Ceramics International 42 (2016) 11732–11738

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Calcium aluminate cement-based compositions for biomaterialapplications

R.M. Parreira a, T.L. Andrade a, A.P. Luz b, V.C. Pandolfelli b, I.R. Oliveira a,n

a Institute for Research and Development, University of Vale do Paraíba, Av. Shishima Hifumi, 2911 São José dos Campos, SP, Brazilb Materials Engineering Department, Federal University of São Carlos, UFSCar, Rod. Washington Luiz, km 235, São Carlos, SP, Brazil

a r t i c l e i n f o

Article history:Received 23 March 2016Accepted 18 April 2016Available online 19 April 2016

Keywords:Chemically-bonded ceramicCalcium aluminate cementBiomaterialProperties

x.doi.org/10.1016/j.ceramint.2016.04.09242/& 2016 Elsevier Ltd and Techna Group S.r

esponding author.ail address: ivoneregina.oliveira@gmail.com (I

a b s t r a c t

Calcium aluminate cement (CAC) Calcium aluminate cement (CAC) is classified as a hydraulic binderpresenting various advantages, such as fast hardening at room temperature and suitable rheologicalproperties, when compared to traditional materials. Based on this, CAC has been investigated as an al-ternative biomaterial in order to overcome some drawbacks presented by commercial products usuallyapplied in the dentistry (mineral trioxide aggregate¼MTA and glass ionomer) and orthopedics (poly-methyl methacrylate¼PMMA) fields. In this work, the properties of CAC-based compositions containingdifferent amounts of additives (i.e., alumina, zirconia, zinc oxide, hydroxyapatite, tricalcium phosphate,chitosan and collagen) were evaluated and the attained results were compared to those of MTA, PMMAand two glass ionomers (Meron and Vidrion F). The characterization of the selected materials comprisedtheir particle size distribution, as well as the cold crushing strength, apparent porosity, pore size dis-tribution and radiopacity. Plain CAC presented higher crushing strength than the commercial productsused in dentistry and the blend of this cement with 4 wt% of additives (alumina, zirconia, zinc oxide,tricalcium phosphate or hydroxyapatite) resulted in improved mechanical performance when comparedto PMMA (cement for bone repair). The addition of zinc oxide and hydroxyapatite to CAC also gave rise tosamples with low porosity levels and smaller pore sizes after their contact with simulated body fluidsolution over 7 days at 37 °C. Conversely, collagen and chitosan-containing compositions showed higherporosity and lower mechanical strength. Regarding the radiopacity results, the evaluated compositionspresented better results than the commercial products, except for MTA.

& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

1. Introduction

Cements used in orthopedics must fulfill many requirements,such as low curing temperature (to prevent thermal necrosis of thebone tissue during setting), suitable setting time and high crush-ing strength (in order to withstand the compressive load devel-oped during daily activities). The most used cement for bone re-pair is comprised mainly by polymethyl methacrylate (PMMA),which presents excellent mechanical properties when comparedto other polymeric materials [1]. However, this product still pre-sents drawbacks regarding its handling behavior (too low con-sistency and strong odor) and biocompatibility. Additionally, thereactions associated to this cement are exothermic, resulting inlocal heat increase which can damage the surrounding tissue.Based on these aspects, the aim is to find and develop new alter-native sources to overcome these limitations [1].

.l. All rights reserved.

.R. Oliveira).

Other classes of materials that have been investigated fordentistry and orthopedic fields are the chemically-bonded cera-mics (CBC), whose setting behavior is controlled by specific che-mical reactions that can take place at room temperature [2]. Forinstance, calcium phosphates and calcium sulfates are extensivelyused to fill bone defects and stabilize fractured vertebrae [1,2].Zinc phosphate, on the other hand, is commonly used as dentalcement, despite its low mechanical strength, poor chemical sta-bility and esthetics for a permanent filling product [2]. It must behighlighted that dental materials must present improved me-chanical performance under compressive strength in order towithstand the stresses derived from the long term application.

