1

Chemical admixtures:

Essential Components of

Quality Concrete

C. Jolicoeur, N. Mikanovic, M.-A. Simard and J. Sharman

Modern-day concrete frequently incorporates one or more

chemical admixtures to achieve specified material

properties. In a context where binder systems have become

increasingly complex, either due to addition of pozzolans

(for example, silica fume), or partial cement replacement by

supplementary cementitious materials (fly ash, blast furnace

slag, etc), or addition of fillers, and where concrete

performance requirements are increasingly demanding,

chemical admixtures are rapidly gaining in importance and

in diversity. For concrete practitioners, the variety of

concrete admixture types, and the diversity of admixtures

within each type, create a rather complex environment. This

paper attempts to present an overview the field of chemical

admixtures and provide some perspective on the need for

these admixtures, their function and benefits in application.

A particular emphasis is given to those admixtures — water

reducing and colloidal — which influence the rheological

properties of fresh concrete.

In recent years, the use of chemical additives in concrete has

grown considerably, for virtually all types of concrete and

their applications. These additives commonly referred to as

'concrete chemical admixtures' now comprise a wide variety

of chemicals which are usually introduced during the

batching of concrete (occasionally after the batching

process, or before, in the dry cementitious binder) to

enhance specific properties of the fresh or hardened

concrete material. This rapidly expanding field has already

been the subject of several monographs and it is consistently

monitored by various standards and regulatory agencies

(ASTM, ACI, RILEM, etc), which overview testing

methods and codes of practice1-8

.

Concrete chemical admixtures are usually classified

according to the specific functions they are intended to

perform; hence, the following groups of admixtures are

commonly designated:

water-reducing admixtures

set modifiers (retarders, accelerators)

air-entraining agents

anti-bleeding/segregation admixtures

corrosion inhibitors

curing and shrinkage (drying) reducing admixtures

water-proofing admixtures

anti-freezing admixtures

admixtures controlling alkali-aggregate reaction

(AAR)

Each of these families of concrete admixtures may comprise

sub-classes, for example: low-range and high-range water-

reducing admixtures (WR and HRWR). While the general

function of each admixture group is obvious, some

admixtures may exert more than one function, for example,

water-reducing and set-retarding. Their ultimate role and

impact on concrete materials and technologies can be best

appreciated through an overview of the main requirements

for durable concrete and the current trends in construction

practices.

Indian Concr. J. 76 (2002) 537-549.

2

Need for admixtures

The pressing need for higher quality concrete (in terms of

strength and durability) inevitably points towards low

porosity materials; these imply minimum water content and

the use of ultra-fine supplementary materials (fillers or

pozzolans). In order to meet these requirements, while

retaining adequate concrete workability, high-range water

reducers must be employed. The material requirement is

exacerbated by the evolution of concrete handling and

placing technologies, which now largely favour the use of

flowing (pumpable) concrete.

In other recent developments, the requirements on the

properties of flowing concrete have been raised to achieve

'self-placing' and 'self-levelling' abilities. The latter are to

ensure that fresh concrete can flow with near-zero yield

stress in order to reach all areas, regardless of the degree of

congestion, and yield level surfaces requiring minimum

finishing. The self-placing requirement implies concrete

with a high fluidity, which will then be prone to bleeding

and segregation. To minimise the latter effects, admixtures

have been designed, the main function of which is to

increase the bulk viscosity of the cement paste at low shear

rate, in order to minimize water migration and segregation

effects, that is anti-bleed/segregation, or colloidal

admixtures.

As emphasized in recent literature, shrinkage-related effects

represent another significant limitation to concrete strength

and durability 9. Whether the shrinkage is autogeneous, or

the result of capillary pressure as the solution leaves the

micropores (drying shrinkage), the phenomenon increases

the microcracking tendency of the concrete, particularly at

early ages. To minimise these effects, an optimal control of

water migration in the capillary pores of the cement paste

during setting and hardening is essential. This may be

achieved through a combination of in-situ 'curing'

admixtures incorporated in the cement paste, and a proper

moisture supply during the drying and hardening phases.

Numerous other admixtures find use in concrete, exerting

specific functions in certain types of applications such as:

air-entraining admixtures to protect concrete from

internal stress due to volume changes during the

freezing and thawing of the concrete pore solution;

corrosion inhibitors to minimise degradation of steel

reinforcement in elements exposed to harsh

environments;

AAR control admixtures to minimise expansion

from the reaction of siliceous aggregate with lime,

leading to bulk deterioration of concrete elements;

water-proofing admixtures to minimise water

permeation and uptake by the hardened concrete.

Another important development in the cement and concrete

industry is a major current drive to minimise the amount of

cement used in concrete mix, largely through the

replacement by supplementary cementitious materials,

mostly fly ash and blast furnace slag. This effort produces

several significant benefits: enhancement of concrete

durability; reduction of CO2 emission associated with the

production of portland cement; beneficiation of secondary

industrial materials. This widely expanding practice

involves a significant increase in the chemical complexity of

the binder system, which affects all properties of concrete:

rheological, bleeding and segregation behaviour, setting,

strength development, etc. This further enhances the need

for chemical admixtures, in order to accommodate changes

in the binder system and to optimise concrete mix design.

Critical admixtures for high performance

materials

The above overview on the need for concrete chemical

admixtures may be further focussed by examining the

concrete features which are most critical for achieving high

performance, that is, high strength and durability. This, in

turn, will identify corresponding ‗critical admixtures‘.

As is now widely realised, the properties of concrete which

determine high performance (as defined above) include:

rheology of fresh concrete; homogeneity in the plastic and

hardened states10

; porosity; dimensional stability through

the setting and hardening stages. Beyond variations in

concrete mix design parameters, these properties may be

controlled and optimised by various combinations of

admixtures typically:

rheology: HRWR and colloidal admixtures

homogeneity: HRWR and colloidal admixtures

(viscosity enhancing admixtures)

porosity: WR and HRWR

dimensional stability: shrinkage-reducing

admixtures (in-situ curing agents)

This promptly suggests that water-reducing admixtures,

colloidal agents and shrinkage control admixtures are three

‗critical‘ groups of admixtures for the design and production

of high quality concrete. For concrete in cold weather

environments, air-entraining agents must also be considered

as a critical admixture.

