Hydroxyethyl methyl cellulose as a modifier of gypsum properties
Journal of Thermal Analysis and Calorimetry
Hydroxyethyl methyl cellulose as a modifier of gypsum properties
Patrycja Mro´ z 0 1
Maria Mucha 0 1
Crystallization Setting 0 1
0 Faculty of Process and Environmental Engineering, Lodz University of Technology , Wo ́lczan ́ska 215, 90-924 Ło ́dz ́ , Poland
1 & Patrycja Mro ́z
The study is focused on the influence of a water-soluble polymer (in weight fraction up to 1.5%), cellulose derivativeshydroxyethyl methyl cellulose, on gypsum properties. Gypsum setting involves two processes: gypsum hydration/crystallization and probably formation of a polymer film in material pores. The processes are studied by various methods such as setting time and mechanical measurements, scanning electron microscopy and differential scanning calorimetry. The additive acts as a retarder (an increase in setting time), and it modifies the morphology of calcium sulfate dihydrate crystals, leading to the change in mechanical properties-an increase in bending stress. The mechanism of gypsum crystal growth during hemihydrate hydration is predicted to be a nucleation control process (the Avrami equation is applied). The value of nucleation rate constant decreases with an increasing additive content.
In recent years, interest in modification of building
materials such as gypsum or cement by mixing with polymers
has been observed. Polymers in small amounts may be
applied as admixtures, dispersants or plasticizers. In all
systems, the polymers act in different ways. In contrast to
polymer dispersion, water-soluble polymers are dissolved
in the mixing water on molecular scale and can form
transparent films in gypsum pores [
Commercial gypsum boards are widely used in the
building industry as facing materials for walls and ceilings
due to their very good mechanical and thermal properties,
as well as fire endurance [
]. The application of polymer
admixtures to gypsum and cement is increasingly
important. Currently used building materials, despite their
advantages, may be mechanically more durable and
resistant to moisture due to the addition of cellulose derivatives.
These polymers are completely safe for health which
makes them even more attractive [
HEMC is one of the cellulose ethers which affects the
properties of construction materials and also improves their
]. In the case of cement and other building
materials, cellulose ether can delay hydration. Pourchez
predicts that methoxyl group has the biggest influence on
hydration delay [
Plank et al. [
] and other researchers [
that HEMC improved water retention of cement and
gypsum. Already, a 0.3% dose of HEMC causes 97% of water
retention capability. However, more than 1% of HEMC
admixture can produce extremely viscous cement pastes.
This effect is dependent on molecular weight of the
polymer sample. An increase in filtrate viscosity may play a
role in the working mechanism of HEMC . Currently,
the main applications of HEMC and other cellulose ethers
are in wall plasters, floor screeds, waterproofing
membranes, joint compounds for gypsum board paneling and
cement tile adhesives [
This work is focused mainly on the mechanical
properties of gypsum plaster and hydration/crystallization of
gypsum with various contents of HEMC.
The experimental material was hemihydrate sulfate (b
form) supplied (as gypsum) by Dolina Nidy (Poland). The
material meets the requirements of standard PN-EN
]. It contains 90.98% calcium sulfate
(CaSO4) in gypsum. Other components include CaCO3—
2.79%, SiO2—1.62%, montmorillonite—3.07%, clays—
0.79% and chlorite—0.16%. Distilled water was used to
mix gypsum with admixtures. Hydroxyethyl methyl
cellulose (HEMC) of viscosity equal to 3000 mPa s was
supplied by Atlas Company (Poland).
Figure 1 shows ether cellulose chemical structure with
active groups to turn our account on possible interactions
by hydrogen bonds with calcium sulfate dihydate.
Preparation of samples
The following samples were prepared to use in
calorimetric, setting time and mechanical studies: gypsum without
admixtures and gypsum with various HEMC contents
(0.25–1.5%). The water-to-gypsum ratio was equal to
0.6–0.74. Samples were prepared according to standard
PN-86 B-04360 (Plasters. Test methods. Physical
characteristic determination.) [
]. First, polymer admixtures in
appropriate amounts were dissolved in distilled water at
room temperature. Then, the composition with gypsum was
vigorously mixed for 30 s using a mechanical stirrer to
obtain homogeneous blends.
