A unique relationship fluidity/adsorption for comb−type superplasticizers in sulfated pore solution

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A unique relationship fluidity/adsorption in sulfated pore solution
for comb-type superplasticizers
Florent DALAS
1,2
*
Sylvie POURCHET
1
André NONAT
1
David RINALDI
2
Martin MOSQUET
2
1
Laboratoire Interdisciplinaire Carnot de Bourgogne (UMR 5209), Dijon, France
2
LCR Lafarge, St Quentin Fallavier, France
Abstract
Cement is a vast domain of use for chemical admixtures. The so-called superplasticizers of kind polycarboxylate grafted
poly(ethylene oxide) (PCP) demonstrated their efficiency for fluidity improvement. High costs of these admixtures imply
to reduce added amounts. But the sole adsorbed PCP is efficient for improving the fluidity. To predict the optimum
dosage, it is interesting to study the relationship existing between the rheology and the adsorbed amount. The
adsorption is more representative than the dosage as used in several previous studies. Here we chose a calcite system
to avoid cement reactivity and to mimic properties of a cement paste at the early age of hydration. Comb-type
superplasticizers with various side chains density were studied in sulfated solution. The adsorption was evaluated by
measuring the adsorbed amount and the PCP equilibrium concentration. A mini-cone slump test permitted to deduce
the yield stress of the same paste. We found that the adsorption isotherms depend on the pore solution and the grafting
degree. Without sulfate, the three PCP have the same adsorption ability. On the contrary, an effect of the side chain
density is revealed in a sulfated pore solution. The amount of adsorbed PCP declines because PCP competes with the
sulfate ions. A more charged PCP will be more resistant to the competition with sulfates. By combining the rheology
with the adsorption, we highlighted a unique relationship “log(yield stress) vs. adsorbed PCP” for a sulfated solution
whatever the sulfate concentration [60-120mmol/L] and the ester ratio [20-38%]. However in a sulfate-free solution,
the previous relationship was not valid and thus depends on the ester ratio. A sulfate-free solution is far away from a
cement pore solution. The established relationships are valid for an adsorption in the slope of the isotherm. In this case,
the relationship is slightly curved showing that the first adsorbed PCP are the most efficient to improve the fluidity
whatever the pore solution. When the amount of adsorbed PCP is close to the saturation dosage of the surface, the
relationship becomes more linear and is no more influenced by the pore solution.
Each PCP has the same fluidizing
efficiency when it manages to adsorb in these ionic conditions. The knowledge of the adsorption and the relationship
obtained is a necessary step to reduce dosages and improve fluidizing efficiency.
Originality
The sustainable production and the emission of greenhouse gases are two problems in which the admixtures can
provide solutions. Comb-type superplasticizers, used in this study, enhance the properties of fresh concrete such as the
fluidity. This study was led on an inert calcite system with various amounts of sulfate to mimic the cement and its pore
solution. Our model system leads to highlight a unique relationship between fluidity and adsorption whatever the
sulfate concentration or grafting degree of the comb-type superplasticizer. The improvement of fluidizing efficiency of
this kind of admixtures is directly related to understanding and improving their adsorption.
Chief contributions
The improvement of properties of fresh concrete is an important industrial area for chemical admixtures. The industrial
applications require to predict the approriate amount of PCP for a specified workability. Thus it is interesting to study
the relationship existing between the adsorption and the rheology. In this study, a inert cement-like system permited to
highlight the unique relationship existing between yield stress and PCP adsorbed quantities with various amounts of
sulfate and with different grafting degree. Therefore the understanding of the adsorption is a crucial point to enhance
the efficiency of comb-type superplasticizers. For this purpose, we have demonstrated that the first PCP adsorbed are
the more efficient to improve the fluidity whatever the PCP and the pore solution.
Keywords: Comb-type superplasticizer, relationship fluidity/adsorption, side chains density, sulfate effect
*
Corresponding author: Email florent.dalas@u-bourgogne.fr Tel +3380396147, Fax +3380393819

1. Introduction
Nowadays cement and concrete industry is subjected to two principal constraints: environmental by
reducing the emission of greenhouse gases and technological by increasing the mechanical strengths
of hardened concrete. However both aims are in contradiction with the workability of a concrete in the
fresh state. To overcome this rheological loss, organic admixtures are used as so-called
superplasticizers made of a polymethacrylate backbone partially esterified with poly(ethylene oxide)
(PEO) chains.