The present paper focuses on two of these critical

admixtures: water reducers and colloidal admixtures. As

noted above, these admixtures jointly determine the

rheological properties and the stability of fresh cement-

based systems, as well as the porosity of the hardened

materials. It is interesting that these two types of admixtures

are, in some ways, antagonists: water-reducers act to

fluidise the system, while colloidal admixtures have

thickening properties (viscosity enhancement). A brief

overview of their respective chemistry, mode of action,

3

performance evaluation methods and performance in

application is given below.

Water-reducing admixtures (WR and

HRWR)

Function

As is now universally accepted, the mechanical properties of

concrete are directly related to the porosity (total porosity,

pore structure) of the binder matrix. Since concrete porosity

is highly connected, the permeation of water and other

substances into the concrete matrix is also dependent on

porosity, with obvious consequences on concrete durability

(air voids generated intentionally with air-entraining agents

are not interconnected).

The complete hydration of portland cement requires

approximately 30 percent water. Any water added beyond

this level (say, 40-60 percent), will leave a corresponding

level of capillary porosity. Since, in these systems, the

volume fraction of water is approximately three times its

weight fraction, the pore volume due to excess water can be

very significant. Hence, minimisation of the excess water

through the use of water-reducing chemical additives is

understandably an important issue in concrete technology.

Many types of chemicals, mostly organic compounds, can

reduce the water requirement for a given concrete

workability. In broad terms, these admixtures enhance the

deflocculation and dispersion of cement particles, reducing

the apparent viscosity of the mix; often, such admixtures

temporarily repress the hydration reactions (retardation),

providing an extended control of the concrete workability 4.

Chemical nature and classification

Chemical additives which can perform as concrete water-

reducing admixtures have been classified by ASTM into

two broad categories according to their effectiveness

(ASTM C-494): water-reducers (WR, type A), from 5

percent water reduction to high-range water reducers:

(HRWR, type F) water reduction of more than 12 percent.

Currently employed HRWR exhibit much higher water

reduction, typically up to 30 percent; these are now

commonly referred to as 'superplasticisers'. Water-reducing

admixtures may be further subdivided into specific classes

according to their influence on setting times, for example

type D (WR and retarding), type E (WR and accelerating)

and type G (HRWR and retarding).

Commonly used WR admixtures comprises mainly sugars

and sugar derivatives (gluconates, hydroxy acids), often by-

products recovered from the agricultural and food

processing industries. Lignosulfonates (sodium or calcium

salts), a by-product of the bisulphite wood pulping process

is also widely used as concrete water-reducer 11

. The latter

consists of sulphonated polymers of substituted phenyl

groups connected by short alkyl, or ether linkages. Due to

the inherent molecular complexity of natural lignin, and

because of the non-specific character of the lignin

sulphonation / de-polymerisation / solubilisation reactions,

the admixture consists of a complex mixture of molecular

species. Fig 1 schematically illustrates the chemical

structure of gluconates and lignosulfonates

The class of HRWR, or superplasticisers, comprises a

variety of synthetic water-soluble organic polymers bearing

ionisable groups, mostly sulfonates (SO3) and / or

carboxylates (COO). The most widely used of such

polymers is polynaphthalene sulphonate (PNS) (sodium, or

calcium salt), the average chemical structure of which is

illustrated in Fig 2(a), together with a schematic

representation of the polymer as used in later illustrations. A

second important type of sulphonated superplasticiser

polymer (not shown) is polymelamine sulphonate (PMS). In

recent years, a wide range of polyacrylate co-polymers has

been proposed as HRWR. A major subgroup of the latter

comprises a variety of polyacrylate molecules partly

esterified with poly-ethyleneglycol side chains; hence their

designation as polyacrylate esters (PAE). Their chemical

structure and schematic representation is shown in Fig 2(b).

The water-reducing ability of PNS superplasticisers depends

on their molar mass — the higher the mass, the better their

performance. The weight average molar mass of

commercial PNS has been determined to be between 10 and

65 kDa 12

. For PAE-type superplasticisers, the performance

is not directly related to the molar mass, since many other

molecular parameters can influence the performance. For

various PAE, weight average molar masses between 20 and

100 kDa have been reported 13

O

H3CO

O

OH

H3CO

SO3Na

HO

nLignosulfonate

CH2

H

OH

H

OH

OH

H

H

OH

C ONa

O

HO

Gluconate

Fig 1 Schematic illustration of gluconates and

lignosulfonates water reducers

4

Mode of action

As noted above, WR and HRWR act to deflocculate and

disperse cement particles, thus enhancing the fluidity of the

cement paste. The molecular mechanisms through which

WR and particularly HRWR can disperse cement particles

in the paste are illustrated in Fig 3. The WR molecules

adsorb onto the surface of the hydrating cement grains,

conveying to these surfaces a negative electrical charge

(potential). The latter generates an electrostatic repulsion

between neighbouring cement particles, promoting

deflocculation and dispersion of these particles; the

phenomenon is depicted schematically in Fig 3(a) for PNS

molecules for which electrostatic dispersion is important.

In addition to the electrostatic forces, the dispersion of

particles is further assisted by repulsive forces originating in

'steric effects': the adsorbed polymer (neutral or charged)

constitutes a physical barrier to particle-particle contact, that

is, when the dangling chains of polymer adsorbed on two

adjacent surfaces begin to entangle, the resulting loss in

their entropy is highly unfavourable 14

. This is illustrated

schematically in Fig 3(b) with PAE molecules for which the

steric repulsion is likely to be dominant.

The physical effects underlying the mode of action of

HRWR, as described above, are complemented by 'chemical

effects', as illustrated in Fig 3(c). The chemical specificity

in HRWR is first manifested in the adsorption process; for

example, sulphonate-based polymers interact preferentially

with the aluminate phases of the cement, that is, by analogy

with the sulphate ions. In addition, several types of WR and

HRWR molecules are known to inhibit the nucleation and

growth of hydration products (gypsum, ettringite, or other

hydrates). While this may not directly improve dispersion of

cement particle, it prevents their reagglomeration and thus,

preserves the rheological properties of the cement paste.

Through specific chemical interactions, water-reducers may

exert set retardation effects depending on the nature of the

admixtures and the dosage. Lignosuphfonates and PAE-type

admixtures are inherently more retarding than PNS or PMS;

in practice, the retardation must be compensated through

addition of accelerating admixtures.