The rate of setting time was determined at room
temperature using the Vicat’s device (Master, Italy) with a
stainless steel needle and ebonite ring. Contact surface of
the needle was polished after every measurement. The
study consists in a measure of free needle penetration into
the gypsum paste every 30 s. The study was conducted
according to standard PN-86 B-04360 (Plasters. Test
methods. Physical characteristic determination) [
Mechanical bending tests for measure of flexural stress
value r was carried out using Instron 3345 (Instron, USA).
Samples were prepared for the bending test by spreading a
hemihydrate paste on rubber molds attached to glass plates.
Mold dimensions were 50 mm/18 mm/5 mm. The tests
were repeated 6 times for paste of the same composition.
Before the experiment, the entire mold was smeared with
oil. The applied samples were set for 48 h at room
temperature. The study was conducted according to standard
PN-86 B-04360 (Plasters. Test methods. Physical
characteristic determination) [
Calorimetric studies of the samples
The hydration/crystallization process was followed by
isothermal calorimetry. Milligrams of the prepared paste
were placed in the calorimeter. The instrument was kept at
controlled constant temperature of 30 C. Heat flow was
recorded up to 2 h. After that time, it became very low. The
cumulative heat proportional to the degree of hydration
was calculated with reasonable accuracy. A Mettler
calorimeter (DSC), model FP90, was applied.
A scanning electron microscope (SEM) model 1430 VP
(LEO Electron Microscopy Ltd, England) was applied to
make microphotographs of gypsum fractures after the
flexural mechanical tests. Variable vacuum mode of 50 Pa,
a BSE detector was used.
Results and discussion
Samples of gypsum plasters with initially various
water-togypsum ratios which contained HEMC admixtures in
weight fractions from 0.25 to 1.5% were prepared. Setting,
mechanical and DSC tests were made. The results are
shown in the following figures. The effect of HEMC
admixture on the kinetics of gypsum
hydration/crystallization process was discussed.
Setting of material is a transition period during which the
physical state of material changes from liquid to solid. This
transformation occurs as a result of the development of
hydrated products which cause rigid connections between
hydrating grains. It is usually characterized by two points
in the hydration process, namely initial and final setting
time. The development of connected hydration product
which reflects the transition of material state was measured
by the penetration resistance technique.
Selected results of the Vicat needle test are shown in
Figs. 2 and 3. The beginning of the setting period was
estimated as the first inflection point on h curve. The time
of setting was measured as the time of the intersection of
straight line on the curve. The time from the onset of the
experiment to the beginning of the setting period is known
as the induction period.
Figure 2 shows setting of the samples with
water-togypsum ratio x from 0.6 to 0.74. In the case of higher water
content (in initial gypsum mixture), the initiation of the
setting process (initiation time ti) followed later than in the
case of low water content. Gypsum setting time ts also
increased with increasing amounts of water. Excess water
caused prolongation of the nucleation process and also
adsorption/accumulation of ions or other molecular units at
the interface due to lowering of the oversaturation degree.
Figure 3 illustrates the setting process of gypsum for
samples with w/g = 0.66 and various HEMC contents. In
Fig. 3, for comparison, a sample without any admixtures
with w/g = 0.66 is also presented. The addition of a small
amount of HEMC admixture (lower than 1%) caused an
insignificant prolongation of the setting and induction time.
Results for the samples with 0.25, 0.5 and 0.75% are very
similar. Compared to the samples with a smaller content of
admixtures and those without any admixture, the addition
of 1% prolongs the setting time. HEMC in the amount of
1.5% causes a significant (almost double) increase in the
setting and induction time. The applied polymer prevents
diffusion of water molecules and anions to the binder
surface due to relatively rigid polymer molecule
conformation in the water phase. No electrostatic interactions of
polymer active groups with the binder surface of gypsum
Fig. 3 Setting process of gypsum samples with w/g = 0.66 for
various HEMC contents (0.25–1.5%) and for sample without
were predicted because it was found elsewhere that the
gypsum zeta potential was close to zero [
Dependence of ti and ts on the w/g ratio was presented in
our previous paper [
The results of mechanical tests are shown in Figs. 4 and 5.