Calcium silicate (CSC) and calcium aluminate (CAC) cements(also classified as CBC materials or hydraulic binders that formhydrates after its dissolution in water) can be used as dental re-storative products [1,3]. The latter presents a suitable performanceas a root-end filling component, as it can overcome some draw-backs of commercial CSC-based compositions and MTA (mineraltrioxide aggregate), such as: long setting time, high porosity and

Table 1Additives and total solid contents in the prepared suspensions containing collagenand chitosan.

Additives Content (wt%) Solid content in the prepared suspension (%)

Collagen 1 732 684 636 5810 53

Chitosan 4 776 7510 72

R.M. Parreira et al. / Ceramics International 42 (2016) 11732–11738 11733

low mechanical strength. CAC also presents interesting features asa biomaterial, for example: (i) better flowability and handlingproperties [4,5], (ii) adjustable rheology and a setting time at roomtemperature, resulting in high initial mechanical strength [2], (iii)biocompatibility when tested in subcutaneous tissues of ratswithout inflammatory reactions [6,7], (iv) ability to induce in situhydroxyapatite generation [2,8], (v) it can act as a barrier pre-venting bacterial microleakage [9], and (vi) low expansion favoringgood retention and adhesion to the teeth [2].

CAC also has the potential to be used in orthopedic applicationsdue to its high viscosity (which injects the prepared mixture justafter the mixing step), low heat release associated to the chemicalreactions of this product with aqueous solution during setting andhigh biocompatibility [1,10].

Some studies have reported on calcium aluminate cement ap-plications to repair bone defects, based on the fact that its che-mical composition and thermal expansion coefficient are similar toteeth and human bones [3,11]. CAC-based zirconia-containingmaterials has also been developed to stabilize compressive ver-tebrae fractures, leading to mechanical strength values similar toPMMA, good stability after 6 months in contact with phosphatebuffer solution and porosity levels in the range of 10–15%. Whenevaluated in sheep vertebrae, CAC-based mixtures showedless inflammation and better adaptation to the bone tissue thanPMMA [1].

Alumina, zinc oxide, calcium phosphates, collagen and chitosanare other alternative additives that can be incorporated into CAC-based compositions. In addition to suitable biocompatibility, cor-rosion and wear resistance, chemical stability and high crushingstrength, zirconia and alumina have the advantage of acting asbioceramic reinforcements for cements [12]. Some investigationshave focused on the development of biphasic calcium phosphatecomposites containing alumina, as they can present improvedmechanical properties, biocompatibility and bone formation incombination with adjacent hard tissues [13]. Zinc oxide presentsantibacterial ability and biocompatibility [14] allowing for thedesign of engineered zinc-containing ceramic composites for bonetissue repair, and it has the following benefits: (i) the presence ofapatite in tricalcium Zn-phosphate may induce cell proliferation[15], and (ii) it can stimulate osteoblast cells in aluminate-basedmaterials, favoring the mineralization process [16].

Calcium phosphate ceramics are used to repair bone defects,increase and maintain alveolar bone crests, relocation of dentalroot, ear implants, lining metallic implants, and others. Hydro-xyapatite and tricalcium phosphate are the most common com-pounds of this category [17]. As reported by Roemhildt et al. [18],cement-based compositions (presenting high mechanicalstrength) comprising a blend of calcium phosphatesþcalciumaluminate have been developed for bone and joint repair. In thiscase, calcium phosphate improves the biological activity of thematerial, whereas calcium aluminate is responsible for increasingthe mechanical strength of this composite.

Collagen and chitosan are biopolymers [19,20] that have re-ceived great attention. The increasing use of the former is relatedto the low immunological reaction index and its ability to generatefibers from soluble mixes, whose properties are similar to the onesfound in tissues. On the other hand, chitosan presents non-toxicity,biocompatibility, antioxidant and antimicrobial abilities, as well ashealing and anti-inflammation characteristics. These featuresmake the later able to be used in surgical sutures, dental implants,bone reconstruction, contact lenses, controlled drug delivery, en-capsulation material, etc. [21]. Kishen et al. [22] stated that theaddition of chitosan nanoparticles to dental cements led to a majorimprovement in antimicrobial properties and leaching ability ofthe antibacterial components in the evaluated systems. However,chitosan also has some drawbacks regarding its mechanical

strength and biological performance, which requires blending in-organic materials (i.e., hydroxyapatite, calcium phosphate and si-lica) with this biopolymer [23].