Water-reducing admixtures, by virtue of their combined

hydrophilic-hydrophobic character, can frequently act as

surfactants (soap) and induce excessive air entrainment in

concrete. Again, lignosulphonates and PAE admixtures are

inherently more active to entrain air than PNS- and PMS-

type compounds. This undesirable effect can usually be

minimised by the use of defoaming admixtures.

Performance evaluation

The performance of water-reducing admixtures is evaluated

from observations of their influence on the rheological

properties of the fresh cementitious system, grout, paste,

mortar, or concrete. For the complete description of

systems, as warranted in the development or optimisation

work, the detailed rheological behaviour (that is, shear stress

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(a) (b)

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- ----=

>

CH2

SO3Na

n

=

>

- - - - - - - - - - - -

CH2 C

R1

COONa

CH2 C

R1

CO

O

n m

R

CH2 C

R1

X o

(a)

(b)

Fig 2 Schematic illustration of commonly used

superplasticizers; (a) polynaphthalene sulphonate (PNS)

(b) polyacrylate ester (PAE)

Fig 3 Mode of action of HRWR; (a) electrostatic

repulsion illustrated with PNS (b) steric repulsion

illustrated with PAE (c) inhibition of nucleation and

growth processes, either from a reacting surface or from

solution.

5

versus shear rate) can be determined through appropriate

viscometers. With pastes and grouts, various commercial

rheometers are adequate for studies on small samples;

however, relevant measurements on mortars and concrete

require specially-designed rheometers 15-20

. For comparative

evaluation purposes, numerous methods have been designed

which rely on the measurement of one or more parameter

related to flow, in ways more or less related to the universal

concrete slump test (ASTM C 143).

With grouts, the Marsh flow cone method (flow time

through a funnel) provides a reliable means to compare

admixtures and determine their optimal dosage (that is,

saturation dosage) 10

. The method reflects the grout viscosity

at intermediate shear rates.

For pastes, the most commonly-used test for relative

evaluation of admixtures is the ‗mini-slump‘ test — a

miniature version of the standard concrete slump test 10,21

. In

the mini-slump test, the flow-spread area (or diameter) is

measured, which relates to viscosity under very low shear

rates. A series of results on Type-10 and silica fume cement

pastes with a PNS superplasticiser, obtained as function of

concentration and temperature, is illustrated in Fig 4 22

.

For the relative evaluation of water-reducing admixtures in

mortars, an impact flow table is widely used (ASTM C

230); in the latter, the flow spread from a normalised cone is

measured following a fixed number of impacts onto the

table. This method reflects mortar flow starting from zero

shear rate, thus emphasising yield stress.

For comparative, or screening purposes, the performance of

HRWR is readily evaluated through the mini-slump test on

cement pastes. Remarkably, the results of the latter,

obtained as function of dosage and time, accurately predict

the trends in the slump values and the slump retention of

fresh concrete.

Performance in application

Typical mini-slump data comparing three superplasticisers

in pastes having w/c of 0.30 and 0.20 respectively are

shown in Fig 5 23a

. The results demonstrate two important

features of these admixtures:

(i) PAE-type superplasticizers are more effective than

the reference PNS (or PMS) at the same dosage;

(ii) the benefit of PAE-type admixtures is particularly

evident at very low w/c (and relatively low

admixture dosages). These trends are generally

observed in concrete data as well, the differences

being again most noticeable at very low w/c.

Selected results on ultra-high strength concrete ( >150 MPa)

are illustrated in Fig 6 23b

: again, at comparable dosages (in

this case very high, 2 percent), PAE-type admixtures

provide higher concrete fluidity and slump retention than

the PNS and PMS-type admixtures. Other concrete data

0

50

100

150

200

0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2%

PNS Dosage (Wt% dry basis)

Min

i-sl

um

p a

rea (

cm

2)

OPC

OPC + SF

40°C

20°C

Fig 4 Mini-slump spread in T-10 portland cement and

HSF cement pastes shown at 10 minutes, as a function of

temperature and superplasticiser (PNS) dosage, w/c =

0.35 (Adapted from 22)

0

50

100

150

200

250

300

350

400

0 0.2 0.4 0.6 0.8 1

SP dosage (Wt%, dry basis)

Past

e f

low

(m

m) PE W/C 0.3

PE W/C 0.2

PC W/C 0.3

PC W/C 0.2

NS W/C 0.3

NS W/C 0.2

Fig 5 Paste flow data obtained on the same cement paste

with several different HRWR as a function of dosage

and w/c; NS: PNS, PC: polycarboxylate, PE: polyether.

(Adapted from reference 23a)

0

5

10

15

20

25

30

0 30 60 90 120 150 180 210 240

Time (min)

Slu

mp (

cm

)

CAE

NSF

MSF

Fig 6 Ultra-high strength concrete slump in the presence

of various superplasticisers at 2 percent dosage, w/c =

0.225; NSF= PNS, MSF= PMS, CAE= PAE (Adapted

from 23b)

6

have been reported comparing different HRWR at their

respective saturation dosage (determined from Marsh cone

fluidity), rather than at equal dosages. As the results show in

Fig 7, under these conditions, all the superplasticisers tested

showed similar performances 24

.

The actual performance of different types of HRWR in

specific cementitious systems is generally found to depend

on numerous variables, either inherent to the admixture

itself, or to the properties of the binder system (for example,

cement type, fineness, chemical composition, etc.). This

combined variability may lead to incompatibility situations

(cement with HRWR, or HRWR with other admixtures),

such as that due to soluble sulphates; through competing

interactions, the latter plays an important role in the

adsorption equilibrium of PNS, or of PAE-type admixtures

25,26.

In broad terms, the potential of HRWR admixtures can be

exploited along several practical schemes 11, 27

. The

schematic diagram reproduced in Fig 8 first illustrates how

the water reduction induced by the HRWR can be applied

either, to increase the slump of the concrete mix to obtain a

more fluid concrete, or to reduce the water content at

constant slump to achieve a higher quality concrete. A more

detailed inventory of the possibilities for exploiting the

beneficial effects of HRWR was outlined by Collepardi and

may be summarized as 11

:

Improving the workability of a given mix for ease

of placing, particularly in congested areas

Improving strength and durability at a given

workability by reducing water content

Reducing cement content for fixed strength and

workability, or for producing ‗low heat‘ concrete

Increase cement content at low w/c, while adding

other pozzolans (that is, silica fume) to produce

high performance concrete.

