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Influence of bentonite addition on the handling and physicalproperties of tricalcium silicate cement
Donghoon Kang, DDS (1), Takahiro Wada, PhD (2), Motohiro Uo, PhD (2,3), andTakashi Okiji, DDS, PhD (1)(1) Department of Pulp Biology and Endodontics, Graduate School of Medical and Dental Sciences,
Tokyo Medical and Dental University, (2) Department of Advanced Biomaterials, Graduate School of
Medical and Dental Sciences, Tokyo Medical and Dental University, (3) Department of Materials
Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan
Purpose: In this study, the influence of bentonite addition on the handling and physical properties of tricalcium
silicate is comprehensively investigated.
Materials and Methods: Bentonite and tricalcium silicate powder were used in this study and 0, 10, 20, 30 wt%
of bentonite added tricalcium silicate cements were prepared. Microstructural and morphological changes intricalcium silicate cements are evaluated with X-ray diffraction and scanning electron microscopy. To evaluate
the handling properties of the cement paste, flowability and injectability, as representative handling properties,
are evaluated. The compressive strength and setting time are measured using the ISO-9917 methods.
Results: Our analysis shows that as the amount of added bentonite increases, both flowability and injectability
improve significantly. Further, no significant difference is observed in the compressive strength and setting timeafter the bentonite addition.
Conclusion: Bentonite addition improves the handling properties of the tricalcium silicate paste without causing
a significant deterioration in its physical properties. These results provide important information for thedevelopment of novel tricalcium silicate-based cements with enhanced properties.
(Asian Pac J Dent 2018; 18: 37-44.)
Key Words: bentonite, handling property, physical property, tricalcium silicate

Introduction
Mineral trioxide aggregate (MTA) was originally introduced as an endodontic material for perforation repair androot-end filling in 1993 [1,2]. Since its introduction, it has also been applied to direct pulp capping andregenerative endodontics [3-5]. MTA consists of Portland cement, bismuth oxide as a radiopacifier, and gypsum
dihydrate. The main components of Portland cement are tricalcium silicate (TCS) and dicalcium silicate, which
are set via the hydration reaction. MTA shows good sealing ability [6], biocompatibility [7], antibacterial
activity [8], and hard tissue formation induction [9,10]. The Portland cement, which is the major component of
MTA contains not only calcium silicate, calcium aluminate, and tetracalcium aluminoferrite but also several
impurities such as ferric oxide, arsenic, and heavy metal elements. The contents of arsenic and other heavy
elements are low, but their toxicity is of concern [11,12]. The first commercial MTA product was gray, andwhite MTA was developed only later to improve the esthetic appearance. White MTA was obtained bypurification, which reduced the content of iron oxide, which is the main cause of the gray color, and other heavy
elements (cadmium, chromium, copper, and zinc). However, the contents of arsenic, nickel, and manganese
became comparatively higher than those in the gray MTA [13]. In terms of clinical properties, significantly lessleakage was observed for gray MTA when used as an apical barrier than for white MTA [14]. Therefore, a fillingmaterial that would possess simultaneously improved barrier properties and low impurity content is required.
TCS is the main component of Portland cement and exhibits the self-setting property upon hydration. TCS
has been applied for preparing bone cement [15], dental restorative material [16], and root filling material[17,18]. TCS can be manufactured using a conventional calcination process from a mixture of calcium carbonate
and silica. Recently, TCS preparation via the sol-gel process was also reported [19]. Therefore, synthetic TCS
could be a viable alternative to MTA material without impurities. However, its long setting time [20] and poor

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handling properties [21] have been of concern and several efforts have been conducted to improve theseproperties. For example, Ber et al. [22] reported that improvement in its handling properties and setting time canbe accomplished via the addition of methylcellulose and CaCl2. However, TCS-based cement may also sufferfrom similar limitations in properties, as exemplified by the fact that the final setting time of a TCS cement has
been measured as 180 min [23]. An improvement in the setting time with nanosilica addition was reported for
MTA and calcium silicate cement without any adverse effect on the mechanical strength [24,25]. Primus et al.
[21] reported an improvement in the handling properties of the experimental TCS-based cement without adecrease in its mechanical strength. However, current MTAs and TCS-based cements are less flowable thanother dental cements for fillings [21]. Therefore, the carriage of the cement to deep and narrow lesions is noteasy, and an improvement in the handling properties is required.
Clay is known to be a useful additive that can improve the rheological properties of industrial materials.
Bentonite (Bt), which is a type of a smectite clay, is widely used as a rheology modifier for print inks, cosmetics,
and industrial cement [26,27]. In particular, Bt is an inorganic rheology modifier for water-based systems [26],and thus, the application of Bt to the TCS-based cement (also an inorganic and water-based system) should notinhibit TCS hydration or setting behavior as much as organic rheology modifiers such as glycerol [28]. Bt is atypical clay mineral consisting of montmorillonite, which is a hydrous aluminum silicate with small amounts ofalkali and alkali-earth metals [26]. The unit structure of montmorillonite consists of two silica tetrahedral sheets
and an alumina octahedral sheet [26]. Water is absorbed into the inter-layer space and increases the c-axis
dimension of the clay. The absorbed water acts as a lubricant for the layers and the plasticity of the clay
increases with the increasing amount of water absorbed [26]. Taken together, we draw a hypothesis that Bt
addition into TCS-based cements improves their rheological and handling properties.
The main objective of this study is to investigate the influence of bentonite addition on the handling (i.e.,
flowability and injectability, as representative handling properties) and mechanical properties of synthesized
TCS-based cement.