Considering these aspects, this paper focuses on the prepara-tion of various CAC-based mixtures containing different amountsof additives (alumina, zirconia, zinc oxide, hydroxyapatite, tri-calcium phosphate, chitosan and collagen) in order to evaluate andcompare their properties with commercial products commonlyapplied in dentistry (MTA and glass ionomers) and orthopedics(PMMA).

2. Materials and techniques

The following materials were used in this work: CACH, pre-pared using the dry-mixture of calcium aluminate cement (Ker-neos Aluminates, France [5]) with a polyglycol-based dispersant(0.6 wt%, Basf, Germany) and a plasticizer CaCl2 �2H2O (2.8 wt%,Labsynth, Brazil) in a ball mill for 1 h, and the additives: (1) cal-cined alumina (CT3000SG, Almatis, USA), (2) monoclinic zirconia(CC-10, Saint-Gobain, France), (3) zinc oxide (Synth, Brazil),(4) hydroxyapatite (Sigma-Aldrich 21,223, USA), (5) tricalciumphosphate (Cadisa, Brazil), (6) chitosan (Polymar, Brazil) and(7) bonive collagen (type I, JBS, Brazil). The selected compoundspresented high purity and their particle size distribution wasevaluated using S3550 equipment (Microtrac, USA) after keepingthe samples for 15 min in an ultrasonic bath (Sonics Vibra-cell,USA, model VCX 500) for the powder deagglomeration. Ad-ditionally, commercial products were analyzed and used as re-ference materials: ortophedic PMMA cement (Bio mecânica, Bra-zil) and dentistry cements, such as white MTA, (Angelus, Brazil),glass ionomers for base and lining (Vidrion F, SS White, Brazil) andfor cementing and restoration (Meron, Voco, Germany).

A total of 1, 2, 4, 6, and 10 wt% of the selected additives wereadded to CACH and these compositions were mixed in a ball millfor 1 h. After that, aqueous suspensions (80 wt% of solids) wereprepared using a lab mixer and molded as cylindrical samples(diameter¼16 mm and height¼18 mm). Collagen and chitosan-containing compositions required extra water to mold the sam-ples. Table 1 shows the resulting solid content for each preparedsuspension with these additives.

The samples were kept at 37 °C for 24 h in a water saturatedenvironment, demolded and dried at 110 °C for another 24 h. Ap-parent porosity measurements were carried out in dried materials,which were classified as “without treatment” tests. Other cast cy-linders were also placed in contact with simulated body fluid (SBF)[24] solution at 37 °C for 7 days. Humid samples were then sub-jected to uniaxial compression tests, whereas others were dried at110 °C for 24 h and their apparent porosity was evaluated (“afterSBF treatment” measurements).

Table 2Reagents used for the preparation of 1 L of simulated body fluid solution.

Reagents SBF 1.5

water 400 mLNaCl 11.992 gNaHCO3 0.529 gKCl 0.335 gK2HPO4 0.261 gMgCl2 � 6H2O 0.458 gHCl 0.1 M 15 mLCaCl2 �2H2O 0.551 gNa2SO4 0.107 g(CH2OH)3CNH2 0.05 M Required amount to adjust pH¼7.25HCl 0.1 M Required amount to adjust pH¼7.25

Fig. 1. Particle size distribution of calcium aluminate cement (CAC), alumina, zir-conia, zinc oxide, hydroxyapatite, tricalcium phosphate, chitosan and collagen.