Considerable literature is now available on the use of WR,

particularly superplasticisers in many different types of

concrete applications.

Prospective developments

WR, especially HRWR/superplasticisers, have enabled a

quantum leap in the development of concrete technologies.

The search for new, more cost effective, admixtures in this

group will obviously continue, but the influence of new

admixtures is likely to be less dramatic than the impact of

the few key products which supported the early

development, namely, PNS and PMS. Future work in this

field is likely to yield more incremental benefits to cope

with shortcomings of currently available admixtures, as well

as the requirements from development in cement and

concrete technology; such products would typically:

have prolonged influence on concrete rheology

exert minimal effect on the cement hydration

reactions and set

be tolerant towards variations in the binder

composition (fly ash, slag, others)

be compatible with a maximum variety of other

chemical admixtures

be applicable in a broad range of different types of

concrete and applications.

Given the rapid evolution of binder systems, the inherent

composition variability of these systems, and the increasing

variety of concrete applications, it can readily be predicted

that a key requirement of future chemical admixtures will be

their ability to perform under the widest possible range of

parameters (binder composition, temperature, other

admixtures), that is, tolerance, robustness.

100

150

200

250

300

0 10 20 30 40 50 60 70

Time (min)

Slu

mp (

mm

)

N

M1

M2

CAE

Fig 7 HPC concrete slump for different superplasticiser

at saturation dosage, w/c 0.30; N= PNS, M= PMS, CAE=

PAE. (Adapted from reference 24)

30

40

50

60

120 140 160 180 200 220 240

Water content (kg/m3)

Flo

w t

able

spre

ad (

cm

)

with SP

without SPIncreased

strength

Increased

workability

Fig 8 Illustration of the use of WR admixtures: diagram

mapping the range of slump-strength (water content)

conditions achievable (Adapted from reference 27)

7

Colloidal admixtures

Function

As noted above, the increasing use of flowing, self-placing

(self-compacting, self-consolidating) concrete calls for

highly fluid mixes, which are inherently more prone to

separation effects: bleeding, surface segregation,

sedimentation of aggregate, segregation of aggregate

according to size and density, etc. This situation can be

alleviated through the use of additives commonly referred to

as ―colloidal admixtures‖. The latter include a variety of

products capable of enhancing the viscosity of concrete —

hence their alternate designation as viscosity-enhancing

admixtures (VEA). Their function is to maximise concrete

viscosity, in order to oppose any separation/segregation,

while retaining the desirable features of the free-flowing,

self-placing concrete. Since colloidal admixtures increase

the cohesion of cement-based systems in general, they can

also function as anti-washout admixtures (for underwater

concreting), or to reduce rebound and sagging in shotcrete

applications.

Chemical nature and classification

The type of admixtures which can impart the desired

rheological properties to self-placing concrete are currently

hydrophilic, water-soluble polymers having high molecular

weight. Under appropriate conditions, such polymers can

form a network of large molecules extending throughout the

mass, analogous to that found in a gel. Very small

interacting particles (in the nano-micrometer range) can also

generate gel-type behaviour. The dimensions of the

polymers or particles suitable for this type of application are

in the colloidal range, hence their designation as ‗colloidal

admixtures‖.

In comparison to water reducing-admixtures, colloidal

admixtures (viscosity-enhancing, anti-washout) are

relatively new; the main groups of polymers currently

proposed or used in such application are listed below 28

:

(i) Natural polymers including starch, welan gum,

xanthan gum and other natural gums, as well as

plant proteins.

(ii) Semi-synthetic polymers that include modified

starch and its derivatives, cellulose-ether

derivatives, such as hydroxypropyl methyl

cellulose (HPMC) and carboxy methyl cellulose

(CMC), sodium alginate and propylene glycol

alginate.

(iii) Synthetic polymers such as polyethylene oxide,

polyacrylamide, polyacrylate, and polyvinyl

alcohol.

As noted earlier, the stability of highly fluid cementitious

systems may also be improved by incorporating materials

which contain colloidal size particles such as fly ash, finely

divided calcium carbonate, blast furnace slag, diatomaceous

earth, etc, or materials having high surface area, or unusual

surface properties such as very fine clays, silica fume,

colloidal silica, milled asbestos 29

.

Colloidal admixtures commonly used in cement-based

systems include derivatised cellulose (ethers) and

polysaccharides from microbial sources, such as welan gum.

The latter is an anionic, high molecular weight (~2.x10

6

g/mol) polysaccharide. The molecular structure of welan

gum is illustrated in Fig 9(b). Cellulose derivatives have

molecular weights between 105 – 10

6 g/mol. The structure

of non-ionic cellulose ether, which is a principal component

in cellulose-based colloidal admixtures, is shown in Fig

9(a).

An important feature of these polymeric admixtures is their

variable stability and performance under high pH conditions

and in the presence of calcium ions, or at high temperature.

In some cases, auxiliary agents are added which react with

the polymer molecule to increase its molecular weight,

thereby improving its cohesion-inducing properties.

Mode of action

Polymeric colloidal admixtures increase the cohesiveness of

cementitious-based systems through a combination of

several effects, depending on the type and concentration of

the polymer used; three contributing mechanisms, illustrated

in Fig 10, have been proposed, which may be summarised

as 30, 31

:

(a)

(b)

R= -CH3 : Methyl Cellulose

R= -CH2 – CH2 – OH : Hydroxyethyl Cellulose

R= H : Cellulose

Fig 9 Molecular structure of common colloidal

admixtures. (a) cellulose ethers; (b) Welan gum.

8

(i) Water adsorption: The hydrophilic polymer

molecules adsorb free water molecules; in doing

so, they tie down a part of the mixing water and

their apparent volume increases by swelling.

(ii) Association: Adjacent polymer chains can

develop attractive forces, resulting in the

formation of a gel-like network, thus further

blocking the motion of water, and increasing the

viscosity of the whole system.

(iii) Entanglement: At low shear rates, and especially

at high concentrations, the polymer molecules can

entangle, resulting in an increase in the apparent

viscosity.

