Evaluation of Calcium Aluminate/Calcium Phosphate Based ... 2692-2706.pdf · different forms of calcium phosphates as it is widely used as a bioactive ceramic powder in bone repairs

ORIENTAL JOURNAL OF CHEMISTRY

www.orientjchem.org

An International Open Free Access, Peer Reviewed Research Journal

ISSN: 0970-020 XCODEN: OJCHEG

2017, Vol. 33, No.(6):Pg. 2692-2706

Evaluation of Calcium Aluminate/Calcium PhosphateBased Biocements as Root-end Filling Material

M.M.RADWAN1, H.K.ABD EL-HAMID1*, H. H. ABO-ALMAGED1,HAIDY N.SALEM2 and NADA OMAR2

1Refractories, Ceramics and Building Materials Department,National Research Centre (NRC), Dokki, 12622 Cairo, Egypt.

2Restorative and Dental Materials Department,National Research Centre (NRC), Dokki, 12622 Cairo, Egypt.

*Corresponding author E-mail:[email protected]

http://dx.doi.org/10.13005/ojc/330602

(Received: September 28, 2017; Accepted: October 20, 2017)

ABSTRACT

Nano-particles of pure single phase calcium aluminate (CA) were prepared via solid statereaction at high temperature. An ultra-pure nano-size hydroxyapatite (HA) prepared by sol-gelmethod was used to formulate a composite of 1:1 CA: HA. Setting times, micro-hardness, bulkdensity, calcium ion and phosphorus ion concentration, XRD, FTIR and scanning electronmicroscopy (SEM) were investigated. The results showed that the substitution of 50% by weightof pure calcium aluminate cement (CA) by nano-size hydroxyapatite (HA) has a retardation effecton setting process. The use of artificial saliva solution as hydration medium enhances the hydrationprocess of both synthesized calcium aluminate based materials due to the formation of morehydrated compounds. Curing of calcium aluminate cement in hot and humid conditions, resultingin hardness loss and high porosity due to the conversion process, in which the compounds CAH

10

and C2AH

8 (hexagonal crystals) are converted into the more stable compound C

3AH

6 (cubic

crystals). The SEM studies indicated that the presence of nano-particles of hydroxyapatite in CA/HA composite facilitated the precipitation of hydroxyapatite-like active layer, suggesting that pureCA cement has some distinctive physico-mechanical properties make it suitable for use as rootend filling materials in dental applications.

Keywords: Nano-calcium aluminate – Nano-hydroxyapatite –Gibbsite –Root end material.

INTRODUCTION

Bioceramics that have been developedduring the last five decades are especially used forrepairing the skeleton bone systems as well as indental applications1. These bioceramics comprising

five main chemically bonded ceramics groups; Ca-phosphates, Ca-silicates, Ca-aluminates, Ca-carbonates and Ca-sulphates2,3.This investigationwill be dealt with only two systems of the abovementioned five groups; those are calcium aluminateand calcium phosphate.

2693RADWAN et al., Orient. J. Chem., Vol. 33(6), 2692-2706 (2017)

The calcium aluminate phase used in thisinvestigation was prepared in the laboratory fromchemically pure starting materials by solid statereaction at elevated temperatures. The aluminatephase prepared by this method has a specialbenefit that is biocompatible. This type of calciumaluminate cement has a superior mechanicalproperties, especially after the first 24 h of hydrationreaction that make it better material for load bearingapplications as in dentistry. The curing of calciumaluminate cement occurs with high waterconsumption during setting and hardening ofprepared pastes. The relatively high strength valuesof calcium aluminate pastes compared to those ofcalcium phosphate and also wide range variationsin the consistency and formulations make it suitablefor a wide range of applications3,4. There are fivephases of calcium aluminate cements, that are C3A,C12A7, CA, CA2 and CA6, the most reactive one isthe monocalcium aluminate CA (CaAl2O4)

5.