R.M. Parreira et al. / Ceramics International 42 (2016) 11732–1173811734

The SBF solution was prepared as described in Table 2, usingthe reagents amount

inorganic ions concentrationratio higher than 1.5 (SBF 1.5) in order to

speed up the apatite generation.Uniaxial cold crushing strength tests (ISO 9917-1) of five sam-

ples of each prepared composition were carried out under acrosshead rate of 0.15 mm/min in mechanical testing equipment(DL 10000, EMIC, Brazil). The modulus of rupture (sR, MPa) wascalculated as follows:

σπ

=( )

PD

41R 2

where, P (N) is the maximum applied load and D (mm) is theaverage sample diameter.

Apparent porosity (AP) was measured via the Archimedesmethod (ASTM C830), where the dried (Ms), immersed (Mi, at-tained after keeping the samples for 1 h under vacuum in kero-sene, ρ¼0.8 g/cm3) and humid (Mu) masses of three samples foreach composition were considered in order to calculate thisproperty (Eq. (2)).

= −−

×( )

⎛⎝⎜

⎞⎠⎟

M MM M

AP 1002

u s

u i

Samples containing 4 wt% of the additives were evaluated viamercury intrusion porosimetry technique (Autopore IV 9500equipment, Micromeritics, USA) in order to identify their pore sizedistribution before and after treatment with SBF. This method isbased on the Washburn equation (Eq. (3)), where D is the porediameter, P is the applied pressure (Pa), γ is the mercury surfacetension (J/m2) and ϕ is the contact angle assumed to be (130°)between the mercury and the sample. The mercury volume (V)that penetrates the pores is directly measured as a function of theapplied pressure.

γ φ= ( ) ( )−D P4 cos 31

Aqueous CACH suspensions containing 4 wt% of additives(alumina, zirconia, zinc oxide, hydroxyapatite and tricalciumphosphate) or 1 wt% of collagen or chitosan were also prepared,molded as cylindrical samples (d¼10 mm and h¼1 mm) and keptat 37 °C for 24 h in a saturated environment (relativehumidity¼100%). After that, these materials were demolded andplaced inside the chamber again at 37 °C under a saturated en-vironment for a total of 3 days, which was followed by a dryingstep at 37 °C (unsaturated environment) for another 24 h for theradiopacity tests.

Radiopacity measurements consisted of placing the samples onaluminum stair steps (99%, alloy 1100) with a thickness of 1–10 mm [in order to compare the steps of the stair (which wascontinuously incremented by 1 mm) and the prepared materials]on an occlusal film (Insight Carestream dental, Kodak) IO-41Oclusal (REF 1169143, Lot 56301405). The tests were carried out in

X ray equipment (Procion, ION 70X, Brazil) working with 70 KVpradiation and 8 mA. 20 cm was selected as the focus-object dis-tance and the exposition time was 0.32 s The films were manuallyprocessed in a dark room according to the time/temperature ratiorecommended by the manufacturer (Kodak, Manaus, Brazil),where they were initially kept for 3 min in a X-ray developingsolution, washed in water for 30 s, placed in a fixing solution for3 min, washed again in water for 10 min and dried. Based on theattained radiographs, the optical density (OD) of the samples andof each step of the aluminum stair was defined using a photodensitometer (MRA Industry Electronic Equipment Ltd., RibeirãoPreto, Brazil). The optical density results presented in this workrepresent the average of 4 measurements carried out for eachcomposition and the collected data were also adjusted by a poly-nomial fitting in order to obtain the equivalent value of radiopacityin mm Al.

It must be pointed out that the same tests described abovewere also carried out in the commercial products selected as areference. In this case, the samples were prepared according to theinstructions provided by the manufacturers.