In addition to viscosity enhancement effects, long-chain

viscosity agents such as cellulose derivatives may also

behave as flocculating agents, that is, adsorbing

simultaneously onto neighbouring cement particles forming

a bridged structure 32, 33

. This effect results in enhanced

particle-particle interactions and the formation of flocs in

which free water may be entrapped. Since this flocculation

effect opposes the dispersion function of HRWR used in

flowing concrete, the use of non-adsorptive colloidal agents

is generally preferable.

As with water-reducers, colloidal admixtures may exert

secondary effects on the behaviour of the cementitious

system. Due to their sugar-based structures, polysaccharide-

or cellulose-type admixtures can delay cement hydration

and increase the concrete setting time. The effect depends

on the type and concentration of the colloidal admixture, the

type and dosage of HRWR, as well as on the cement

composition and w/c 28-30, 34-36

.

Also, by analogy to water reducers, some cellulose

derivatives, for example, hydroxy-propyl methyl cellulose

and some synthetic polymers such as polyethylene oxide

may entrain substantial volumes of air in fresh concrete.

This effect can be counteracted with an appropriate de-

foaming agent 28

. Concrete containing colloidal admixture

generally requires higher dosages of air-entraining

admixtures to achieve a given air content 30, 34

.

Performance evaluation

Colloidal admixtures are used to control the rheological

properties of concrete in order to achieve self-levelling, self-

placing properties, and to ensure its stability towards

segregation effects. The performance assessment of

colloidal admixtures therefore requires various methods

which aim to measure the following properties: free flow,

constrained flow, dynamic stability and static stability. As

with water-reducing admixtures discussed above, the

detailed rheological influence of colloidal admixtures can be

described from studies of their shear stress versus shear rate

curve (or stress amplitude versus rate) using specially-

designed rheometers 18-20

. For relative practical

comparisons, numerous other methods have been proposed.

The free flow behaviour of concrete can be determined

through Abram‘s cone (ASTM C 143) or through a L-

shaped container, while measuring both the spread area and

the spreading time 37

. To measure constrained flow, several

devices have been reported, which comprises various types

of geometries and obstacles intending to create conditions

relevant to adverse field situations, for example, high re-bar

congestion 38,39

. The flow time, and filling capacity of the

concrete in these devices provide a measure of their ability

to perform as self-compacting concrete.

To evaluate the dynamic stability of concrete, and the

influence of colloidal admixtures on this property, a simple

test has been proposed which involves evaluation of the

paste-aggregate adhesion through aggregate sieving. The

static stability of concrete, particularly segregation effects,

is more difficult to quantify in the fresh state. The extent of

compaction of the concrete can be evaluated through a test

which relates to the behaviour of the fines in the

cementitious system 40

. The overall static stability in terms

of bleeding, compaction and segregation can also be

solvation

and swelling

gel

formation

molecular

entanglement

Fig 10 Schematic illustration of mechanisms

contributing to the function of colloidal admixtures (a)

water adsorption and retention (b) gel formation

(c) molecular entanglement.

(a)

(b)

(c)

9

determined through a multiple-electrode conductivity

method develop recently 41

.

Performance in application

The main function of colloidal admixtures is to enhance the

cohesion and viscosity of fluid concrete at low shear rate,

while maintaining a relatively low resistance to flow at high

shear rates, such as encountered during mixing, pumping

and casting. In rheological studies, the colloidal admixtures

were found to raise the yield stress value, and increase the

apparent viscosity at all shear rates 34,35,42

. However, systems

modified with a colloidal agent exhibit a shear thinning (or

pseudoplastic) behaviour, that is, the apparent viscosity

decreases with increasing shear rate. The importance of this

effect depends on the type and dosage of the polymer; it can

be seen in more pronounced manner for mixes containing

welan gum, than for those containing cellulose derivatives,

under the same conditions 34,43

. Pseudoplastic behaviour is

believed to be the result of disentanglement of polymer

molecules and their alignment in the direction of flow at

higher shear rates.

In practice, highly flowable cement-based materials with

adequate resistance to bleeding, segregation, settlement,

etc., may only be obtained by proper combinations of

colloidal admixtures and HRWR. In view of the partially

opposing effects of these admixtures, the required dosage of

the latter may increase with increasing dosage of the

colloidal admixture 35

.

With respect to HRWR / colloidal admixture combinations,

the cellulose-based colloidal admixtures yielded erratic

cohesion and flowability behaviours with PNS-type HRWR,

but were compatible with PMS-type admixtures 29,44-46

.

Similar studies carried out with welan gum showed no

apparent incompatibility with either melamine-based or

naphthalene-based HRWR 46

.

Fig 11 illustrates the function and performance of colloidal

admixtures, used jointly with HRWR 42

. The apparent

viscosity measured at two different shear rates (8 and 256

rpm) are illustrated for the pure cement paste, the paste with

the HRWR or colloidal admixture added separately, and the

paste containing both admixtures. As is readily evident, the

influence of the admixtures is most pronounced at low shear

rate, the pastes exhibiting considerable shear thinning. It is

also apparent that, in this case, the combination of

admixtures exhibits synergy with respect to the increase in

apparent viscosity, that is, the viscosity increment for the

combination is higher than the sum of the individual effects.

A second illustration of the application of colloidal

admixtures on high fluidity concrete is taken from

investigations by Khayat et al. on bleeding, segregation and

overall stability of self-compacting concrete 41

. Fig 12

shows the variations of external bleeding (ASTM C-232)

and the segregation of the coarse aggregate of two

traditional flowable concretes (C1 and C2) and two highly

flowable self-compacting concretes (C3 and C4) containing

welan gum as colloidal admixture (C4 is typical of materials

used for underwater concreting).

The C1 and C2 mixtures had similar compositions, as well

as the C3 and C4 mixtures, except for w/c values and the

concentrations of chemical admixtures; the latter were

0

500

1000

1500

2000

8 rpm 256 rpm

Appare

nt

Vis

cosi

ty (

cP

)

C

CS

CG

CGS

C CS CG CGS

Fig 11 Apparent viscosity of pastes in the presence of

HRWR and of a colloidal admixture; C: cement, S: PNS,

G: polysaccharide gum (Adapted from reference 42)

0

0.01

0.02

0.03

0.04

20 220 420 620

C2 : W/C = 0.55 Slump = 220 mm

C4 : W/C = 0.48 Slump flow = 650 mm

Exte

rna

l bee

din

g (

ml/

cm2)

Elapsed time (min.)

