Chemical Admixture

transcript

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

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

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

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

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

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

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

Consider a thin layer of liquid between two parallel plates

psurf =αr=

2 α cosθd

Chemical Admixture

Ice Formation in Concrete

Chemical Admixture

Air Voids

Chemical Admixture

Ice Forming in Air Voids

Chemical Admixture

Air-Entraining Surfactants

Salts of wood resins, protainaceous materials and petroleum acids, and some synthetic detergents.

Chemical Admixture

Air-Entraining Surfactants

Chemical Admixture

Advantages of Using Air-Entrained Surfactants

Freezing and thawing cycles

Improve workability

Reduce tendency for segregation and bleeding

Chemical Admixture

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

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

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

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

Examples

Chemical Admixture

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

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

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

Lignosulfonate as a Water Reducer

H 3COSO 3Na

Courtesy from Carmel JOLICOEUR

Chemical Admixture

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

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

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

Chemical Admixture

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

Typical Scale of Components in SF-Cement Paste

Cement

Chemical Admixture

Influence of PNS on Ettringite Morphology

Without PNS With PNS

Chemical Admixture

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.

Chemical Admixture

OPTIONS IN SP APPLICATIONS

120 140 160 180 200 220 240

Water content (kg/m3)

with SP

without SPIncreasedstrength

Increasedworkability

Chemical Admixture

Mechanism

Chemical Admixture

Mechanism 2

Chemical Admixture

Mechanism 3

Chemical Admixture

Mechanism 4

Chemical Admixture

HRWR or Superplasticizers:Synthetic Water-Soluble Polyelectrolytes

Type of monomer

Length (Mw)

Branching, cross-linking

Charge, counter-ions

- - - - - -

Co-polymers

(building blocks)

Chemical Admixture

Structure of PolyNaphthaleneSulfonate (PNS)

SO 3Nan

Chemical Admixture

Structure of PolyMelamineSulfonate (PMS)

N NHNH

SO 3Na

Chemical Admixture

Structure of a Co-Polymer

CH CH 2

SO 3Na

Chemical Admixture

Structure of PolyAcrylic Esters (PAE)

CH 2 C

Chemical Admixture

Set-Controlling Chemicals

Rheological changes in a fresh concrete mixture

Stiffening: loss of consistency by the plastic cement paste

Chemical Admixture

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

Classification

Accelerator: decreases the setting time.

Retarder: increases the setting time.

Chemical Admixture

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

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

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

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

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

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

Accelerating Admixtures

Chemical Admixture

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

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

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

Chemical Admixture

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