These comb-like copolymers are referred to hereafter by the acronym PCP. The main
constraints are the cost of admixtures and the improvement of fluidizing efficiency. The sole adsorbed
PCP is efficient for improving the fluidity. In fact an added amount of PCP shares between the pore
solution and the adsorption on surfaces. Because of the alkaline pH of the cement suspension (pH
>12), the non-esterified methacrylic groups (MAA) are ionized. Thus these groups have a role to
adsorb on the solid surfaces covered by Ca
++
ions. Graft chains generate steric hindrance effects which
overcome the attractive forces responsible for the flocculation of cement particles and hydrates (e.g.
Uchikawa et al., 1997). The polymer adsorption is very sensitive to the concentration of divalent ions
like sulfate ions as a consequence of their competitive adsorption (e.g. Yamada et al., 2001). The
initial workability of the cement paste made with PCP is consequently dramatically affected in
presence of high sulfate ions concentrations (e.g. Hanehara et al., 1999). Numerous studies already
reported the influence of the characteristic of the polycarboxylate grafted poly(ethylene oxide) (PCP)
such as the molar mass, the number and the length of the PEO side chains (e.g., Yamada et al., 2000)
on the rheology omitting results analysis in sight of the adsorption behavior.
Industrial problems are the prediction of the superplasticizer added amount to keep the fluidity
constant up to several hours. To have good slump retention, PCP should cover surfaces of anhydrous
and hydrated phases. During the hydration of cement, the dissolution of anhydrous phases leads to the
precipitation of hydrated phases. Thus these hydrates will not be covered and a slump loss can occur.
The prediction of the dosage involves knowing the surface of hydrated phases and the relation
fluidity/PCP adsorption. Our study focuses on the relationship fluidity/adsorption. A linear correlation
between the paste flow and the adsorption per gram of cement was shown by Schober (2003) and by
Flatt (2006) for various structures of PCP. Hanehara (2007) highlighted a linear relationship between
the adsorption amount of PC per surface area of hydrated cement and slump flow depending on the
water to cement ratio. These previous studies were done for a unique dosage as the entire surface was
covered
.
The originality of our work is to study the contribution of the first adsorbed PCP on the
relationship between the polymer adsorption and dispersion ability in presence of sulfate ions.
2. Materials
2. 1. Minerals
Yamada (2003) has shown the difficulty to study the relationship fluidity/adsorption by working with
nine kinds of cement. To work solely on this relationship without the effects of hydration, an inert
system, calcium carbonate, was chosen to faithfully duplicate rheological properties of a cement paste.
Calcium carbonate gets surface and colloidal properties close to cement particles at the first steps of
hydration. Calcite has got in saturated lime water solution a zeta potential and a specific surface area
(both main parameters governing the adsorption) similar to unhydrated cement (Mikanovic et al.,
2006). Thus aqueous calcite (CaCO
3
) suspensions can provide an inert system model for early age
cementitious materials by simulating superplasticizer/cement interactions and their consequences on
rheology without the chemical reactivity of a cement. The calcite betocarb P2-OG was supplied by
OMYA France. The specific surface area determined from nitrogen adsorption using the BET
equations was found to be equal to 0.77 m
2
g
-1
.
2. 2. Superplasticizers
In order to study the effect of the anionic groups, three different PCP were synthesized by
conventional radical polymerisation of PEO methacrylate and methacrylic acid (MAA) with an

average molecular weight close to 30 000g/mol. The length of the side chains was set at a constant
value of 1100 g/mol. Then the chosen grafting ratio or ester ratio (POE/(POE + MAA)= XX %mol)
was 20%, 30% and 38% respectively designated by PCP-20, PCP-30 and PCP-38.
3. Sample preparation and methods
3. 1. Synthetic pore solution:
All the solutions used are saturated with respect to calcium hydroxide in order to keep the surface
charge of calcite unchanged. For the initial solutions, this was done by adding 1,8g of CaO to 1L of
distilled deionised water. In order to evaluate the effect of the sulfate ions on the performances of the
three PCP, three solutions containing different sulfate concentrations were prepared. The first solution
consists in a previous CaO saturated solution. The solutions with various sulfate concentrations were
obtained by adding respectively 10,8g and 19,9 g of Na
2
SO
4
to 1L of the CaO saturated solution.