C1 : W/C = 0.41 Slump = 220 mm

C3 : W/C = 0.42 Slump flow = 650 mm

C2 : W/C=0.55; 0.15% HRWR

Slump =220 mm

; 0.5% HRWR

C4 : W/C=0.48; 0.39% HRWR +

0.14% welan gum

Slump flow = 650 mm

C3 : W/C=0.42; 0.50% HRWR + 0.14%

welan gum; Slump flow = 650 mm

20 25 30 35 40

0

10

20

30

C3 (1.9%)

C4 (3.1%)

C1 (segregation coeff. = 4.3%)

C2 (6.0%)

5

15

25

Percentage of aggregate • 5 mm

Dis

tan

ce f

rom

bott

om

(c

m)

Fig 12 Relative stability of fluid concrete mixes. C1-2:

flowing concrete, C3-4: self levelling concrete. Top:

External bleeding versus time, Bottom: Seggregation of

aggregate. (Adapted from reference 41)

10

adjusted to allow comparisons between two flowable (250

mm slump) and two highly flowable (650 mm slump flow)

concretes. It can be seen that the external bleeding is much

lower and it occurs significantly later for self-compacting

concretes containing welan gum. Also, the mixtures with

greater w/c exhibit higher bleeding. The relative segregation

behaviour of these materials followed the same pattern as

the bleeding rates; mixtures containing welan gum were

found to be more stable overall.

In the same study, the vertical heterogeneity resulting from

bleeding and segregation phenomena was monitored

continuously in the fresh concrete samples through an in-

situ conductivity method developed recently 41

. From

conductivity measurements taken at different levels and as

function of time, the stability of each system is reflected by

the standard deviation of the conductivity values, ,

relative to the average conductivity; an apparent stability

index can thus be defined as Is= (1- / av). Through this

approach, the stability of cementitious systems can thus be

assessed quantitatively in real time and in a non-disruptive

manner.

Prospective developments

The beneficial impact of colloidal agents on the flowing,

pumping, filling capacity, bleeding, segregation, and other

properties of self-placing concrete are now widely

acknowledged. The main obstacle to their broader

acceptance appears to be their cost 47,48

. However, savings

from increased productivity, improvement of working

environment, improved service life and aesthetic are also

recognised and may actually override the increased initial

material costs for production of self-compacting concrete. In

this context, new low-cost colloidal agents such as modified

starch, precipitated silica and new biopolymer molecules are

actively being developed and introduced to the concrete

industry 49-53

.

A complementary approach to cost reduction of these

systems is through incorporation of large volume of fine

powdered materials such as fly ash, silica fume, limestone,

blast furnace slag, etc. to decrease cement content and

admixture demand 36,54-56

. It has been shown, however, that

highly flowable concretes formulated without colloidal

admixtures can be more sensitive to variability to its

formulation, raw materials and other factors such as

aggregate moisture content, cement fineness, temperature,

etc 57

.

A more technical limitation to the development of new

chemical admixtures and formulations of highly flowable

cement-based systems, as well as the general adoption of

these materials as mainstream construction materials, is the

absence of normalised test methods, especially for assessing

their resistance to segregation 47,58

. Considerable efforts are

thus required to develop testing/monitoring methods, as well

as guidelines or standards, to ensure adequate

characterisation of these materials and support their

optimum application. The multiple-probe conductivity

method referred to above for in-situ monitoring of bleeding,

segregation and strength development seems a promising

development 41,59,60

.

Conclusion

Given their important functional properties and their rapidly

increasing acceptance, chemical admixtures may rightly be

considered an integral, even an essential part of modern-day

concrete. This is particularly evident for some types of

admixtures such as HRWR and colloidal admixtures; used

in conjunction, these two families of admixtures allow ―fine

tuning‖ of the rheological properties and stability of flowing

concrete, even at the low water contents required for

optimum performance and durability.

The diversity of cementitious systems currently in use or

development, as well as the rapidly growing variety of

chemical admixtures proposed in each category of

admixtures, lead to a broad range of physical and chemical

interactions, some of which have been shown to be

deleterious (for example, incompatibility of cement-

admixture pairs, or between admixtures). Hence, to reap

adequate benefit of these ongoing developments, and

minimise any negative impact, researchers must

continuously seek to elucidate the phenomena in which the

various admixtures take part in cementitious systems, while

developing more efficient diagnostic tools. Concrete

practitioners should, on the other hand, resort to extensive

QC of the concrete components and performance testing on

reference mixes.

Acknowledgements

The authors gratefully acknowledge the financial support of

the Natural Sciences and Engineering Research Council of

Canada, Handy Chemicals Ltd and the Institute for

Intelligent Materials and Systems of the Université de

Sherbrooke. Numerous fruitful discussions with Prof Kamal

Khayat and Dr Monique Pagé are also gratefully

acknowledged.

References 1. Concrete Admixture Handbook. V.S. Ramachandran ed., Noyes

Publications, Second Edition, 1995,1153 p.

2. Rixom, R and Mailvaganam, N. Chemical Admixtures for Concrete.

E & FN Spon, Third Edition, 1999, 417 p.

3. Dodson, V. Concrete Admixtures. Van Nostrand Reinhold, 1990, 211 p.

4. Ramachandran, V.S., Malhotra, V.M., Jolicoeur, C. and Spiratos, N.

Superplasticizers: Properties and Applications in Concrete. CANMET, 1998, 404 p.

5. Admixtures for Concrete: Improvement of Properties. Proceedings of

the International RILEM Symposium, Barcelona, 1990, E. Vazquez ed., Chapman and Hall, 586 p.

11

6. Application of Admixture in Concrete. RILEM Report 10, 1995, A.M.

Paillère ed., E & FN Spon, 131 p.

7. The Role of Admixture in High Performance Concrete. Proceedings

of the. International RILEM Conference, Monterrey, Mexico, 1999,

J.G. Cabrera and R. Rivera-Villarreal eds, RILEM Publication S.A.R.L., 506 p.