Materials and Methods
Materials
Synthentic TCS (Nippon Shika Yakuhin Co., Ltd., Shimonoseki, Japan) was used as the base material for theexperimental cement. The crystal phase and the particle shape of the synthesized TCS were estimated by X-ray
diffraction (XRD, Miniflex, Rigaku Corp., Tokyo, Japan) and scanning electron microscopy (SEM, S-4500,
Hitachi High-Technologies Corp., Tokyo, Japan). Bt (Kaneatsu-Yuyaku, Toki, Japan) was used as theconsistency control agent. The experimental cements were prepared by replacing 0, 10, 20, and 30 wt% of TCS
with Bt; the corresponding cement samples are denoted as 0Bt, 10Bt, 20Bt, and 30Bt, respectively.
Relative flowability
The relative flowability was evaluated on the basis of the cement paste spreading area under compression
according to a previously reported method with some modifications [29]. Each experimental cement specimen
was mixed with distilled water (DW) in a water/powder ratio of 0.6 by weight. The mixture (6 mm in diameter, 1
mm in thickness) was placed at the center of a glass slide using a stainless steel mold. The amount of the cement
placed was accurately weighed using a precision balance (AG245, Mettler Toledo, Greifensee, Switzerland).
Subsequently, another slide glass was place on the material and 1 kg of weight was applied on the glass slide for38

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60 s at room temperature (24˚C). The load was applied within 2 min after mixing. After the load application, thearea of the spread mixture was estimated using the flat bed scanner (600 dpi in resolution, ApeosPort-IV C3375,
Fuji Xerox, Tokyo, Japan) and the image analysis software (ImageJ, Ver.1.49u, National Institute of Health,Bethesda, MD, USA). The relative flowability of the mixture was estimated as the spread area normalized
relative to the weight of the mixture as follows:
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒  𝐹𝑙𝑜𝑤𝑎𝑏𝑖𝑙𝑖𝑡𝑦 =𝑎𝑟𝑒𝑎  (𝑚𝑚 ! )    𝑤𝑒𝑖𝑔ℎ𝑡  𝑜𝑓  𝑡ℎ𝑒  𝑝𝑎𝑠𝑡𝑒  (𝑔)Relative injectability
The relative injectability of the experimental cement mixture was estimated using a modification of the syringepush-out test described in reference [30]. Each experimental cement specimen was mixed with DW in awater/powder ratio by weight of 0.5. Approximately, 0.28 g of the mixture was filled into a 1-mL disposable
syringe (SS-01T, Terumo, Tokyo, Japan). The syringe was placed in a universal testing machine (EZ LX,
Shimadzu, Kyoto, Japan), and the syringe plunger was pushed with a test speed of 15 mm/min to push out the
mixture from the syringe head at room temperature (24˚C). The push out test was carried out until the applied
force reached 50 N. The relative injectability was estimated as the percentage of the weight of the mixture
ejected from the syringe tip relative to the total weight of filled mixture.
𝐼𝑛𝑗𝑒𝑐𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦   % =𝑤1  ×  100  𝑤0where w0 is the total weight of the filled mixture and w1 is the weight of the mixture ejected from the syringetip.
Compressive strength
The compressive strength was determined according to ISO 9917-1; 2007. Each experimental cement specimen
was mixed with DW in a water/powder ratio by weight of 0.5. The mixture was added to a separable cylindricalstainless steel mold (4 mm in diameter and 6 mm in height). After filling the mixture, both sides of the moldwere covered by two glass plates, and kept at 37˚C for 1 day under 100% relative humidity (RH) until setting.
The specimen was removed from the mold after setting. A part of each specimen was immersed in DW at 37˚C
for 28 days. Those specimens were applied for the compression test using the universal testing machine (EZ LX,
Shimadzu), with a test speed of 0.75 mm/min.
Setting Time
The setting time was determined according to ISO 9917-1; 2007. Each experimental cement specimen was
mixed with DW in a water/powder ratio by weight of 0.5. The mixture was added into a stainless steel mold (10mm × 8 mm in cross section, 5 mm in depth) and kept at 37˚C at 95% RH. The setting time was estimated using
the modified Vicat needle penetration test (indenter mass of 400 g, needle tip has a flat circular end with 1 mmof diameter). The indenter was carefully brought into contact vertically with the mixture surface with loading.
This measurement was repeated every 60 s until setting was complete. The setting time was determined as theperiod from the end of mixing until the time that no clear indentation mark was observed on the mixture surface
after loading.
Statistical analysis
Statistical analysis was performed by 1-way analysis of variance (ANOVA) and Tukey-Kramer test was used toperform multiple comparison tests with a significance level of 0.05.