These suspensions were continuously stirred at 25°C and then filtered through 0.3 µm millipore filters.
The ionic concentrations were then determined by atomic emission spectroscopy:
-without sulfate: [Ca
2+
] = 21±1mmol/L
-with sulfate: [Ca
2+
] = 15±1mmol/L and [sulfate] = 60±2mmol/L or [Ca
2+
] = 12±1mmol/L and
[sulfate] = 120±3mmol/L
3. 2. Calcite suspension:
The suspension is prepared using a Waring blender and according to the following procedure:
- 27.2 mL of initial solution is poured into the blender.
- 80g of calcite and 150 mg of calcium hydroxide (added to compensate calcium consumption due to
Ca
2+
adsorption on calcite) were mixed dry for about 5 seconds by hand outside the blender.
- The powder is then poured into the blender.
- Mixing is then performed according to the next procedure: 1 minute speed 5000 rpm, 1 minute
without mixing, 1 minute speed 5000 rpm.
Using this procedure, the liquid/solid ratio of the suspension is equal to 0.34. According to Hanehara
(2007), the higher the solid volume fraction is, the higher the sensitivity of the relationship
fluidity/adsorption is. The liquid to solid ratio was chosen to be the most sensitive for the rheology
experiments.
3. 3. PCP adsorptions measurements:
The PCP adsorption onto the mineral particles in the different pastes was determined by measuring the
residual concentration of polymer in the liquid phase of the suspensions. Again, immediately after the
rheology experiment, a part of the paste was centrifuged and the filtered supernatant was also acidified
with H
3
PO
4
to avoid carbonation. The total organic carbon (TOC) of the pore solution was then
measured by using a Shimadzu TOC analyser 5000A. The amount of PCP adsorbed is therefore
deduced from the reference TOC measurements of initial solution prepared before the calcite addition.
3. 4. Rheology experiments:
A part of the paste was used for adsorption measurements and the other part permitted to evaluate the
yield stress (τ
0
) by using a mini-cone test. The slump or spread measurement involves filling a
normalized cone (38mm lower and 20mm upper diameter and a height of 55mm) then lifting it up at a
constant speed and finally measuring the diameter (R) of the paste spread disk onto the glass plate. A
preliminary study had confirmed that the relation between the yield stress and the slump can be
predicted by the law (e.g., Zimmermann et al., 2009):
V
R
RVR
Vg
2
352
2
0
3
128
225
1128
225
λ
π
π
ρ
τ
−





⋅+
=
−
(1)

where: ρ = paste density, V = volume of the cone and λ = parameter which takes into account the wet
angle and the liquid-vapor interfacial energy (it was fixed at 0,003).
The slump measured values were converted into yield stress thanks to the previous relation (1).
Results of the previous studies on the relationship fluidity/adsorption (e.g. Schober, 2003, Flatt, 2006)
were plotted in paste-flow in millimeter. The use of the yield stress makes the comparison of results
easier.
4. Results and discussion
The adsorption and rheology measurements will be analyzed according to the sulfate concentration
and the grafting degree. Each pore solution is different by its ionic strength, pH and ions
concentrations, conditioning the interfacial properties of the calcite.
4.1. Adsorption isotherm
The adsorption isotherms of the three PCP are plotted according to the sulfate concentration: without
sulfate (fig. 1), 60mmol/L of sulfate (fig. 2) and 120mmol/L of sulfate (fig. 3).
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
adsorbed PCP (mg/g of calcite)
PCP equilibrium concentration (g/L)
PCP-20
PCP-30
PCP-38
Without sulfate
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
adsorbed PCP (mg/g of calcite)
PCP equilibrium concentration (g/L)
PCP-20
PCP-30
PCP-38
[sulfate] = 60mmol/L
Figure 1 Figure 2
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
adsorbed PCP (mg/g of calcite)
PCP equilibrium concentration (g/L)
PCP-20
PCP-30
PCP-38
[sulfate] = 120mmol/L
Figure 1: Adsorption isotherm without sulfate. The
three PCP have the same plateau value and a strong
affinity for the surface of the calcite.
Figure 2: Adsorption isotherm with 60mmol/L of
sulfate. The adsorbed amount and the affinity for the
surface decrease with an increase of the side chains
density.
Figure 3: Adsorption isotherm with 120mmol/L of
sulfate. The adsorbed amount and the affinity for the
surface decline more strongly than in presence of 0 or
60mmol/L of sulfate.