8. ACI Committee 212, Chemical Admixtures for Concrete. ACI

Manual of Concrete Practice, Part 1, 2001.

9. Nmai, C.K., Tomita, R., Hondo, F. and Buffenbarger J. Shrinkage-

reducing admixture. Concrete International, April 1998, Vol. 20, pp.

31-37

10. Aïtcin, P.-C. High-Performance Concrete. E & FN Spon, 1998, 591

p.

11. Collepardi, M.M. Water reducer/retarders. Chapter 6 of ref. 1, pp. 286-409.

12. Piotte, M., Bossanyi, F., Perreault, F. and Jolicoeur, C.

Characterization of poly(naphthalenesulfonate) salts by ion-pair chromatography and ultrafiltration. Journal of Chromatography A,

1995,Vol. 704, pp. 377-385.

13. Yamada, K., Takahashi, T., Hanehara, S. and Matsuhisa M. Effect of the chemical structure on the properties of polycarboxylate-type

superplasticizer. Cement and Concrete Research, 2000, Vol. 30, pp.

197-207.

14. Uchikawa, H., Hanehara, S. and Sawaki, D. The role of steric

repulsive force in the dispersion of cement particles in fresh paste

prepared with organic admixture. Cement and Concrete Research, 1997, Vol. 27, pp. 37-50.

15. Banfill, P.F.G. and Gill, S.M. Superplasticizers for ciment fondu. Part 1: Effects on rheological properties of fresh paste and mortar.

Advances in Cement Research, 1993, Vol. 5, pp. 131-138.

16. Lewis, J.A., Matsuyana, H., Kirby, G., Morissette, S. and Young, J.F. Polyelectrolyte effects on the rheological properties of concentrated

cement suspensions. Journal of the American Ceramic Society, 2000,

Vol. 89, pp. 1905-1913.

17. Tattersall, G.H. Workability and Quality Control of Concrete.

Chapman & Hall, 1991, 262 p.

18. Wallevik, O.H. and Gjorv, O.E. Development of coaxial-cylinders viscometer for fresh concrete. Proceedings of the RILEM Colloqium

on Properties of Fresh Concrete, 1990, Chapman & Hall, pp. 213-

243.

19. De Larrard, F., Szikar, J.C., Hu, C. and July, M. Design of a

rheometer for fluid concrete. International RILEM Workshop on

Special Concrete: Workability and Mixing, 1993, pp. 125-134.

20. Beaupré, D. and Mindess, S. Compaction of wet shotcrete and its

effect on rheological properties. Proceedings of the International

Symposium On Spray Concrete, Fagernes, (Norway), pp. 167-181.

21. Kantro, D.L. Influence of water-reducing admixtures on properties of

cement paste - A miniature slump test. Cement, Concrete and

Aggregates, 1980, Vol. 2, pp. 95-101.

22. Jolicoeur, C., Sharman, J., Otis, N., Lebel, A., Simard, M.-A. and

Pagé, M. The influence of temperature on the rheological properties

of superplasticized cement pastes. Proceedings of the Fifth CANMET/ACI International Conference on Superplasticizers and

Other Chemical Admixtures in Concrete, ACI SP-173, Rome, 1997,

V.M. Malhotra ed, pp. 379-405.

23. a) Shonaka, M., Kitagawa, K., Satoh, H., Izumi, T. and Mizunuma,

T.Chemical Structures and performance of new high-range water-

reducing and air-entraining agents. Ibid 22, pp. 599-614.

b) Guerrini, G.L., Biolzi, L., Cassar, L. and Colombet, P.

Superplasticizers and mechanical properties of a very high

performance concrete. Proceedings of the Fifth CANMET/ACI

International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, Supplementary Papers, Rome, 1997, V.M.

Malhotra ed, pp. 1-11.

24. Nkinamubanzi, P.-C., Baalbaki, M. and Aïtcin, P.-C. Comparison of the performance of four superplasticizers on high-performance

concrete. Ibid reference 23 b, pp. 199-206.

25. Kim, B.-G., Jiang, S., Jolicoeur, C. and Aïtcin, P.-C. The adsorption behavior of PNS superplasticizer and its relation to fluidity of cement

paste, Cement and Concrete research, 2000, Vol. 30, pp. 887-893.

26. Yamada, K., Ogawa,S. and Hanehara, S. Controlling of the adsorption and dispersing force of polycarboxylate-type

superplasticizer by sulfate ion concentration in aqueous phase.

Cement and Concrete Research, 2001, Vol. 31, pp. 375-383.

27. Meyer, A. Experiences in the use of Superplasticizers in Germany.

Proceedings of the First CANMET/ACI symposium on

superplasticizers in Concrete, ACI SP-62, Ottawa, 1978, V.M. Malhotra ed., pp. 21-36

28. Kawai, T. Non-dispersible underwater concrete using polymers.

Marine concrete. International Congress on Polymers in Concrete, Brighton, UK, 1987, Chapter 11.5.

29. Mailvaganam, N.P. Miscellaneous admixtures, Chapter 15 of ref 1,

pp. 939-1024.

30. Khayat, K.H. Viscosity-enhancing admixtures for cement-based

materials – An overview. Cement, Concrete and Composites, 1998,

Vol. 20, pp. 171-188.

31. Izumi, T. Special Underwater Concrete Admixtures. Concrete

Engineering, 1990, Vol. 28 (3), pp. 23.

32. Yammamuro, H., Izumi, T., Mizunuma, T. Study of non-adsorptive

viscosity agents applied to self-compacting concrete. Ibid reference

22, pp. 427-444.

33. Nawa, T., Izumi, T. and Edamatsu, Y. State-of-the-art report on

materials and design of self-compacting concrete. Proceedings of

International Workshop on Self-Compacting Concrete, Japan, 1998,

pp. 160-190.

34. Khayat, K.H. Effects of Anti-washout admixtures on fresh concrete

properties. ACI Materials. Journal, 1995, Vol. 92, pp. 164-171.

35. Khayat, K.H. and Yahia, A. Effects of welan gum – high range water

reducer combination on rheology of cement grout. ACI Materials

Journal, 1997, Vol. 94, pp. 365-372.

36. Khayat, K.H., El Gattioui, M. and Nmai, C. Effect of silica fume and

fly ash replacemen on stability and strength of fluid concrete

containing anti-washout admixture. Ibid reference 22, pp. 695-718.

