Effect of Nano-sized Titanium Dioxide on Early
Age Hydration of Portland Cement
A.R. Jayapalan, B.Y. Lee, and K.E. Kurtis1
Abstract. The effect of nano-scale non-reactive anatase titanium dioxide (TiO2)
on early age hydration of cement was experimentally studied. Isothermal calorimetry was performed on cement pastes with two different particle sizes of TiO2
at replacement levels of 5, 7.5 and 10%. The addition of TiO2 to cement increased
the heat of hydration and accelerated the rate of reaction at early stages of hydration. This increase was found to be proportional to the percentage addition and the
fineness of TiO2. These results demonstrate that the addition of non-reactive nanoscale fillers could affect the rate of cement hydration by heterogeneous nucleation.
1 Introduction
The early age hydration of Portland cement remains of interest to researchers and
to industry because of potential implications relating to setting time, dimensional
stability and strength development. Researchers have noted an acceleration of cement hydration when fine fillers are added to cement [1, 2]. It has been observed
that fine (0.5µm to 4µm) powders of limestone [3], quartz [4], silica fume [5] and
pulverized fly ash [6], when added up to 15% cement replacement levels, can increase the rate of the cement hydration.
Addition of a fine non-reactive filler to cement modifies the hydration rate primarily due to dilution, modification of particle size distribution and heterogeneous
nucleation [4]. For increasing dosage rates of inert filler (when used as a partial
A.R. Jayapalan
Georgia Institute of Technology
e-mail: [email protected]
B.Y. Lee
Georgia Institute of Technology
e-mail: [email protected]
K.E. Kurtis
Georgia Institute of Technology
e-mail: [email protected]
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replacement of cement), cement dilution results in an increase of the water-tocement ratio (w/c) and a decrease in total cement content when water-to-solids
ratio (w/s) is kept constant. The modification of particle size distribution due to
chemically inert filler addition changes the system porosity, but the effect on cement hydration does not seem to be well documented in literature. The surface of
the fine fillers has been shown to provide sites for nucleation of cement hydration
products (C-S-H) and catalyzes the reaction by reducing the energy barrier [4].
The effectiveness of this catalysis depends on fineness and dosage of the filler [4].
In addition, other phenomena may occur, including water ab/adsorption by the
nanoparticles, interactions with nanoparticle surface treatments, and reaction of
materials previously presumed to be inert.
Most of the fine fillers previously examined react chemically to some extent in
the cement hydration process. Limestone may be slightly reactive with the Portland cement forming a monocarbonate phase [3]. Silica fume reacts with calcium
hydroxide by pozzolanic reaction. Considering the increasing interest in inert additives to cement, such as titanium dioxide (TiO2), there needs to be a study that directly focuses on the effect of non-reactive filler on cement hydration. Titanium
dioxide (TiO2) is added as a filler to cement for its photocatalytic activity. Research has shown that the photocatalytic activity is superior in nano-crystalline
TiO2 and that it exhibits maximum efficiency in anatase phase compared to rutile
or brookite phase [7, 8]. When added to Portland cement, TiO2 is considered to act
as inert filler and has not been believed to take part in the hydraulic reaction of
Portland cement.
The nano-size of the TiO2 particles could significantly affect the rate of hydration reaction especially in the early stages. Most of the previous researches on the
effect of fine inert fillers on cement hydration were conducted using micrometer
sized particles in the range of 0.5µm to 4µm. Jo et al. used nano-particles (of average particle size 40nm) of silicon dioxide (SiO2) [9], but SiO2 could react with
cement during early hydration due to its high pozzolanic activity.
Thus, the objective of this research was to better understand the effect of
addition of nano-sized and presumably non-reactive anatase TiO2, on the early hydration of Portland cement. Specifically, the effects of variation particle size and
percentage addition of TiO2 nanoparticles were examined as a part of this research. Isothermal calorimetry was used to characterize the influence of these factors on early age cement hydration.
2 Research Methodology
2.1 Materials
Six blended cements were prepared and examined to study the effect TiO2 dosage
rate and particle size on the cement hydration process. The potential Bogue composition of the ordinary Portland cement used for making all TiO2–blended
Effect of Nano-sized TiO2 on Early Age Hydration of Portland Cement
269
cement was 51.30% C3S, 19.73% C2S, 8.01% C3A and 9.41% C4AF and 0.40%
Na2Oeq. Two TiO2 powders (T1 and T2), of different surface areas were labblended with ordinary Portland cement, each at 5, 7.5 and 10% replacement by
mass. The properties of the TiO2 powders, obtained from Millennium Inorganic
Chemicals are given in Table1.
Table 1 Properties of anatase TiO2 added to cement (as obtained from the manufacturer)
2
Crystal Size (nm)
Agglomerate Size (µm)
Surface Area (m /g) pH
Purity (%)
T1
20-30
1.5
45-55
3.5-5.5
>97
T2
15-25
1.2
75-95
3.5-5.5
>95
2.2 Sample Preparation
A w/s of 0.50 was used for all the mixes. The mixing tools and materials were
stored at a constant temperature of 23°C for 24 hours before mixing. TiO2 powder
was added to the water and mixed using a handheld mixer for 60 seconds to disperse the agglomerates. The cement was then added, mixed by hand using a stirrer
for a maximum of 10 seconds and mixing continued with the handheld mixer.
The entire mixing period was maintained within 60±5 seconds. The paste was then
carefully poured into calorimeter capsules and the weight of the cement paste in
the capsule is noted.