Calcium phosphate cements are highlybiocompatible as they are composed of cations andanions generally found in physiological media.These bioceramics have been widely used in bothdental and orthopedic applications6,7.Hydroxyapatite (HA) is an important phase amongdifferent forms of calcium phosphates as it is widelyused as a bioactive ceramic powder in bone repairsand as coatings for metallic prostheses in order toimprove the biological properties8, 9. HA has achemical structure very similar to that of humanbone with a stoichiometric Ca/P ratio of 1.6710. Itcould be also considered the main phase of bonesand tooth structure11. Synthetic HA that has thesame chemical composition of mineralized humanbone and teeth shows great affinity to host hardtissues. The excellent stability of HA above pH 4.3and the chemical bond that is formed with thehuman tissue makes HA the ideal phase in clinicaland dental applications in the human body8,12.Otherwise, mechanical reliability of pure HAceramics is not sufficient, especially in wetenvironments as HA cannot be used as heavy loadedimplants, such as artificial teeth or bone. Its medicalapplications are presently limited to smallerunloaded implants, powders, coatings and loaded porous implants13. Therefore, HA should be mixed withother load bearing material to formulate a compositecharacterized by good biological activity and bettermechanical strength of cement 14.

This investigation aims at studying theevaluation of pure CA base material as a root-endfilling biocement. The influence of adding nano-hydroxyapatite on sealing ability of the synthesizedcalcium-aluminate cement as root end fillingmaterial. Studying to some physicmechanicalproperties and hydration characteristics of twoexperimental calcium aluminate basedformulations (CA and CA/HA)cured in two differenthydration media (distilled water and saliva solution)by studying setting times, micro-hardness test,calcium and phosphorus ion concentration inaddition to X-ray diffraction analysis, infraredspectroscopy and scanning electron microscopy.

MATERIALS AND METHODS

Two different synthesized materials wereprepared in laboratory by two different methods.The first denoted CA was composed of 80% ofmonocalcium aluminate CA and 20% of BiO2 (asfiller to achieve radio opacity). The second, denotedCA/HA was consisted of 40% of monocalciumaluminate CA, 40% of hydroxyapatite HA and 20%of BiO2.

Materials preparation and characterizationPreparation of monocalcium aluminate (CA)

For the formation of CA phase, a dry-mixed1:1 mixture of CaCO3 and Al2O3 (highly purelimestone (≈ 99.8%) and Alumina (99.6% Al2O3)were used. The two materials were mixed andpressed into cubes and fired at 1000oC for 2 h, Theproduct was ground, remolded with the aid ofcarbon tetrachloride and fired at 1500°C for 6 hagreed with all other studies, and mean that thelevels of C12A7 and CA2 decreased while the CAincreased with increasing temperature and time15,16.The synthesized powder was investigated by X-ray diffraction to verify the identity of the synthesizedcompound, using a copper target with radiation;wavelength=0.154nm, X-ray was generated at 40kV with a current of 2–5-mA (Fig.1). The synthesizedmaterial was grinded for 15 h in an agate mill untilthe desired grain size(15-32 nm). It was investigatedby a Transmission Electron Microscope (JEM-1230)at 100 KV to evaluate the particle diameter (Figure.2.a).

Preparation of Hydroxyapatite (HA)Calcium nitrate tetra hydrate, di-ammonium

hydrogen phosphate and ammonium hydroxide

2694 RADWAN et al., Orient. J. Chem., Vol. 33(6), 2692-2706 (2017)

were used for preparation of HA17. The stoichiometryof calcium nitrate and di-ammonium hydrogenphosphate solution was adjusted to get (Ca/P) molarratio 1.68. HA was performed by slow addition of(0.6 M) of di-ammonium hydrogen phosphate to (1M) of calcium nitrate under continuous stirring. ThepH of the reaction mixture was adjusted at 11 by theaddition of NH4OH. The milky gelatinous precipitatewas left to digest under reflux at 65°C for 1 hours.The prepared HA undergo thermal treatment at1000°C for 2 hours. The formed phase wasinvestigated by X-ray diffraction analysis (Fig.3). Theaverage size of HA particles lie in the nano rangebetween 18 to 31 nm (Figure. 2.b).

Two types of aqueous solutions, namely,distilled water and saliva solution were used ascuring liquids for the pastes. The chemicalcomposition of the artificial saliva solution is givenin Table 1.18. Distilled water was used as a mixingliquid to prepare pastes of calcium aluminate (CA)and calcium aluminate/hydroxyapatite composite(CA/HA). Trials were carried out with differentamounts of distilled water to achieve the bestworkable consistency. The speculation of the powderincorporated into the liquid phase was done assuccessive fractions, one sixth of the powder wasadded every 15 s and kneaded with a spatulabetween addition to produce a paste with aworkable consistency. The sample exhibited a stringof one inch as the spatula was raised slowly fromthe glass slab.

