Indian Journal of Engineering & Materials Sciences

Vol. 17, February 2010, pp. 27-33

Effects of SNF and LS superplasticizer on cement paste using

electrical measurement

Xiaohui Zenga,b

* & Tongbo Suib

aDepartment of Civil Engineering, Central South University, Changsha 410 075, Hunan, P R China bChina Building Materials Academy, Beijing 100 024, P R China

Received 20 October 2008; accepted 10 November 2009

A non-contact electrical resistivity measurement is used to study the effects of sulfonated naphthalene formaldehyde

condensates (SNF) and lignosulphonate (LS) superplasticizers on cement paste. Other experiments like fluidity test and test

of the amount of superplasticizer adsorbed on cement particles are also carried out. Results show that SNF superplasticizer

has different effects on the resistivity of cement pastes comparing with LS superplasticizer. The plot of initial resistivity

versus SNF dosage can be divided into 4 zones. Each zone reflects the characteristics of SNF as a superplasticizer of cement

paste, respectively. The curves of the resistivity developing with time; ρ(t), and their derivatives; dρ(t)/dt(t), have distinct

changes when the dosages of SNF and LS superplasticizer are high. Electrical method is potentially a very useful method in

concrete designing.

Keywords: Cement, Electrical resistivity, Superplasticizer, Fluidity, Adsorption, Dissolution

The main purpose of using superplasticizer in

concrete is to improve the paste’s fluidity. With

superplasticizer, cement particles can be better

dispersed and the water required can be reduced.

Since high water-cement ratio have negative effects

on concrete durability and strength, the use of

superplasticizer plays an important role.

Sulfonated naphthalene formaldehyde condensates

(SNF) and lignosulphonate (LS) superplasticizers are

two types of superplasticizers which are still widely

used in China today. Both of them are

polyelectrolytes. By their main polymer chains, with

attached sulphonate groups (SO3-) adsorbing onto the

surfaces of cement particles, producing negative

charged particles, and hence the dispersing of

particles occurs by electrostatic repulsion between

cement particles1. The two superplasticizers differ in

various ways. Generally, LS superplasticizer has

problems of super-retarding setting or never-setting,

and the naphthalene-based superplasticizer is more

incompatible with cement2-10

.

Electrical conductivity method had been used to

study the interaction between superplasticizer and

cement at early ages11–18

. Levita et al.11

studied the

electrical, setting and hydration heat behaviours of

cement paste with different amounts of

superplasticizer; similar works were carried out by

Heikal et al.12

. Xu et al.13

presented the effects of

superplasticizer on the electrical resistivity and

interfacial transition zone (ITZ) characteristics of

mortars. Xiao et al.14

used electrical resistivity

method to select suitable superplasticizer for certain

cement. However, since superplasticizers are mainly

used to improve the fluidity of the paste, the study on

the relationship between fluidity and electrical

property would provide important information. Little

literature has been published on this subject.

In this paper, various amounts of SNF and LS

superplasticizers were added to cement pastes. The

electrical resistivity changes with time of the pastes

were recorded, and the fluidity tests were carried out

on the same material. An adsorption and dissolution

mechanism of superplasticizer was proposed to

explain the resistivity change with the dosage of

superplasticizer. The relationship between initial

resistivity and fluidity of the cement pastes was also

analyzed. The formulations were varied to obtain the

optimal dosage of superplasticizer in concrete

preparing.

Methods and Materials

Materials

The cement used in the study was the Chinese

Standard Cement (produced by Lafarge Co.,) with ——————

*Corresponding author (E-mail : zxhzlh@yahoo.com.cn)

INDIAN J. ENG. MATER. SCI., FEBRUARY 2010

28

specific area of 333 m3/kg (Blaine method), which is

specially designed for testing the properties of

admixtures (for example, in the arbitration). Table 1

shows the characteristics of clinker of this cement.

Superplasticizers used were sulfonated

naphthalene formaldehyde condensates (SNF) (solid

based sodium salt, produced by Zanjiang Admixture

Company) and lignosulphonate (LS) (solid based

calcium salt, produced by Yanbian Shishan Bailu

Paper Company).

The dosages of SNF superplasticizer in the cement

pastes various of 0.05, 0.1, 0.2, 0.3, 0.5%, 0.7, 1.0, 1.5

and 2.0% were used. And dosages of LS

superplasticizer in cement pastes at concentrations

0.05, 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0% were used. All

the dosages were calculated by weight. The water

used was tap water with resistivity of 21.97 Ω·m.