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Results
Microstructure of TCS powder and set cement, and results of XRD analysis
Figure 1 shows the SEM images of TCS powder and commercial MTA (ProRoot MTA, Dentsply, Johnson City,
TN, USA) powder. Particles of the TCS powder are 6-40 µm in diameter and generally spherical, as shown in
Figs. 1(A) and 1(B). The MTA particles are 10-60 µm in diameter and cuboid in shape. In addition, theradiopacifier particles of MTA appear white, as shown in Figs. 1(C) and 1(D). Figure 2 shows the hardened
specimens of TCS and MTA after curing for 7 days. The TCS specimen exhibits aggregated spherical particles,
while the MTA specimen exhibits granular appearance together with scattered radiopacifier particles. As shownin Fig. 3, as per XRD analysis results, TCS was confirmed to be mainly composed of the tricalcium silicate(Ca3SiO5) phase.

Fig. 1 (left) SEM images of (A,B) TCS and (C,D) MTA powder (ProRoot MTA). TCS particles are 6-40 µm in
diameter and generally exhibit a spherical shape. MTA particles are 10-60 µm in diameter and exhibit a squareshape. The radiopacifier of MTA appears as white particles in SEM images.
Fig. 2 (right) SEM images of (A,B) hardened specimen of TCS after curing for 7 days and (C,D) hardened
specimen of MTA (ProRoot MTA) after curing for 7 days. Clusters of particles are visible in each image.

Fig. 3 (left) XRD analysis of TCS powder. ▼ indicates peaks characteristic for Ca3SiO5 phase.
Fig. 4 (right) Relative flowability of the cement paste. The 30Bt sample shows significantly improved flowability.
All values are expressed as mean ± standard deviation (n = 14). *p < 0.05

Relative flowability and injectability
Figure 4 shows the effect of Bt addition on the relative flowability of the experimental cement. The relative40

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flowability of the sample with 20 wt% of Bt (20Bt) showed small but significant increase (p < 0.05) comparedwith that of the control (0Bt); the sample with 30 wt% of Bt (30Bt), by contrast, showed significant
improvement compared to all other groups (p < 0.05).
Figure 5 shows the typical load/plunger stroke curves of the syringe injection tests. The load required to
stroke the plunger decreased with increasing wt% of Bt. In the case of the 20Bt and 30Bt samples, plateau
regions were observed, which show that plunger stroke with an almost constant force had to be applied. In
particular, the 30Bt sample exhibited a long plateau region and the plunger was continuously pushed at around
20 N.

Fig. 5 (left) Load/plunger stroke curves of the cement pastes with added bentonite, as measured by the injection
test. The area spanning the plateau region in the curve increased with increasing amount of Bt added.
Fig. 6 (right) Results of injectability analysis. Bt addition increases the injectability of the cement paste. All values
are expressed as mean ± standard deviation (n = 5). *p < 0.05

Fig. 7 (left) Results of compressive strength after 1 day. Compressive strength of the 10Bt sample after 1 day
exhibited a significant increase when compared to that of 0Bt. All values are expressed as mean ± standard
deviation (n = 5). *p < 0.05
Fig. 8 (right) Results of compressive strength after 28 days. No significant differences were observed between thesamples examined. All values are expressed as mean ± standard deviation (n = 5). *p < 0.05

This tendency was also observed in Fig. 6, which shows the dependence of the relative injectability on the Bt
content. Relative injectability increased significantly with increasing wt% of Bt (p < 0.05). In the case of the
30Bt sample, most of the mixture in the syringe could be ejected.

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Compressive strength
Figures 7 and 8 show the compressive strength of TCS and Bt-added experimental cements after 1 day and 28
days. After 1 day, 10Bt showed only a slight increase in compressive strength, while 20Bt and 30Bt samples did
not show a significant change. After 28 days, no significant difference was observed between the groups.
Setting time
Figure 9 shows the setting time of the cements. It is apparent that there is no significant difference between thesetting times of each group.