Setting timeFive scoops (1 scoop = 0.2 gm)of the

powder were mixed with five drops of each liquid tofill a mold of dimensions; 10 mm±0.1-mm height.Mixing was done on a clean glass slab with a clean

spatula, for 30 seconds. Each mold was placed ona glass plate 1 mm thick × 25mm wide × 75 mmlong and filled with the tested paste to a level surface.

The stopwatch was started at the beginning ofmixing. After 120±5 s from the beginning of mixing,the assembly was placed in a cabinet maintainedat 37 ± 1°C temperatures and 95% relative humidity.The Gilmore needle (having a flat end diameter of

2mm±01mm) was carefully lowered on to the horizontalsurface of the material. Indentations were repeated at30-second intervals until the indenture fails to make acomplete circular indentation in the cement.

Bulk densityBulk density was determined for the paste

samples before the hardness test. Hardened cementsamples were weighed suspended in water and inair as saturated surface dry. That was followed bydrying the sample at 105oC for 24 hours. The driedsamples were weighed in air after cooling indesiccators19.

Hardness testThe prepared pastes of both synthesized

materials were poured into a cylindrical brass mold(diameter = 10 mm, height = 2 mm). The paste wasplaced in the mold into two approximately equallayers. Each layer was compacted and pressedalong the surface of the mold until homogenousspecimen was obtained. Immediately after molding,the samples were cured in a humidity chamber at100% relative humidity at a constant temperatureof 37 °C for 24 h. At the end of the moist curingperiod the samples were demoulded and then curedunder the two types of aqueous solutions (distilledwater and saliva) until the time of testing such as 1,3, 7and 14 days at a constant temperature of 37°C.The hardness test20 was carried out on three cubesof each case of the hardened cement pastes. Thetest was performed with a Vickers indentationhardness tester at five points on one side of eachsample. A load of 1 Kg was applied for 15 seconds.The depth of indentation was then recorded.

Calcium and phosphorus ion concentrationThe concentrations of calcium and

phosphorus ion released of the teeth injected bythe two experimental pastes were measured byatomic absorption spectra (Savant AA, GBC,Australia). A total of 20 prepared teeth were used tomeasure the concentrations of calcium andphosphorus ion released of the experimental teethinjected after one week and one month weremeasured by atomic absorption spectra.

ATR/FTIR spectroscopyIR spectra were recorded on some selected

samples with the aid of JASCO 4600 model FTIRspectrometer.

Scanning electronmicroscopy (SEM)Scanning electron microscopy (SEM)

model Philips XL30 was used to examine themarginal adaptation for group A filled with CA and


in group B filled with CA/HA at root-end cavitiesafter coating with gold thin films.Qualitativeevaluation of the dentine cement interface wascarried out. The penetration of the root-end fillingmaterials in the dentinal wall irregularities,presence of gaps between the root-end materialsand the dentinal walls, presence of voids or airbubbles, adaptation of the root-end materials andits proximity to the walls, the homogeneity of theroot-end materials through its thickness and thepresence of cracks in the root-end materials bulkwere studied.

Root-end filling proceduresA total of 20 vital, freshly extracted and

caries free human single-root teeth were collectedfrom Cairo dental clinic at the National ResearchCentre. Teeth were stored in saline solution beforethe root-end filling procedures.

The teeth were decoronated by sectioningthe coronal part of each tooth using a sectioningdisc Toolouip, Germany, mounted on a low speedhand piece accompanied with water coolant. Thiswas done for ease of manipulation and tostandardize the tooth length at 15 mm21.

Preparations of the teeth were carried out by thesame operator as follow:

Canal patency and working length wereestablished using a size 15 K- file that was insertedinto the root canal until the tip became visible at theapical foramen. Working length was thenestablished at 1mm short of the measured length.The root canals were instrumented with protapernickel-titanium rotary system, in an electricendodontic motor (X-Smart) set at 300 rpm andtorque 1.2 Ncm. Each canal was enlarged to size#F3. During instrumentation, 1 ml of 5.25% NaOClwas used to irrigate the canal between file sizes.Upon completion of instrumentation, 5 mL of NaOCl,followed by 5 mL of 17% EDTA were used toremove the smear layer. Finally, fluid flush withdistilled water was then carried out. Canals weredried with absorbent paper points prior toobturation.The root canals were filled withguttapercha (GP) points F3. AH plus sealer wasmixed according to manufacturer’s instructions.Master apical cone of size #F3 was coated with thesealer and slowly inserted into the canal to theworking length. A spreader #30/0.02 taper was usedfor lateral compaction. Spreader was inserted inthe canal alongside the master cone. Pressure wasapplied apically to push the spreader in as far aspossible. Guttapercha cones (#20/0.02 or #25/0.02