Superplasticizers were added into the tap water before

mixing with cement. The w/c (water-cement) ratio in

all pastes was 0.4.

Electrical resistivity

The electrical resistivities of the pastes were tested

by a non-contacting electrical resistivity measurement

(Fig. 1)19-22

. The transformer principle of mutual

inductance of electricity and magnetism was adopted

in this apparatus. When voltage is applied to the

primary coin (transformer, Fig. 1), a toroidal voltage

V will be generated in the ring cement specimen,

which acts as the secondary coil of the transformer.

The toroidal current I can be measured by a leakage

current meter, and hence the resistance of the concrete

can be calculated through V and I, using Ohm’s law22

,

and then resistivity by considering the geometry of

the specimen. There are no electrodes in the

apparatus, which solves successfully the contact

problem and monitors accurately the resistivity ρ(t) of

cementitious material. The sensitivity of the device

was calibrated using standard KCl solution

(concentration 0.1 and 1 mol/L), their standard

conductivity of 1.276 Ω-1

/m and 11.082 Ω-1

/m at

24.5°C (relative errors were less than 0.4%).

The pastes were cast in moulds as soon as possible

after it was prepared and then the moulds were

vibrated for 10 s. The initial resistivity was recorded

by the computer. The w/c ratio is relatively high

(w/c=0.4), therefore the quality of cement paste was

relatively easy to control; as a result, the error of

initial resistivity is in an acceptable scope. Fluidity

The paste fluidity experiments were carried out

using a mini-slump test. The mini-cone used was of

top diameter of 36 mm, bottom diameter of 60 mm

and height of 60 mm. The cement paste was poured

into the cone quickly after mixing, and then the cone

was vertically pulled out. The spread diameter of the

cement paste after 30 s was the fluidity. The paste was

mixed by a blender initially mix slowly for 2 min, and

then mixing quickly for 3 min. Adsorption measurement

Superplasticizers adsorption on the surface of

cement particles were evaluated by measuring the

amount of superplasticizers in the solution extracted

from fresh cement paste samples. The solutions were

extracted from the cement pastes through filtering by

a vacuum pump. The original solutions were diluted

to a suitable concentration and a UV-visible

spectrometer (PGENERAL TU1901) was used to

determine the superplasticizers concentration in the

solution. The water consumed by the early hydration

of cement was ignored in the calculation. Setting time

The setting time of pastes was monitored using Vicat needle measurement. The needle diameter was 1.1 mm. The initial setting time was determined when the distance of the Vicat needle penetrated into the sample was less than 5 mm to the mold’s base. The

Table 1— Chemical and mineral composition of Chinese Standard Cement Clinker (%).

SiO2 CaO Al2O3 Fe2O3 MgO SO3 Na2Oeq f-CaO C3S C2S C3A C4AF

21.88 65.89 4.61 2.65 1.76 1.77 0.59 0.98 60.22 17.33 7.63 8.52

Fig. 1— Non-contact electrical resistivity set-up

ZENG & SUI : EFFECTS OF SNF AND LS SUPERPLASTICIZER ON CEMENT PASTE

29

final setting time was that when needle makes an impression on the surface of the paste without penetration.

Results and Discussion

The main minerals of cement clinker are

tricalcium silicate (C3S), dicalcium silicate (C2S),

tricalcium aluminate (C3A) and triclacium alumino

ferrice (C4AF). Alkali salts like sodium and potassium

from impure raw minerals are present also in a glass

phase of the clinker. Furthermore, free calcium oxide

(f-CaO) is present due to incomplete sintering

reaction. At the end, gypsum is ground with the

cement clinker for the adjusting setting time purpose

in the cement production23

.

When cement contacts with the water, its easily

dissolving component like alkali salts and gypsum

will dissolve in water instantly, and its most of

reactive minerals like C3S, C3A and f-CaO will start to

hydrolyse reaction. In a few minutes, the solution will

be full of Na+, K

+, SO4

2- Ca

2+, OH

- and Al(OH)4

- ions.