Fig. 9 The results of setting time analysis
No significant differences were observed
between the samples examined. All values are
expressed as mean ± standard deviation (n = 3).
*p < 0.05

Discussion
The relative flowability and injectability were estimated as representative handling properties. The results can besummarized as (1) the poor flowability of the TCS paste was improved with Bt addition of more than 20 wt%(Fig. 3); and (2) the TCS paste obtained measurable injectability by the addition of ≥10 wt% Bt (Fig. 4). Thus,
our hypothesis that “Bt addition improves the rheological and handling properties of TCS pastes” was accepted.
Injectability was measured using a long and narrow syringe at a rapid test speed (15 mm/min) and TCS paste
showed no extrusion from the syringe. On the other hand, the addition of Bt to the TCS paste at a quantity higher
than 10 wt% improved the injectability. In particular, the 30Bt sample showed a clear plateau region in its
injection curve under a constant load (about 20 N). Montufar et al. [30] reported the injection properties ofcalcium phosphate cement; in their report, the load to syringe stroke dependency appeared to show three phases.
In the first phase of the injection curve, the applied load increased rapidly until a yield point was reached. Afterthe yield point, the cement paste was extruded at a constant load (i.e., the second phase). After most of the paste
was extruded, the applied load increased rapidly again in the third phase. The 30Bt sample examined in thisstudy displayed similar three phases. Consequently, the results of the syringe tests suggest that Bt addition atover 30-wt% imparts the TCS paste with excellent injectability properties.
Both the relative flowability and injectability improved after the Bt addition, and exhibited similar trends.
These improvements can be attributed to the good plasticity of wet clay. The results of the comprehensive
analysis confirm that the improvements observed in the flowability and injectability of the Bt-containing TCS
paste are a direct consequence of the beneficial properties of bentonite.
In this study, Bt addition caused no significant decrease in the compressive strength of TCS despite of adecrease in the TCS content which is higher compressive strength than Bt. In fact, in the 10Bt sample, a slightincrease in compressive strength after curing for 1 day was observed. In the cases of the 20Bt and 30Bt samples,42

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there was no significant difference in the compressive strength and the handling properties improved moresignificantly. The compressive strength results obtained for the Bt-added cement can be attributed to the dense
condensation of the cement in the mold, caused by the rheological improvements through the Bt addition. The
compressive strength of commercial MTA has been reported as; 14.3 and 43.4 MPa after 24 h and 21 days,
respectively, for white MTA Angelus [31]; 45.84 and 86.02 MPa after 3 and 28 days, respectively, for ProRoot
MTA [32]; and 27.41, 43.65 and 30.77 MPa after 1, 3, and 7 days, respectively, for ProRoot MTA [33]. These
data suggest that the setting reaction, which involves the hydration of calcium silicates, which are the main
component of MTA, continues after the initial setting and leads to an increase in strength. The TCS-based
cement samples examined in this study showed a relatively higher compressive strength than the commercial
MTA after 24 h. However, after 28 days, their strength was lower than that of the commercial MTA. This result
is directly linked to the fact that MTA cements are based on Portland cement, which consists not only of TCS
but also dicalcium silicate, calcium aluminate, and other compound oxides. TCS shows a faster hydration
reaction when compared to others such as dicalcium silicate [34]; consequently, the TCS-based cement in thisstudy showed a higher compressive strength after 24 h.
Further, the Bt addition induced no acceleration or retardation effect on the TCS setting process. Bt does notundergo hydration reaction, and thus, the setting behavior depends on the TCS hydration reaction only.
Therefore, the lack of dependence of the setting time on the Bt content is a reasonable and predicted outcome.
Further, the setting times of commercial MTAs have been reported as 48.3 min for white MTA-Angelus [31],140 min for ProRoot MTA white [32], and 219 min for ProRoot MTA [33]. The estimated setting time measured
in this study was about 130–140 min, regardless of the Bt content and was identical to the value reported for
ProRoot MTA.
The flowability and injectability of the TCS paste was improved by the addition of increasing amount of Bt.
In particular, addition of 30 wt% of Bt conferred both flowable and viscous properties on the TCS paste with nosignificant change in the setting and mechanical properties. Thus, this study demonstrated that Bt is a promising
additive of TCS-based cements for improved handling properties.
Acknowledgments
This work was supported by JSPS KAKENHI (Grant No. JP16H02688).
The authors would like to acknowledge Mr. Kazuyoshi Yokota of the Nippon Shika Yakuhin Co., Ltd. for the synthesis of
TCS.
Conflicts of Interest
The authors declare no conflicts of interest.

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Correspondence to:Dr. Takahiro Wada
Department of Advanced Biomaterials, Graduate School of Medical and Dental Sciences,
Tokyo Medical and Dental University
1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-5467 E-mail: wada.abm@tmd.ac.jp
Accepted November 30, 2018.
Copyright ©2018 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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