Fig. 2: Transmission electron photomicrograph of the two nano-sizedsynthesized materials: (a) monocalcium aluminate and (b) hydroxyapatite

Fig. 1: XRD pattern of monocalcium aluminate(CA) powder fired at 1500°C/6h


Table. 1: Composition of the artificial salivasolution

Ingredients Concentration(g/ L)

NaCL 0.4KCl 0.48CaCl2. 2H2O 0.795Na2S.H2O 0.005NaH2PO4.2H2O 0.78Urea 1Phosphoric acid to adjust pH to 6.4Distilled water

taper) were coated with the sealer and used asaccessory cones, until there was no more room inthe canal for additional accessory cones.

After sealing of all access cavities withColtosol, an apical resection at 90o to the long axisof the tooth was made at 3 mm from the end of theroot. Root-end cavities of 3 mm were prepared withan ultrasonic tip (S12/90D-Gnatus, Brazil). Onceprepared, teeth were randomly divided in twogroups A and B. Root-end fillings were performedin group A with CA, and in group B root-end cavitieswere filled with CA/HA. All samples were stored inhumid environment at 37 oC for one week to allowsufficient time for the root-end filling materials toset.The marginal adaptation was evaluated ingroups A and B.

RESULTS

Setting timeThe setting time values of the pure CA

phase and the prepared 1:1 CA/HA composite arepresented in Fig. 4. The data shows a little delay insetting time of CA material upon substitution it by50 wt% HA. This minute increase in setting time ofCA in the presence of hydroxyapatite may beacceptable when using these materials in dentalbiomedical applications as calcium aluminate phasehas a relatively fast curing process during the first24 h that help minimizing the fluid leakage22.

Bulk densityThe influence of using artificial saliva

solution as a curing medium in comparison with

distilled water on some physico-mechanicalproperties of the prepared pastes of bothsynthesized materials was studied. Fig.5 indicatesthat the density values of pastes of pure CA increasefrom 1 to 3 days followed by a little decrease from 3to 14 days, these values were found to be higher forsamples cured under saliva solution. For pastesprepared form CA/HA composite, there is acontinues increase in the density values from oneup to 14 days curing period, but for pastes cured for7 and 14 days in saliva medium, there was a slightincrease in density values.

Micro-hardness testThe measured hardness values of both

investigated materials cured under distilled waterand saliva solution are graphically represented inFig.6 (a&b). The samples of pure CA showed(Fig.6.a) a steady decrease in hardness Vickersvalues with curing periods for both hydration media,while, those prepared from CA/HA composite(Fig.6.b) there is an increase in the hardness valuesup to 3 days followed by a decrease at 14 dayscuring age hydration media. Fig.6.b indicates thatthe hardness values of samples cured using salivaare higher than those cured under distilled water atall curing ages.

Calcium and phosphorus ion concentrationFig. 7 illustrates the variations in calcium

ion concentrations of samples cured for one weekand one month. The data show a trace amount ofCa2+ ion in the hydration medium ranging from ≈0.5to ≈2ppm. At one week curing age, samples of pureCA paste showed less Ca2+ ion than CA/HA

Fig. 3: XRD pattern of Hydroxyapatitepowder fired at 1000 °C/2h


composite, while at one month, the Ca2+ ion valueof CA/HA samples was less than those of CAsample. The values of phosphorus ion for pastesmade of CA/HA composite and cured for one weekand one month are represented in Fig. 8. The datarevealed an increase in the P3- ion concentrationwith the curing age from a week up to a month dueto the dissolution of the apatite (HA) in the hydrationmedium at 37oC.