As shown in Figs 2 and 3, for the dissolution of cement minerals and superplasticizers, the electrical resistivitiy of the pastes range from 1.2 to 1.8 Ω·m at the beginning; much lower than the tap water of 21.97 Ω·m. The resistivity development with time, ρ(t), consistent with literatures values

15,20,24-25, that for the

increasing concentration of ions, the resistivity of the paste decreases with time in the early stage. However, when the solution is saturated with ions the crystallization of hydration products occurs, and the resistivity increased for the crystallization reactions consume ions and water. The increasing of solid phases which are insulators also enhances the resistivity for the decrease of liquid phase volume fraction.

Before the crystallization reaction process has

initiated, there is an induction period23

before

significant hydration had been established. In part

explained by the fact that the crystallization needs

some extent of supersaturation; this but

supersaturation inhibits ions dissolution required for

hydration. In Fig. 2a, it is clear that the resistivity

early have no obvious change after the quick decease

period. And the more the superplasticizer, the longer

the period. The induction period is related to the

cement’s setting time as seen in the curve of ρ(t), the

higher the superplasticizer concentration the longer

the setting retarding, as consistent with literature work

(Fig. 4).

Fig. 2— Effect of varying dosage of SNF superplasticizer on the curves of ρ(t) (w/c=0.4). Ⅰ: Starting period; Ⅱ: induction period;

Ⅲ: acceleration period26. (a) Whole view and (b) partly magnified. Curve 6# with dosage of 0.5% is quite different with the blank one

Fig. 3— Effect of varying dosage of LS superplasticizer on the on

the curves of ρ(t) (w/c=0.4). Curve 6# of dosage of 0.5% is quite

different with curve 3# of dosage of 0.3%. The peaks at the quite

early time on curve 7# and curve 8# might be caused by the

accelerated early ettringite gel formation, which consent with

Monosi and Collepardi et al.27,28.

INDIAN J. ENG. MATER. SCI., FEBRUARY 2010

30

The behaviour of superplasticizer in paste can be

divided into two periods, adsorption on cement

particles followed by dissolution. Figure 2a shows the

curve ρ(t) of the blank sample is different from that of

0.5% superplasticised sample in the period II.

Figure 5a shows there is considerable SNF in the

solution when the dosage of superplasticizer is 0.5%.

Cement hydration reactions involves various solution

reactions. It seems that the superplasticizer in solution

act by disturbing these normal cement hydration

reactions.

Figure 3 shows that ρ(t) peaks very early; when

the dosage of LS is higher than 0.5%. Early peak(s) is

purported to be caused by the accelerated ettringite

(AFt) formation. LS superplasticizer has sulphonate

groups (SO3-), and may act as SO4

2-, which may react

with C3A and water, and precipitate AFt29

:

2-3 4 2

2 3 4 2

C A+SO +H O3CaO Al O 3CaSO 32H O(AFt)

→

i i i

The higher the superplasticizer concentration the

sharper the peak appears.

Figure 6 are the derivative curves of resistivity

development with time: dρ(t)/dt(t). The derivative

curves have two large peaks when dosages of SNF

superplasticizer are higher than 0.5%, but below 0.5%

the curves have only one large peak. Literature30-32

indicates that double peaks are associated with

the transformation of the ettringite (AFt) to

monosulfoaluminate (AFm):

2 3 4 22-

2 4 2 3 4 2

3CaO Al O 3CaSO 32H O(AFt)

Ca(OH) +SO +3CaO Al O CaSO 12H O(AFm)

→i i i

i i i

As the reaction of AFt advances SO42-

ions are

released and the conductivity of the paste increases, as

Fig. 4— Initial setting time and final setting time as a function of superplasticizer’s dosage (a) SNF superplasticizer and (b) LS

superplasticizer.

Fig. 5 —The adsorption and dissolution amounts of superplasticizer versus concentration of superplasticizer (a) SNF superplasticizer and

(b) LS superplasticizer.

ZENG & SUI : EFFECTS OF SNF AND LS SUPERPLASTICIZER ON CEMENT PASTE

31

a result the derivative of resistivity decreases and the

double peaks appear. SO42-

ion concentration in

solution is a key factor of such a reaction. When the

concentration is low, the equation will go in the

direction of product formation. May be for the

disturbance of sulphonate groups (SO3-), the SO4

2- ion

concentration in solution is different from that of

under normal condition. Therefore, such a

transformation happened, intensively.

Figure 7 is plot of the initial resistivity (the

resistivity at the beginning) and fluidity versus dosage

of SNF superplasticizer. The curves in Fig. 7 can be

divided into 4 zones, associated with selective

adsorption, effective adsorption, ionization and

saturated ionization, respectively.