37. Izumi, I., Yonezawa, T., Ikeda, Y. and Muta A. Placing 10 000 m3

super workable concrete for guide track structure of retractable roof

of Fukuoka dome. Proceedings of the Second CANMET/ACI International Symposium on Advances in Concrete Technology,

Supplementary Papers, Las Vegas, 1995, pp. 171-185.

38. Bartos, P. Orifice rheometer as a test for flowing concrete. Proceedings of the Second CANMET/ACI International Conference

on Superplasticizers in Concrete: Development in the Use of

Superplasticizers, ACI SP-68, Ottawa, 1981, V.M. Malhotra ed., pp. 467-477.

39. Ozawa, K., Tangtermsirikul, S. and Maekawa, K. Role of powder

materials on the filling capacity of fresh concrete. Proceedings of the Forth CANMET/ACI International Symposium on Fly Ash, Silica

Fume, Slag and Natural Pozzolans in Concrete, Istambul, 1992, V.M.

Malhotra ed., pp. 121-137.

40. Manai, K. Étude de l’effet d’ajouts chimiques et minéraux sur la

maniabilité, la stabilité et les performances des bétons autonivelants.

MSc thesis, Université de Sherbrooke, 1995, 182 p.

12

41. Pavate, T.V., Khayat, K.H. and Jolicoeur, C. In-Situ conductivity

method for monitoring segregation, bleeding, and strength development in cement-based materials. Proceedings of the Sixth

CANMET/ACI International Conference on Superplasticizers and

Other Chemical Admixtures in Concrete, ACI SP-195, Nice, 2000, V.M. Malhotra ed, pp. 535-559.

42. Ghio, V.A., Monteiro, P.J.M. and Demsetz, L.A. The rheology of

fresh cement paste containing polysaccharide gums. Cement and Concrete Research, 1994, Vol. 24, pp. 243-249.

43. Skaggs, C.B., Rakitsky, W.G. and Whitaker, S.F. Application of

rheological modifiers and superplasticizers in cementitious systems. Proceeding of Forth CANMET/ACI International Conference on

Superplasticizers and Other Chemical Admixtures in Concrete, ACI

SP-148, Montréal, 1994, V.M. Malhotra ed., pp. 189-207.

44. Saucier, K.L. and Neeley, B. D. Anti-washout admixtures for

underwater concrete. Concrete International, 1987, Vol. 9, pp. 42-47.

45. Anderson, J.M. Remote controlled hydrocrete. Concrete, 1983, Vol. 17, pp. 12-15.

46. Kawai, T. and Okada, T. Effect of superplasticizer and viscosity-

increasing admixture on properties of lightweight aggregate concrete. Proceedings of the Third CANMET/ACI International Conference on

Superplasticizers and Other Chemical Admixtures in Concrete, ACI

SP-119, Ottawa,, 1989, V.M. Malhotra ed, pp.583-604.

47. Skarendahl, A. market acceptance of self-compacting concrete, the

Swedish experience. Proceedings of the Second International

Symposium on Self-Compacting Concrete, Tokyo, 2001, pp. 1-12.

48. Ozawa, K. Utilization of new concrete technology in construction

project – Future prospects of self-compacting concrete. Ibid reference 47, pp. 57-62.

49. Rols, S., Ambroise, J. and Pera, J. Effects of different viscosity agents

on the properties of self-leveling concrete. Cement and Concrete Research, 1999, Vol. 29, pp. 261-266.

50. Pera, J., Ambroise, J., Rols, S. and Chabannet, M. Influence of starch

on the engineering properties of mortars and concretes. Proceedings of the 20th International Conference on Cement. Microscopy, Lyon,

1998, pp. 120-128.

51. Kanematsu, M. and Takafumi, N. A study on the basic properties of a

self-compacting concrete containing the -polyglutamic acid-type

segregation inhibitor. Ibid reference 47, pp. 221-228.

52. Sakata, N., Yanai, S., Yoshizaki, M., Phyfferoen, A. and. Monty, H. Evaluation of S-657 biopolymer as a new viscosity-modifying

admixture for self-compacting concrete. Ibid reference 47, pp. 229-

236.

53. Sari, M., Prat, E. and Labastire, J.-F. High strength self-compacting

concrete - Original solutions associating organic and inorganic

admixtures. Cement and Concrete Research, 1999, Vol. 29, pp. 813-818.

54. Ghezal, A. and Khayat, K.H. Optimization of cost-effective self-

consolidating concrete. Ibid reference 47, pp. 329-338.

55. Bettencourt Ribeiro, A. and Gonçalves, A. A low-cost self-

compacting concrete. Ibid reference 47, pp. 339-348.

56. Khurana, R. and Saccone, R. Fly ash in self-compacting concrete.

Seventh CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, ACI SP-99, 2001,

Vol. 1, pp. 695-718.

57. Yurgu, M. and Sakai, G. Viscosity agent and mineral admixtures for highly fluidized concrete. Concrete Under Severe Conditions:

Environment and Loading, Vol. 2, E&FN Spon., 1995, pp. 995-1004.

58. Rooney, M.J. and Bartos, P.J. Development of the settlement column segregation test for fresh self-compacting concrete. Ibid reference 47,

pp. 109-116.

59. Jolicoeur, C., Khayat, K.H., Pavate, T. and Pagé, M. Evaluation of effect of chemical admixture and supplementary cementitious

materials on stability of cement-based materials using in-situ

conductivity method. Ibid reference 41, pp. 461-483.

60. Pavate, T., Mikanovic, N., Khayat, K.H. and Jolicoeur, C. Non-

published results.

Dr Carmel Jolicoeur, is professor in the

department of chemistry, Université de

Sherbrooke, Québec, Canada. His research

interests include the solution and colloid

chemistry of cementitious systems and chemical

admixtures for cement-based materials.

Mr Nikola Mikanovic, is a Ph.D. student in the

department of chemistry at the Université de

Sherbrooke, Quebec.

Mr Marc-André Simard, is a research chemist

in the department of chemistry at the Université

de Sherbrooke, Quebec.

Mr Jeff Sharman, is a research chemist in the

department of chemistry at the Université de

Sherbrooke, Quebec.