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al. Armii Ludowej 16, p. 551, 00-637 Warszawa, POLAND; tel.: (+48 22) 825-76-37, fax: (+48 22) 825-75-47, e-mail:[email protected]

POLITECHNIKA WARSZAWSKA WYDZIAŁ INśYNIERII LĄDOWEJ KATEDRA INśYNIERII MATERIAŁÓW BUDOWLANYCH

WARSAW UNIVERSITY OF TECHNOLOGY FACULTY OF CIVIL ENGINEERING

DIVISION OF BUILDING MATERIALS ENGINEERING

Lech Czarnecki

Paweł Łukowski

Andrzej Garbacz

Bogumiła Chmielewska

Building chemistry – laboratory exercises

collaborative work under the chairmanship of Lech Czarnecki

12. CHEMICAL MODIFICATION OF CONCRETE

Theoretical background

Practical task 1. Assessment of dispergation properties of concrete admixtures

Practical task 2. Effectiveness assessment of concrete surface hydrophobization

Laboratory of Building Chemistry, Division of Building Materials Engineering, WUT

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12. CHEMICAL MODIFICATION OF CONCRETE

THEORETICAL BACKGROUND

Classification of chemical modifiers of concrete

Construction materials are modified to improve their present properties or to give them

new ones, related to a change or extension of their application range. In the case of concrete,

which is a material commonly used in construction, modification concerns both technological

properties of a concrete mixture and functional characteristic of hardened concrete. Chemical

modifiers consisting of up to 5% of cement mass are defined as additives, and below that

amount – as concrete admixtures. Due to their chemical composition, concrete additives and

admixtures can be divided into organic and inorganic (mineral).

Polymer-cement concretes

Organic modifiers are foremost high-molecular compounds or otherwise, polymers.

Concretes containing polymer additives are called polymer-cement concretes (PCC). In such a

material the polymer creates a separate phase, interacting with the cement binder. The

influence of the modifier can be of physicochemical character (“pre-mix” type modifiers , i.e.

added to a concrete mix in already polymerized state and not transformed any further) or of

chemical character – then the polymer functions as a co-binder (“post-mix” modifiers, i.e.

polymerizing after they have been mixed in, while the cement binder is setting). Acrylic and

epoxy resins as well as latexes of synthetic rubbers are some of the most commonly used

polymer additives. The modifier content in polymer-cement concretes is usually 10% to 20%

in relation to the cement mass. Such concretes, as compared to unmodified concretes, possess

higher mechanical properties, better adhesiveness to other materials and tightness.

Concretes modified by mineral additives

The most important mineral additives are silica fume, fly ash and metallurgical slag.

Silica fume is a by-product obtained in the production of ferrosilicon. It contains 85% to 98%

of pure amorphous silica of very fine grain (its diameter is below µm). Silica fume displays

pozzolanic properties (reactivity in relation to calcium hydroxide), and at the same time it acts

as a micro-filler, sealing up concrete structure. Due to this fact it is possible to obtain

concretes of very high strength and chemical resistance.

Fly ash (coal burning waste) and ground slag (metallurgical waste) are foremost used

as cheap substitutes of part of the Portland cement. With proper selection of binding

composition, concrete made from these substitutes may show desirable strength

characteristics and good tightness.

Concretes with admixtures

Concrete admixtures are the most popular chemical modifiers of a concrete mixture

and concrete used today (in highly developed countries 80% of concretes are produced with

admixtures). There are many kinds of admixtures of varied chemical composition and the way

they affect a concrete mixture and concrete (Table 12.1). Among the most popular are agents

affecting the consistency of a concrete mixture, air entrainment agents and agents controlling

the binding time of a cement binder.