X-Ray DiffractionThe X-ray diffraction patterns of pastes of

the two synthesized materials cured under distilledwater and saliva solution for 1 and 14 days are

represented in Fig.9(a&b). The X-ray patterns ofpure CA phase Fig.9.a showed a decrease in thepeak height of the anhydrous monocalcium

aluminate (CA) with curing period from 1 to 14 daysin both distilled water and saliva solution. Thecharacteristic peaks of the two main hydration

products (CAH10 and C2AH8) could be detectedeither in a short height or overlapping on those ofthe anhydrous phase. X-ray peaks characteristicsfor Gibbsite (AH3) and hydrogarnet (C3AH6) are

clearly found as a result of the conversion processof the principal hydration compounds, CAH10 andC2AH8, at 37oC. The XRD data of the selected

hydrated pastes reveals that the use of salivasolution enhances the hydration process. On theother hand, Fig.9.b that displays the diffraction

patterns of the CA/HA composite showed anincrease in all peak height of the anhydrous CAmaterials and enhancement of the hydroxyapatite(HA) characteristic peak height.

IR spectroscopyFT-IR spectroscopy was employed to

investigate the hydrated phases of some selectedhydrated phases. Fig. 10 and Table. 2 illustrate thechanges of the IR bands with hydration periods for

pure CA phase cured in saliva solutions anddistilled water. Fig. 10 shows the typical band ofCAH10 at 3500 cm-1 due to the OH- valence vibration

and the band in the range 1650-1600 cm-1 for theH-O-H bending vibration. These two bands showtheir maximum at 3 days for CA pastes cured in

distilled water (Fig. 10(a)) that is due to the increasein the hydration products, then these two bandsdiminish at 7 days followed by an increase at 14days curing period, while in case of pastes cured insaliva solution there is a steady decrease in that IRbands. The Al-O vibration bands at about 520-550cm-1 and 800-850 cm-1 were detected for CA pastescured in both distilled water and saliva (Fig. 10

Fig. 4: The setting time values of the pure CAphase and the prepared CA/HA composite

Fig. 5: Bulk density of; (a) pure CA and (b) CA/HA compositecured in distilled water and artificial saliva


(a,b)), these IR bands are decreased with curingperiod. The IR spectrums given in Figs.10& 11 (a,b)also show the band corresponding to the v3 CO3

2- at1500-1400 cm-1and the duplets at 500-400 cm-1 dueto the partial carbonation of the hydration products.The typical absorption bands of α-Al(OH)3 could bedetected at about 1100-1000 cm-1 and small one at970 cm-1 as can be seen in Figs. 10&11(a,b). Thevibrations of the C3AH6 are detected in the rangebetween 600-500 cm-1 and may overlap with that ofthe Al-O6 in the range which will increase with thehydration reaction time. Fig. 11 (a,b) illustrates theIR spectrums of pastes of composite bio-ceramicCA/HA cured in distilled water and the artificial salivasolution for 1, 3, 7 and 14 days. All the mentioned IRbands that are found in Fig. 10 (a,b) could be

detected for pastes of CA/HA composite in additionto those corresponding to typical IR bands of PO4

3-

. The IR band corresponds to CAH10 at 3500 cm-1

due to the OH- valence vibration and that at 1650-1600 cm-1 for the H-O-H bending vibration show adecrease from 1 to 3 days and then an increaseagain for 7 and 14 days curing age of the twohydration media (Fig. 11 (a,b)), but this behavior isnot noticeable for paste cured in saliva solution.The typical IR band of the v3 CO3

2-at 1500-1400 cm-

1and the duplets at 500-400 cm-1 are also found inFig. 11(a,b) due to the partial carbonation of thehydration products. The main IR bandscorresponding to hydroxyapatite due to thesymmetric stretching of v3 PO4

3-at 1100- 1060 cm-1

and v4 PO43- bending at 650 and 560cm-1 could be

Fig. 7. Calcium ion concentration of pure CAand CA/HA composite cured in distilled water

Fig. 8. Phosphorus ion concentration ofCA/HA composite cured in distilled water

Fig. 6. Micro-hardness data of (a) pure CA and (b) CA/HA compositecured in distilled water and artificial saliva


detected in both graphs and overlapping with thoseof absorption bands of α-Al(OH)3 at about1100-1000 cm-1 and small one at 970 cm-1 and thevibrations of the C3AH6 in the range between600-500 cm-1. 23. All the aforementioned IR bandsgiven in Figs. 10,11 show a decreasing trend withthe curing age from 1 up to 14 days for the twohydration media except for pastes cured for 3 daysin saliva solution, a deep diminishing of all IR bandsof all hydrated phases followed by an increase for 7and 14 days samples.