Figure 4 presents the dependence of bending stress r on
HEMC content for water-to-gypsum ratio equal to
0.6–0.74. The bending stress increases with increasing
HEMC content for chosen w/g. The higher values of r are
received for a smaller water-to-gypsum ratio.
Figure 5 presents bending stress r versus
water-togypsum ratio w/g. In both cases (samples without
admixture and with 1% HEMC), the bending stress r decreases
with increasing water content. It results from a change in
the sample morphological structure and increasing pore
content. Greater HEMC content causes the growth of
overlapping crystals which leads to significantly increasing
bending stress of the sample.
Fig. 2 Setting of gypsum obtained with various water-to-gypsum
ratios (w/g = 0.6–0.74). Weight fraction of HEMC is equal to 0.5%. ts
example of setting time measurements, ti induction time of setting
Fig. 4 Bending stress r versus HEMC content for various w/g ratios.
Error is equal to 1 MPa
with 1% HEMC
where Q, the amount of heat evolution, depends on a
number of factors.
The reaction of gypsum with water [
] is divided
into three main stages which involve a set of coupled
A nucleation period starts immediately after the
hemihydrate powder sample is mixed with water
solvent and dissolved. The dissolution involves
detachment of molecular units from a solid surface in contact
with water as well as diffusion and transport of
solution components into volume paste. The solution
becomes supersaturated with respect to Ca2? and
SO42- ions, which leads to the precipitation of solid
grains of calcium dihydrate due to the nucleation
An acceleration period in which a complex reaction
between ions or solid complexes adsorbed on solid
surfaces due to the crystallization/hydration process is
A deceleration period of very slow reactions of
adsorption and accumulation of ions or other molecular
units at an interface. The late stage of hydration is thus
controlled by the diffusion process.
The wide range of properties can be observed as
hydration proceeds, including heat of hydration, porosity
and setting time as well as phase volume fraction.
The rate of all the reactions can be changed by the
presence of a polymer admixture. For example, the
induction period can be prolonged with:
a. Reduced diffusion of water and calcium ions at
gypsum surface because the adsorbed polymer (if so)
hinders the process,
b. Formation of a complex between calcium ions and the
polymer in the pore solution,
c. Change in the growth kinetics and morphology of
hydrated phases caused by the dispersive action of the
Experimental observations have suggested that the
formation of a cementitious hydrated product is a
rate-controlling process at early stages [
]. Furthermore, this led
to the development of hydration kinetics models based on
nucleation and growth phenomena such as presented by
Avrami et al. [
] and also Johnson and Mehl , as well
as Kolmogorov [
], proposed a simple but widely used
equation which is derived using the assumption within the
transforming volume in changing liquid on crystal. Avrami
power law is as follows:
XðtÞ ¼ 1 expð KtnÞ;
where X(t) is the volume fraction of crystalline phase that is
transformed at time t, K is the combined rate constant that
involves the rates of growth and nucleation, and n is a
parameter dependent on the mechanism and dimensionality
Depending on crystal, growth n can reach a value
between 1 and 3. If n & 1, the growth will have
onedimensional (needle) character. In the case of n & 2, two
dimensions and n & 3 isotropic growth (sphere) are
The volume of the transformed phase will increase with
the simple power law (Avrami equation) at early stages of
the process before adjacent regions of the growing product
impinge. Thus, the overall growth rate in the system
decreases with time.
It is commonly assumed that hydration is the diffusion
controlled by the rate at which the reactants can diffuse
through the nanoporous layer of hydration product around
sum itno 0.00
g lla –0.02
lfow rc –0.04
Fig. 7 Isothermal DSC curves: heat flow of gypsum hydration/
crystallization for various w/g (for 0.5% HEMC content)
Fig. 8 a Curves from Fig. 6
transformed into Avrami
plots—degree of hydration/
crystallization X versus time
t. b. Log[-ln(1 - X)] versus
the remaining unhydrated gypsum particles. The point in
which the hydration process shifts away from nucleation
and growth is not well established, but it is an important
aspect of the hydration process.