3. Results and discussions

Fig. 1 shows the particle size distribution of plain CAC and theevaluated additives. Zinc oxide presented the lowest particle size(D50¼0.8 mm) whereas those for collagen (D50¼118.0 mm) andchitosan (D50¼50.0 mm) were higher than plain CAC (D50

¼7.0 mm).The crushing strength of the samples kept in contact with SBF

for 7 days is shown in Fig. 2. The addition of tricalcium phosphate,hydroxyapatite, zirconia, alumina and mainly zinc oxide to theCAC-based composition increased the CACH mechanical strength.Nevertheless, plain CACH still showed higher mechanical re-sistance (6771.8 MPa) than Meron, MTA and Vidrion F

Fig. 2. Crushing strength of commercial products (MTA, Meron, Vidrion F and PMMA) and the prepared compositions: plain CACH or CACHþdifferent contents of alumina,zirconia, zinc oxide, hydroxyapatite, tricalcium phosphate, chitosan or collagen. The samples were previously kept in contact with SBF solution for 7 days at 37 °C.

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(commercial products for dentistry) and the zinc oxide-containingmixture had similar or a slightly better performance than PMMA(cement for bone reconstruction). The high crushing strength at-tained for the evaluated compositions after only 24 h of curing isan important property, especially when considering these mate-rials as likely candidates for orthopedic applications. In this work,zinc oxide-containing mixtures were also evaluated after 24 h ofcuring resulting in mechanical resistance similar to PMMA(7775.1 MPa).

Although hydroxyapatites may be alternative materials for re-placing human hard tissues, they usually present low mechanicalstrength due to their decomposition into phosphate phases, suchas tricalcium and tetra-calcium phosphates [25]. As indicated inFig. 2, the addition of tricalcium phosphate to the evaluatedmixture resulted in lower modulus of rupture than the alumina,

Fig. 3. Apparent porosity of commercial products (MTA, Meron, Vidrion F and PMMA),hydroxyapatite, tricalcium phosphate, chitosan and collagen. The samples were evaluate37 °C for 7 days.

zirconia, zinc oxide and hydroxyapatite-containing compositions,which indicates that this phosphate additive should not effectivelyimprove the mechanical properties of CAC-based systems. For thesamples containing a blend of collagen or chitosan with CACH, thecrushing strength values decreased with the additives content,resulting in a worse performance than the reference material(CACH). Considering that biomaterials for fractured vertebraetreatments must present high mechanical resistance due to thestresses they will face [1], the compositions containing alumina,zirconia, zinc oxide, hydroxyapatite or tricalcium phosphate werethe most promising ones (Fig. 2).

Previous studies [5,27] reported that adding fine oxide powdersto CAC should improve its mechanical strength. Besides that, thepresence of two or more raw materials in a cement-based mixturewill affect the rheological properties and particle packing of the

plain CACH mixture or CACHþdifferent contents of alumina, zirconia, zinc oxide,d (a) before and (b) after their contact with simulated body fluid (SBF) solution at

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system, depending on the physical features of the selected ad-ditives (i.e., size, shape and density). Fine powders tend to fill inthe voids among the large particles of the mixture decreasing theporosity level of the prepared samples [27]. As a consequence, theporosity results will depend on the features and content of theadditives incorporated into the calcium aluminate cement.

Fig. 3 shows the apparent porosity values for the evaluatedcompositions before and after the samples' contact with simulatedbody fluid (SBF) solution at 37 °C for 7 days. In general, all mate-rials showed similar apparent porosity (A. P.) levels before theircontact with SBF, except chitosan and collagen-containing sampleswhere A. P. increased with the additive content (Fig. 3a). Thisbehavior might be associated to the fact that both of them (chit-osan and collagen) have higher average particle sizes than CAC(Fig. 1), which can result in molded samples with lower packingand, consequently, reduced density. Besides that, a higher amountof water was required during the mixing step of these two sys-tems, as chitosan and collagen usually induce gel-phase genera-tion. Therefore, these aspects favored the higher porosity levels ofthese samples after drying at 110 °C for 24 h. Not only the overallporosity, but also the pore size distribution of the samples con-taining 4 wt% of chitosan or collagen was affected, as shown inFig. 4a, leading to larger pores when compared to the othercompositions before their contact with SBF solution.

Regarding the commercial products, MTA presented the high-est pore content (�34%, Fig. 3a), which is in agreement withprevious studies [5,26] that pointed out that, due to the lowflowability of this material, a large amount of water is requiredduring its preparation resulting in samples with increased porosityand lower mechanical strength.