2.3 Methods
Isothermal calorimetry was performed on triplicate paste samples using an eight
channel micro-calorimeter at 25°C. The capsules were placed inside the isothermal calorimeter within 240 seconds of addition of cement to water. The time of
addition of cement to water is considered as the starting time for each sample. The
data for initial 10 minutes of the tests was discarded to ensure that capsule reached
thermal equilibrium with the calorimeter. The rate of hydration was measured
every 60 seconds as power (mW) and was normalized per gram of cement. Since
the difference between the triplicate samples was not significant, one of the triplicate samples per mix was chosen for the comparative studies.
3 Results and Discussion
Isothermal calorimetry was carried out on the laboratory blended TiO2-cements to
study the effect of TiO2 replacement level and particle size on the early hydration
reaction. The data for the first 48 hours, starting from the time of mixing of cement and water, was analyzed.
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3.1 Rate of Hydration of TiO2-Blended Cements
Figure 1 shows the variation of the rate of hydration of ordinary Portland cement
and the TiO2-blended cements at 5, 7.5 and 10% replacement levels. For the graph
of ordinary Portland cement, the main peak of heat release corresponding to the
reaction of C3S can be observed from ~1 to ~8 hours. This main peak is followed
by a secondary peak corresponding to C3A hydration.
From Figure 1 it can be observed that all the mixes with TiO2 addition showed
accelerated hydration compared to ordinary Portland cement. For example, at 10%
replacement, the peak of C3A hydration was accelerated by around 80 and 180
minutes for the mixes with T1 and T2 respectively compared to the ordinary Portland cement. The increasing dosage of TiO2 was found to accelerate the rate of
hydration of the mixes and increase the peaks for C3S and C3A hydration. For instance, the increase in the peak of the C3A hydration compared to the control mix
was found to be 22.24%, 28.32% and 37.20% for the mixes with 5%, 7.5% and
10% replacement by T2. These results indicate that heterogeneous nucleation effect could be more dominant than dilution effect. Previous research using calorimetry has also shown that heterogeneous nucleation increases the rate of cement
hydration when fine inert materials are added to Portland cement [4].
From Figure 1 it can also be observed that the rate of hydration for the cement
mixes prepared by replacement with finer TiO2 (T2) was higher than all the mixes
8
OPC
7
T1 - 5%
T2 - 10%
Power (mW/g)
6
T1 - 7.5%
T1 - 10%
5
T1 - 10%
4
T2 - 5%
T2 - 7.5%
3
T2 - 10%
2
OPC
1
0
0
10
20
30
Time (hours)
Fig. 1 Rate of hydration of TiO2-blended cements
40
50
Effect of Nano-sized TiO2 on Early Age Hydration of Portland Cement
271
prepared with coarser TiO2 (T1). This shows that the rate of cement hydration in
the presence of TiO2 strongly depends on the size (Table1) of the nano-sized particles that are blended to the cement, with smaller particles accelerating the reaction
much more than larger particles. This reinforces the conclusion that nucleation effect, which depends on the surface area of the particles, could be dominant than
dilution effect in the case of addition of nano-TiO2 particles to cement.
3.2 Cumulative Energy Released by TiO2-Blended Cement
Figure 2 shows the variation of cumulative energy released with time of ordinary
Portland cement and the lab-blended TiO2 cements. For the graph of ordinary
Portland cement it can be seen that after the initial dormant period there is a rapid
increase in the total energy released, which corresponds to the peaks of C3S and
C3A hydration.
From Figure 2 it can be seen that the total energy evolved during the hydration
reaction increased as the percentage replacement of cement with TiO2 was increased. The total energy released also increased with the increasing surface area
of TiO2 used, again reinforcing the significant effect of the addition of nano-sized
particles in the nucleation reaction.
350
T1 - 10%
Energy (Joules/g)
300
T2 - 10%
250
OPC
200
T1 - 5%
150
OPC
T1 - 7.5%
T1 - 10%
100
T2 - 5%
50
T2 - 7.5%
T2 - 10%
0
0
10
20
30
40
Time (hours)
50
60
Fig. 2 Cumulative energy released by OPC and TiO2-blended cements
As explained earlier, the addition of TiO2 to cement paste affects the hydration
rate by dilution effect and heterogeneous nucleation. With increased dosage of
fine filler, the rate of hydration of cement and the total heat evolved will be decreased by the dilution effect and increased by nucleation effect. In all the tests
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that were conducted in this research it was observed that increasing the dosage and
lowering of particle size of filler caused the rate of reaction to increase (Figure 1).
Figure 2 shows that at all replacement rates by TiO2 the cumulative heat evolved is
greater than that of ordinary cement paste indicating an acceleration of cement hydration. This suggests that the heterogeneous nucleation effect is the dominant effect than dilution effect when nano-sized TiO2 particles are added to cement.
It should be noted here that the TiO2 used for these experiments were acidic,
presumably due to surface treatments used in their manufacturing. The dissolution
of surface groups (chlorides, sulphates or ammonium ions) could affect the rate of
reaction of the cement particles which are in a highly basic environment during
normal hydration. Research on the effect of the acidity and surface coating of TiO2
particles on the rate of nucleation is needed. It is our understanding, however, that
titania blended with portland cement in commercial use is not further processed to
remove or alter the surface chemistry; that is, the TiO2 examined herein should be
quite similar to that used in practice.
4 Conclusions
The effect of the addition of nano-sized TiO2 particles on the early hydration
reaction of cement was studied as a part of this research. When TiO2 of different
particle sizes were added to cement, the hydration reaction was accelerated and
the rate of hydration increased. The increase in the rate of reaction was proportional to the dosage of the TiO2. Smaller particles of TiO2 were found to accelerate
the reaction more than larger particles. Heterogeneous nucleation effect was found
to be dominant compared to the effect of dilution when inert TiO2 particles were
added to cement.
Acknowledgments. This research is supported by the National Science Foundation under
Grant Nos. CMMI-0825373 and CMMI-0855034. Any opinions, findings, and conclusions
or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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