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Table 12.1

Function and application of basic admixture types

Admixture type Examples Effects Practical application

plasticizing and

fluidizing

calcium

lignosulfonate,

melamine-

formaldehyde

resin

increased concrete

strength or increased

fluidity of a concrete

mix or decreased

cement consumption

concrete mix of high

fluidity, shotcrete

accelerating of

setting and/or

hardening of

concrete

calcium formate quick strength increase

without thermal

treatment

precast elements,

quick-setting

concretes and

mortars (e.g. for

repairs)

retarding of the

setting

calcium phosphate keeping the mix in

fluid state

placing the concrete

in a heat wave,

transporting a

concrete mix

air-entraining sodium abietate increased freeze

resistance of concrete

concretes exposed to

moisture and

temperatures below

zero degrees

frost resistant sodium thiocyanate enabling to produce

concrete in low

temperature

concrete made in low

temperature – in

winter

sealing silica fume decreased absorbability

of concrete

watertight concretes

Plasticizing and fluidizing admixtures

The application of admixtures changing the consistency of a concrete mixture enables:

• to increase the fluidity of a concrete mix with the same amount of mixing water, which

facilitates mix transport and placement,

• to decrease water content while maintaining stable consistency, due to which concrete of

higher strength is produced,

• to decrease cement consumption (by 10 -20%) at unchanged strength.

There are plasticizing admixtures (plasticizers), allowing to lower water content by 8–

16%, and fluidizing admixtures (superplasticizers), allowing to lower water content by 16–

30%.

The mix fluidizing mechanisms by applying admixtures can be basically of three types

(Fig. 12.1):

• electrostatic mechanism – induction on agglomerated cement grains uniform electric

charges, repelling each other and causing agglomerates decomposition while releasing

water contained inside; this is the way, among others, sulfonated naphthalene-

formaldehyde resins work,

• lubricating mechanism – creating on cement grains a “lubricating” layer of molecular

thickness separating individual grains and creating a slide between particles, which lowers

the internal tension of a concrete mixture; this is the way, among others, melamine resins

work,


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• hydrophilic mechanism – lowering surface tension of water in relation to cement, and thus

improving wettability of cement grains; this is the way, among others, lignosulfonates

work,

The efficiency evaluation of fluidizing admixtures is the subject of practical task 1.

Electrostatic mechanism

agglomerated cement induction of uniform dispersion of cement

grains electric charges grains

Lubricating mechanism

agglomerated cement cement grains covered dispersion of cement

grains by lubricating layers grains

Hydrophilic mechanism

high surface tension surfactant forms an well damped

of water – badly wetted adsorption layer cement grain

cement grain on water surface –

lowered surface tension

Fig.12.1. Mechanism of fluidizing of a concrete mix

- - - -

-

-

-


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Air-entraining admixtures

Air-entraining admixtures are the substances which produce a great number of very

small (20 – 250 µm) and uniformly spread out (150 – 200 µm) air bubbles during the mixing

stage. Bubbles in a hardened material disrupt capillary continuity (Fig. 12.2) and prevent

capillary absorption of water in a material, lowering concrete wettability and susceptibility to

frost – water freezing in capillaries, extending its volume, can squeeze into empty bubbles,

which prevents concrete from cracking.

Air-entraining substances are surface active chemicals of hydrophobic action, e.g.

abietates or stearates. Air-entraining admixtures usually plasticize the mix as air bubbles

reduce internal tension in it. However, the adverse effect here is slightly lower concrete

strength.

Fig.12.2. Structure of aerated concrete: 1 – air bubbles, 2 – capillaries

Admixtures controlling the setting time of a binder

They are substances, mainly mineral, shortening or lengthening setting or hardening

time of cement in a concrete. They affect the hydration temperature of cement and time when

heat is released during hydration (Fig. 12.3). They can also accelerate or retard chemical

reactions between water and cement ingredients.

Fig. 12.3. Heat releasing rate during cement hydration with admixtures controlling setting

time

0

50

100

150

200

250

0 5 10 15 20 25 30

time, h

total heat released, J/kg

with accelerated admixture

without admixture

with retarding admixture


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Until recently the most popular accelerating admixture used has been calcium

chloride. However, due to reinforcing steel corrosion caused by chlorides, at present chloride

admixtures can only be used in a limited way. For instance, instead of calcium chloride,

calcium formate is used, often considered to be the most appropriate substitute. Sodium nitrite

is a strong setting accelerator, not causing steel corrosion; it is a strong poison, though.

Setting retarders belong to a group of products lowering solubility of cement

components, mainly lime and aluminates, and thus decreasing the initial speed of cement

setting. Some of them produce protective layers on cement grains, often reacting with cement

components. Some inorganic compounds possess setting retarding properties, foremost all

phosphates.

