) increase the rate of early strength development so as to permit earlier removal of forms and earlier opening of the construction for service; and
(d) permit more efficient plugging of leaks against hydraulic pressures.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Accelerating Admixtures
Calcium chloride is by far the best known and most widely used accelerator. See Table 8.3 for the potential problems in using such admixture.
There are accelerators that do not contain choride: calcium formate, formic acid.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Accelerating Admixtures
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Retarding admixtures
Compensation for adverse ambient temperature conditions particularly in hot weather. Extensive use is made of retarding admixtures to permit proper placement and finishing and to overcome damaging and accelerating effects of high temperatures.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Retarding admixtures
Control of setting of large structural units to keep concrete workable through-out the entire placing period.
This is particularly important for the elimination of cold joints and discontinuities in large structural units.
Also control of setting may prevent cracking of concrete beams, bridge decks, and composite construction due to form deflection of movement associated with placing of adjacent units.
Chemical Admixture
P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials
Chemical Admixtures
Type A: water-reducing
Type B: retarding
Type C: accelerating
Type D: water-reducing and retarding
Type E: water-reducing and accelerating
Type F: high-range water-reducing
Type G: high-range and retarding