Figure 3
The adsorption isotherms of the three PCP are very similar in a sulfate-free solution (figure 1). The
behavior of the three PCP reveals a strong affinity for the surface. A same plateau value is obtained
whatever the structure of the PCP: 0,56mg/g of calcite. The calcite surface without sulfate is favorable
to the PCP adsorption revealed by the high affinity values.

The influence of sulfate ions will be studied thanks to both adsorption isotherms with 60 (figure 2) and
120mmol/L of sulfate (figure 3). In general, both the adsorbed amounts and the affinity for the surface
i.e. the solution/adsorption distribution decline with the sulfate concentration and/or the ester ratio.
Sulfate ions allow to highlight different adsorption behaviors linked to the PCP structure.
The slope of the linear part of the isotherm describes the affinity for the surface. In both sulfate
concentration, the structure effect is observed because the order of the slope is unchanged: PCP-20 >
PCP-30 > PCP-38. To adsorb onto the surface, PCP must compete with sulfates: the capacity as a
competitor depends on the charge of the PCP. A highly charged backbone (PCP-20) implies a good
resistance to sulfate/PCP competition and thus a low decrease of the affinity is observed contrary to a
poorly charged backbone (PCP-38).
The plateau values for both PCP-20 and PCP-30 seem to be very close. However, this plateau
decreases from 60 to 120mmol/L of sulfate: 0,5mg/g of calcite to 0,45mg/g of calcite. The pore
solution is different by its ionic strength, pH and ions concentrations, conditioning the interfacial
properties of the calcite. Due to the gypsum equilibrium, the sulfate increase will lead to a calcium
lowering (21→15→12mmol/L) and consequently to a surface charge density reduction. By this fact,
the adsorption is going to decrease. PCP-38 shows a different behavior by the shape of the isotherm.
The plateau value and the affinity are quite different from the others PCP.
In the following rheology experiments, the adsorbed amount is always inferior to 0,3mg/g of calcite.
So the dosage range allows to always be in the linear part of the isotherm and to study the contribution
of the first adsorbed PCP on the relationship fluidity/adsorption.
4.2 Rheology experiments
The yield stress of the calcite paste (in logarithmic scale) versus the PCP adsorption in a sulfate-free
solution is plotted in figure 4. It clearly appears that the logarithm of yield stress linearly decreases
with the PCP adsorption. Thus the adsorbed PCP layer has a strong effect by reducing the attractive
forces acting between the calcite particles. The higher the adsorbed PCP amount is, the more separated
the curves are, denoting an influence of the ester ratio. Surprisingly at same adsorption level for high
dosages, PCP-38 leads to a stronger decrease of the yield stress. A better knowledge of the PCP
conformation could possibly explain this behavior.
The yield stress is plotted as a function of the amount of adsorbed PCP (in mg per gram of calcite) for
the three PCP and for the two different sulfate concentrations (Figure 5). A unique relationship
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35
0,1
1
10
100
Yield stress
τ
0
(Pa)
Adsorbed PCP (mg/g of calcite)
PCP-20
PCP-30
PCP-38
Without sulfate
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35
0,1
1
10
100
Yield stress
τ
0
(Pa)
Adsorbed PCP (mg/g of calcite)
[sulfate]=60mmol/L:
PCP-20
PCP-30
PCP-38
[sulfate]=120mmol/L:
PCP-20
PCP-30
PCP-38
Figure 4: Yield stress (in logarithmic scale) decreases
as a function of the amount of adsorbed PCP in a
sulfate-free pore solution. This relationship is affected
by the ester ratio.
Figure 5: Yield stress (in logarithmic scale) decreases
as a function of the amount of adsorbed PCP. This
relationship remains the same whatever the ester ratio
[20-38%] of the PCP studied and the sulfate
concentration [60-120mmol/L].

fluidity/adsorption for a sulfated pore solution is highlighted whatever the sulfate concentration and
the ester ratio except the PCP-38 at 120mmol/L of sulfate. The sulfate concentration (60 to
120mmol/L) of the pore solution does not modify the dispersion ability of the adsorbed polymer. The
ionic strength of both experiments is respectively 0.20 and 0.36mmol/L. These results are consistent
with the conclusions of Borget (2005) who found that the hydrodynamic radius of different PCP in
alkaline solution remains the same in a large range of ionic strength (0.03 < I mmol/L < 0.40) obtained
by modifying the sulfate concentration for example. He concluded then that electrostatic interactions
between the carboxylate groups are completely screened under these conditions. Under the
experimental conditions the yield stress can simply be expressed as an exponential decrease of the
amount of adsorbed PCP, and this relation remains the same whatever the ester ratio of PCP studied
and the ionic strength of the calcite suspension by varying the Na
2
SO
4
contents.