Polymer impregnated concrete

Polymer impregnated concretes (PIC) constitute a separate group of modified

materials, obtained by impregnating hardened concrete with a monomer or prepolymer, which

then polymerizes inside the concrete. This kind of materials is characterized by very high

mechanical strength and chemical resistance. Their application is, however, limited due to

their high cost and complicated impregnation technique. Impregnation with an appropriate

polymer does not change the external appearance of concrete; this can be significant when

renovating the existing objects. Hardened concrete impregnation may be of the through, in-

depth or surface type. In the last case the beneficial effect may be, among others,

hydrophobization of the concrete surface, i.e. giving concrete surface the property “to repel”

water particles, due to which this surface is badly wetted by water – which limits its adverse

effect. The assessment of hydropbobization effectiveness of concrete surface is the subject to

be dealt with in practical task 2.

PRACTICAL TASK 1. ASSESSMENT OF FLUIDIZING PROPERTIES OF CONCRETE ADMIXTURES

The equipment needed for the task:

scales of accuracy 0.1 g bowl 500 cm3 beaker

10 cm3 pipette measuring cone glass plate

ruler

Reagents and other materials used:

portland cement tap water concrete admixtures

Task performance The task consists in measurement of the diameter of the portland cement paste flow.

The higher is fluidity of the paste, the larger is diameter of the flow; therefore, the paste

containing the fluidizing admixture is expected to show the larger diameter of the flow than

the paste without admixture. The efficiency of fluidizing admixture (superplasticizer) can be

assessed this way.

First, 200.0 g of tap water should be weighed in the beaker (do not use the measuring

cylinder). Then, 500.0 g of portland cement should be weighed in the bowl (therefore, the

water/cement coefficient w/c will be equal to 0.4). The water should be added to the cement

and mix together by at least 3 minutes. The obtained paste should be quantitatively transferred

(using a spoon if needed) to the measuring cone. The measuring cone should be placed


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(narrow part up) on the glass plate. The cone should be now vigorously lift up, allowing the

paste to flow freely. After 30 seconds, the diameters of the flowing paste should be measured

in two perpendicular directions, using a ruler with accuracy to 1 mm. The result of the test is

the arithmetical average from both measurements. After the test, the cement paste should be

totally removed from the glass plate to the waste bin (not to the sink!). The glass plate and the

measuring cone should be clean up from the paste residues.

Then, 190.0 g of tap water should be weighed in the beaker and 500.0 g of portland

cement should be weighed in the bowl. 10 cm3 of the admixture A should be taken using the

pipette, added to the water in the beaker and carefully mixed. Then, the water with admixture

should be added to the cement in the bowl. The next operations should be the same as

previously. The diameter of the flow of the cement paste with the admixture should be

measured.

The tests should be repeated in the same way using the admixtures B and C.

Note: The number and type of the admixtures can vary from classes to classes.

The results of testing should be presented in the table 12.2. On that base the efficiency

of the tested admixtures should be compared.

Table 12.2

Diameters of the flow of the tested cement pastes Admixture – A B C

Diameter of the flow, mm

PRACTICAL TASK 2. EFFECTIVENESS ASSESSMENT OF CONCRETE SURFACE HYDROPHOBIZATION

The equipment need for the task:

brush, pipette or dropper, stop watch.

Reagents and other materials used:

hydrophobizing means,

concrete or mortar plates or beams,

distilled water.

Task performance The effectiveness measure of hydrophobization is the wettability of the protected

surface (Fig. 12.4). An ill-protected substrate easily and quickly absorbs water. In order to

define the effectiveness of protection, tested materials should be placed on the surface of

beam with a brush; to avoid mistakes, each beam should be carefully marked and the name of

the applied mean written down. After 15 minutes a second coat of preventive agent may be

applied if needed. After the surface has dried well, the proper test is carried out. A drop of

distilled water is placed on a protected surface with a pipette or dropper. One should notice

the shape that the drop assumes on the protected surface and measure the time until it

disappears. Five such tests should be done for every surface. Measurements taken for surfaces

protected by different means should be compared with analogous observations of an

unprotected surface and the effectiveness of hydrophobization assessed.


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Fig. 12.4. Wettability

poor good medium

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