Figs. 3b and 4b show the reduction in the apparent porosityand pore size distribution of the cement-based samples after thetreatment with SBF solution. This trend was observed for plain

Fig. 4. Pore size distribution of plain CACH or CACHþ4 wt% of alumina, zirconia,zinc oxide, hydroxyapatite, tricalcium phosphate, chitosan or collagen. All sampleswere evaluated (a) before and (b) after their contact with simulated body fluid(SBF) solution at 37 °C for 7 days.

CACH and the mixtures containing 4 wt% of alumina, zirconia,hydroxyapatite, tricalcium phosphate and zinc oxide.

These results can be explained by the cements' bioactivitywhen in a simulated body fluid medium. Bioactivity is defined asthe material's ability to generate a carbonated hydroxyapatitelayer on its surface in order to induce strong interfacial bonds withliving tissues. This carbonated hydroxyapatite layer is chemicallyand structurally equivalent to the bone mineral phase, favoring thechemical bonding between these components [17].

A previous study by the present authors [28] indicated thatCACH mixtures containing commercial or synthesized hydro-xyapatite showed lower porosity and pore size distribution aftertheir interaction with SBF. This behavior was associated to apatiteprecipitation on the materials’ surface, which was detected byfollowing the pH and calcium ions concentration evolution in theliquid medium. Thus, the same effect should take place for theplain CACH and CACHþadditive compositions, as the precipitatedphosphates (derived from the interaction of the cements with thecompounds available in the SBF solution) might coat the solidsurface and fill in the pores. Consequently, the overall pore size ofthe prepared samples was lower when compared to the materialsonly placed in contact with water (Fig. 4).

The lower pore size of the chitosan-containing sample (Fig. 4b)is related to the complexation of calcium ions after its interactionwith simulated body fluid solution [29]. On the other hand, ZnO-containing composites are non-cytotoxic and have the ability toinduce the precipitation of important compounds for the miner-alization process when in the presence of SBF solution. Not onlyZn3(PO4)2, but also hydroxyapatite and other phosphates are ex-pected to be generated when ZnO is applied as a biomaterial.Additionally, the precipitation of zinc phosphate can slow downthe calcium phosphate layer build up on the ZnO-containing ma-terial's surface due to the lower solubility constant of the formerthat affects the nucleation rate of Ca3(PO4)2 and other calcium-based phosphates [17].

Bioactivity is also an important aspect as it provides betterbonding between the dental materials and the tooth structure. Forinstance, the improved bioactivity of a product may help preventthe formation of secondary caries (derived from remaining voidsbetween the filling material and the teeth, allowing the migrationof bacteria to these regions) and consequently the replacement ofthe dental restoration. Moreover, strong bonding and in situ apa-tite generation are parameters that make tooth restoration moresimilar to its original structure, which are desired advantagesespecially when comparing cements to other filling products suchas amalgam [2].

Regarding the evaluated commercial products, PMMA (whichusually does not lead to chemical or biological bonds with bones[30]) was the only one that did not present the pore size decreaseafter SBF treatment (Fig. 5b). On the other hand, a major reductionof pore size was detected for the MTA, highlighting its well-knownand better bioactivity. As reported in the literature [8,31], the si-licate phase (Ca2SiO4) contained in this product favors the hy-droxyapatite generation in SBF medium, which can also stimulatethe deposition of mineralized tissue on the biomaterial surface andinduce further integration of the latter with the surroundingin vivo components.

The presence of polyacrylic acid for the Meron and Vidrion Fcompositions (glass ionomers) inhibits the apatite formation of thesample's surface in SBF liquid [32]. However, the mercury intru-sion porosimetry results indicate the pore sizes decrease in thesematerials (Fig. 5b). This behavior may be associated to the fact thatthe polymeric samples were not dried at 110 °C after contact withthe SBF solution.