Figure 8 shows initial electrical resistivity and

initial fluidity of cement pastes as a function of

dosage of LS superplasticizer (w/c = 0.4). As mention

previously, superplasticizers improve fluidity by

electrostatic repulsion of adsorbed surfaces.

Superplasticizer adsorbed on cement particles will

impede the ions’ dissolution11,18,33

, and increase the

resistivity of the paste, the higher the superplasticizer

concentration, the larger the area covered, and the

higher the resistivity of the admixture. Moreover, the

electrical double layer (formed by adsorption

superplasticizer and cement particle surface) will also

adsorb K+, Na

+, Ca

2+ ions and lead to a decrease in

both of number and mobility of these ions12

. The

deflocculating the cement paste, by the

superplasticiser, increases the tortuosity and decreases

the connectivity of the ionic pathways and therefore

increase the resistivity11

. So in zones a and b (Fig. 7),

the initial resistivity increases with the increasing

dosage of SNF superplasticizer. But unfortunately,

SNF superplasticizer is preferentially adsorbed on

C3A and f-CaO minerals1, which will have no

contribution to the fluidity. And as a result,

the fluidity didn’t have any notable improvement

in zone a.

In zone b, when the C3A and f-CaO minerals

surfaces are saturated, SNF superplasticizer will

adsorb on C3S and C2S minerals, it is that fraction

which improves the fluidity, notably34

. So in zone b,

both of the resistivity and fluidity increase with the

increasing dosage of SNF.

In zone c, the C3A, f-CaO, C3S and C2S minerals

surfaces are all saturated, superplasticizer begin

dissolve in the solution, as a result, the resistivity

decreases for both SNF and LS superplasticizer

Fig. 6— Effect of varying dosage of SNF superplasticizer on the

curves of dρ(t)/dt (w/c=0.4).

Fig. 7— Initial electrical resistivity and fluidity of cement pastes

as a function of dosage of SNF superplasticizer (w/c=0.4).

(a) Zone caused by the selective adsorption, (b) zone caused by

the effective adsorption, (c) zone caused by the dissolution and

ionization and (d) zone caused by the saturated ionization.

Fig. 8— Initial electrical resistivity and initial fluidity of cement

pastes as a function of dosage of LS superplasticizer (w/c=0.4).

INDIAN J. ENG. MATER. SCI., FEBRUARY 2010

32

(Fig. 9). Contrary with respect to SNF superplasticizer

cements, the resistivity of the paste with LS

superplasticizer did not decrease obviously with LS

superplasticizer’s dosage in zone c (see Fig. 8).

Figure 9 shows the resistivity of aqueous solution of

SNF and LS. Figure 9a is for SNF and Fig. 9b for LS.

It can be seen that the resistivity of LS aqueous

solution has a minimum, which might be caused by

forming micelle or entranced air. Electrical

conductivity methods are always used to test the

critical micelle concentration (CMC). At the CMC,

the surfactant will form agglomerate in the solution,

and the electrical conductivity of the solution will

have a break with its increasing concentration.

Therefore the LS superplasticizer, in zone c, causes

the paste’s resistivity to increase with the increasing

dosage of LS.

Since SNF superplasticizer’s selective adsorption

always tends to have incompatible problems with

cement6-10

, the resistivity method has provided a new

way to identify the compatibility between SNF and

cement caused by its selective adsorption. Since

excessive LS superplasticizer always leads to the

problems of super-retarding or never-setting, the

resistivity method provides a way to identify a

suitable dosage that ensures the setting. Therefore,

electrical methods will have potential in the

application field of superplasticizers.

Conclusions

1. The setting retarding effect of superplasticizer can

be seen from the paste’s electrical resistivity

development.

2. The initial resistivity of paste initially increases

with the increase of the dosage of SNF

superplasticizer. Afterwards, it decreases with the

increase of the dosage of SNF superplasticizer.

3. The curves of resistivity development with time

have distinct changes when the dosages of

superplasticizer are high.

Acknowledgments The financial supports form China “Eleven Five”

National Key Technology R&D Program (Award No.

2006BAJ03A09) and China National Nature

Science Foundation (Award No. 50678174) are

acknowledged. The assistances of Dr. Wei from Hua

Zhong University of Science and Technology with the

experimental work are gratefully appreciated.

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