The increasing use of mineral or polymer admixtures in
cementitious materials leads to the question how the
admixtures can affect hydration rate, especially at early
The wide range of sample properties can be observed as
hydration/crystallization proceeds, including heat of
hydrated phase, volume fraction, chemical shrinkage,
percolation of capillary porosity and setting time.
Hydration/crystallization of calcium hemihydrates, both
pure and with admixtures, was conducted by DSC research.
Figures 6 and 7 show curves of the process. Figure 6
presents curves obtained for samples with the
water-togypsum ratio equal to 0.66. Gypsum hydration occurs in
the main three-stage processes, i.e., nucleation (I),
acceleration (II) and deceleration (III). The hydration/
crystallization process occurs faster in the case of sample 6
(without admixtures). The increasing polymer content
causes a delay of the hydration/crystallization process. Not
only the induction period is extended, but also the rate of
the following hydration reaction is slowed down. This is
illustrated by lower values of maximum heat release and
broader exothermal peaks in the calorimetric curves of the
Figure 7 shows samples with various water-to-gypsum
ratios (0.60–0.74) and HEMC content equal to 0.5%. With
an increasing w/g value, the hydration/crystallization
process also delays. Both the presence of polymer in water
solution and increasing w/g ratio leading to the delay of
gypsum hydration/crystallization result from the same
prevention of nucleation process of gypsum species.
Figure 8a presents curves transformed in Fig. 6 to the
plots of the degree of hydration/crystallization X drawn
versus time. Figure 8b shows Log[-ln(1 - X)] versus log
t (obtained according to Avrami equation).
The values of crystal growth rate constant K (dependent
on the nucleation rate) are determined from Fig. 8a, b and
presented in Fig. 9 (parameter n & 1.4). An increasing
polymer admixture content causes a decrease in the
K value. The results indicate that HEMC is an efficient
agent disturbing the nucleation and crystallization of
gypsum (lower K value).
The morphology of calcium sulfate dihydrate crystals
depends on the formation conditions and the presence of
]. SEM microphotography is helpful for
observation of gypsum crystals. Figures 10 and 11 present
microphotographs of bending fracture area of the samples
with water-to-gypsum ratio equal to 0.6 (with the
admixture in Fig. 11—0.5% of HEMC). The crystal habit is
affected by the presence of polymer as may be seen in the
SEM photographs. Crystals in the absence of admixtures
are thin and elongated which is a result of their rapid
Fig. 9 Parameter K versus HEMC content
growth. They are longer than in the case when HEMC is
added. The presence of the polymer in the reacting solution
enhances the agglomeration of crystals. A decrease in the
total pore volume and increase in crystal overlapping lead
to a more impact structure and higher mechanical strength.
Admixtures in the form of water-soluble polymers
change the interaction between Ca2?, SO42- and OH- ions
forming hydrated crystals of gypsum. Polymers are not
built into gypsum crystals but can form a separate phase
(for example thin films in the pores).
Results of the above-presented studies on gypsum
properties and hydration/crystallization process lead to the
Higher supersaturation of gypsum solution (low w/g
ratio) means a quicker growth of gypsum crystals and
shorter setting time (Fig. 2). The increasing w/g leads
to the decrease in system saturation (Fig. 2).
Retardation of the setting and hydration/crystallization
process due to the presence of water-soluble HEMC is
observed (Fig. 3).
The rise of bending stress with increasing HEMC
content (Fig. 4) and w/g ratio (Fig. 5) is due to
changing morphological structure of the sample. The
decrease in pore volume and increase in crystal
overlapping cause a more compact structure and
higher strength (bending stress) of the samples.
Diffusion-controlled crystal growth is disturbed by the
presence of polymer chains in the solution system—a
decreased nucleation rate (Fig. 9).
Additives to gypsum in the form of water-soluble
polymers change the interaction between Ca2? and
SO42- ions forming hydrated crystals of gypsum. They
are not built-in crystals, but form a separate phase
(Figs. 10 and 11).
Acknowledgements The research was funded by the National Science
Centre (OPUS 6) under Project No. UMO-2013/11/B/ST8/04308.
Open Access This article is distributed under the terms of the Creative
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