Radiopacity is an indispensable requirement for dental mate-rials, as it must be radiopaque to locate it precisely, thus

Fig. 5. Pore size distribution of the commercial products MTA, Meron, Vidrion Fand PMMA. All samples were evaluated (a) before and (b) after their contact withsimulated body fluid (SBF) solution at 37 °C for 7 days.

Fig. 6. Average radiopacity (expressed as aluminum thickness, mm Al) and opticaldensity of plain CACH, CACHþ4 wt% of additives (alumina, zirconia, zinc oxide,hydroxyapatite or tricalcium phosphate) or 1 wt% (chitosan or collagen) and thecommercial products (MTA, Meron, Vidrion F and PMMA).

R.M. Parreira et al. / Ceramics International 42 (2016) 11732–11738 11737

professional dentists will be able to identify and delimitate thetooth-restoration interface. Low radiopacity of biomaterials maycause them to be mistaken for caries or voids [33]. Furthermore,high radiopacity is required for some vertebral compression frac-ture treatments that consider cement injections, as proper iden-tification of the biomaterial after application is fundamental toprevent the material leaking to the spine or veins [1].

According to the tests carried out in this work, the commercialproducts presented low radiopacity (except MTA) despite thepresence of barium sulfate (ZBa¼56) for Vidrion F and PMMAcompositions (Fig. 6). Due to the 25 wt% of bismuth oxide con-tained in MTA and the high atomic number of Bi (ZBi¼83), thiscement showed the highest radiopacity levels as a consequence ofthe X-ray absorption ability of bismuth [33].

All prepared compositions in this work presented higherradiopacity than Meron, Vidrion F and PMMA. The most effectiveadditive for improving this property was tricalcium phosphateand, despite the low atomic number of phosphorus (ZP¼15), thehigher concentration of this element (due to its reduced density,0.6 g/cm3) might be responsible for this result. The performance ofthis compound was even better than other potential materials:zinc oxide (ZZn¼30; d¼5.61 g/cm3) and zirconia (ZZr¼40;d¼5.68 g/cm3).

Although bismuth oxide is known as an excellent radiopaquecomponent, some investigations reported that this additive caninduce the increase in porosity and reduce the mechanicalstrength of cement-based samples [34,35]. Hence, in order toovercome this problem, the most recommended alternative is toblend Bi2O3 with other compounds. For instance, a mixture of15%ZnO:10%Bi2O3 added to calcium aluminate cement resulted insuitable radiopacity values (43.0 mm Al) for clinical purposes,without affecting the mechanical properties of the samples [35].

4. Conclusions

Reactions involving calcium aluminate cement (CAC) and aqu-eous solutions are not highly exothermic and this material pre-sents suitable biocompatibility and setting time that can be ad-justed by adding other components to the mixture. Based on that,this work investigated some important properties of this cementfocusing on the requirements to apply it as a biomaterial fordentistry and orthopedic fields. The attained results showed thatplain CAC presented suitable compressive strength when com-pared to commercial products used in dentistry (Meron, MTA andVidrion F) and the mixtures of CACþ4 wt% of different compounds(alumina, zirconia, zinc oxide, tricalcium phosphate and hydro-xyapatite) resulted in samples with similar or higher mechanicalstrength than PMMA. On the other hand, collagen or chitosanaddition to CAC led to the porosity level increase and reduced theoverall crushing strength of the cement.

CAC-based compositions containing zinc oxide also presentedapparent porosity and pore size decrease after the samples inter-action with simulated body fluid solution. This additive is a bio-material due to its ability to induce the precipitation of importantphases for the mineralization process. Consequently, the presenceof these novel phases on the samples' surface and pores affectedthe properties of the prepared cements, leading to porosity valuessimilar to the ones attained for PMMA and improved mechanicalresistance.

Only the tricalcium phosphate additive induced the increase inthe CACH radiopacity and all compositions presented a betterperformance than the glass ionomers (Meron Vidrion F) andPMMA (cement for bone repair).

Acknowledgments

The authors are grateful to Fapesp (2013/22502-8, and 2014/08988-8) and CNPq (300782/2012–8) for supporting this work.

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