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35
0,1
1
10
100
Yield stress
τ
0
(Pa)
Adsorbed PCP (mg/g of calcite)
Relationship with sulfate
Relationship without sulfate
PCP-38: 120mmol/L of sulfate
Figure 6: Yield stress (in logarithmic scale) decreases as a function of the amount of adsorbed PCP. A slightly
curved relationship is obtained for dosages in the slope of the isotherm. On the contrary, an almost linear
relationship fluidity/adsorption is obtained on the plateau of the isotherm as for PCP-38 at 120mmol/ of sulfate.
The figure 6 summarizes trends observed for the relationships fluidity/adsorption with and without
sulfate. The exception of PCP-38 at 120mmol/L of sulfate is also plotted. As stated above, PCP
dosages studied are located in the first part of the slope of the isotherms. In this case, both
relationships “log(yield stress) vs. adsorbed PCP” are slightly curved. On the contrary, dosages studied
for PCP-38 at 120mmol/L of sulfate reach the plateau value of the isotherm and the relationship
fluidity/adsorption is almost linear. Both relationships are compared on figure 6. First the yield stress
measured without PCP is lower in a free sulfate solution. This observation is still valid in presence of
PCP for low adsorption levels (<0,25mg/g of calcite). For an adsorption amount inferior to 50% of the
plateau value, the yield stress is influenced by the adsorbed PCP and also by the colloidal properties of
the free PCP paste. Divalent sulfate ions could cause a further contribution to the attractive forces such
as ionic correlation forces in addition to the attractive forces of van der Waals leading to an increase of
the yield stress. When the adsorption becomes superior to 50% of the plateau value, the yield stress is
then governed by the steric repulsion of POE
This study allows conclusions about the contribution of the first adsorbed PCP. On one hand, when the
amount of PCP adsorbed is inferior to 50% of the value of the plateau, the workability is governed by
the paste properties and by the free PCP pore solution.
However, the first adsorbed PCP are the most
effective in improving the fluidity and the relationship fluidity/adsorption is slightly curved.
On the
other hand, when the amount of PCP adsorbed is getting close to the plateau value, only the amounts
of PCP adsorbed influence the fluidity and a linear relationship “log(yield stress) vs. Adsorbed PCP”
is obtained.
So the improvement of fluidizing efficiency by lowering the dosage requires a detailed
knowledge of the adsorption, which is influenced by the ester ratio, as sometimes omitted in the
rheological studies.

5. Conclusions
To avoid cement reactivity a calcite system was chosen to mimic properties of a cement paste at the
early age of hydratation. This study focused only on the effects of the ester ratio and sulfate ions on
PCP adsorption and rheology behavior. We can draw conclusion about the relationship
fluidity/adsorption and the contribution of the first adsorbed PCP.
Differences on the adsorption behavior have been noticed depending on the ester ratio and the sulfate
concentration. The study without sulfate confirms the high affinity for the calcite surface. The number
of carboxylate groups on the backbone within the range studied does not influence the adsorption.
With an increasing sulfate concentration, the plateau value and the affinity for the surface decline
more dramatically. A more negatively charged PCP will have a greater affinity and plateau because it
competes more effectively with sulfate ions.
By combining the rheology with the adsorption, a unique relationship “log(yield stress) vs. adsorbed
PCP” was highlighted for a sulfated pore solution whatever the sulfate concentration [60-120mmol/L]
and the ester ratio [20-38%]. However in a sulfate-free solution, the previous relationship was not
valid and thus depends on the ester ratio. It is worth noting that a sulfate-free solution is far away from
a cement pore solution. The established relationships are valid for an adsorption in the slope of the
isotherm. In this case, the relationship is slightly curved showing that the first adsorbed PCP are the
most efficient to improve the fluidity whatever the pore solution. When the amount of adsorbed PCP is
close to the saturation dosage of the surface, the relationship becomes more linear and is no more
influenced by the presence of sulfate ions in the pore solution .
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