Scanning electron microscopyThe SEM micrograph given in Fig.12 (a,b)

showed that group A(CA) had no adhesionbetween the material and tooth interface, and gapformation at the interface after 1 week and 1 month.However, the interface between tooth and the CA/HA based fill ing material after 1 week washomogeneous and did not show any gaps,,whileat 1 month interval a tiny gap was observed betweenthe material and tooth interface. Figure. 13(a,b).

Fig. 9: XRD patterns for; (a) pure CA and (b) CA/HA composite curedfor 1 and 14 days in distilled water and artificial saliva

(a)

(b)


Table. 2: Results of FTIR analysis

Wave number (cm-1) Crystal phase

400-5001400-1500 The CO32-Stretching vibrations

600-500 The vibration of C3AH6and may overlap with that of the Al-O6

550-520850-800 The Al-O vibrations1100-1000 The absorption band of α-Al(OH)3

970-9901027 The O-H bending vibration attributed to gibbsite (AH3)3475, 3530 and 3630 The O-H stretching vibration attributed to gibbsite (AH3)524, 3465 and 3670 The bands appearing of the water hydrated CAC paste (CAH10, C2AH8

and C3AH6)1650-1600 The H-O-H bending vibration band of CAH10

1100-1050 The symmetric stretching vibration of v3 PO43-

650-560 The PO43- bending vibration

DISCUSSION

Calcium aluminate cement that is madefrom highly pure limestone or lime with bauxite orother aluminous material has special propertieswhich include rapid strength development and goodresistance to many forms of chemical attack. Theessential aluminate phase that develops the mainhydraulic activity and is consequently responsiblefor the strength development, is monocalciumaluminate, CA. Calcium aluminate cement hydrateswith water and the major crystalline hydrationproducts are CAH10 at low temperatures(5-10oC),C2AH8 and AH3 at 22-35oC and C3AH6 (katoite) andAH3 (gibbsite) at >35oC. Depending upontemperature and curing period, all hydration

products will be converted to C3AH6 and AH3 inprocess called conversion24,25. The strength loss ofcalcium aluminate cement cured in hot and humidconditions is mainly due to the so called, conversionprocess, in which the compounds CAH10 and C2AH8

(hexagonal crystals) are converted into the morestable compound C3AH6 (cubic crystals) inaccordance to the following equations22:

3CAH10 → C3AH6 + 2AH3 + 18H*3C2AH8 → 2C3AH6 + AH3 + 9H

High early strength is one of the majoradvantages of calcium aluminate cement (CAC)over Portland cement. However, its high earlystrength is adversely affected by temperature as it

Fig. 10: IR spectra of pure CA cured for 1, 3, 7 and 14 days in: (a)distilled water and (b) artificial saliva


enhances the conversion process. It was stated thatthe CAH10 and C2AH8 crystals are not stable in a

humid and hot conditions, consequently, they willeventually convert to the more stable C3AH6. This

will result in a strength reduction due to the formationof micro cracks in hardened structure26.

The setting time results, Fig. 4, showed alittle retardation of setting time of CA material upon

Fig. 12: SEM of pure CA group cured for one week (a) and one month intervals (b)

(b)(a)

Fig. 13: SEM of CA/HA composite group cured for one week (a) and one month intervals (b)

(a) (b)

Fig. 11: IR spectra of CA/HA composite cured for 1, 3, 7 and 14 days in;(a) distilled water and (b) artificial saliva


substitution it by 50 wt% HA. This retardation ofsetting time may be acceptable in dental biomedicalapplications as calcium aluminate phase has arelatively fast curing process during the first 24 h27-29.The setting process of CA cement starts immediatelyafter mixing with water through, first, dissolutionfollowed by precipitation of the major hydrationphase CAH10, C2AH8 and AH3, the replacement of50% of the base calcium aluminate cement byhydroxyapatite as a hydrated calcium phosphatewill perhaps adversely affect the physical andmechanical properties of this type of cementingmaterials as a softening and dissolution of HA alsowill start upon mixing with water14. Curing of pastesprepared from the two composites was carried outat 37°C in distilled water and artificial saliva solutionin order to investigate the hydration behavior asinfluenced by hydration medium. Since this materialsets and harden in vivo there is always a certainamount of ion leakage. Earlier studies reported thatthe leakage and hardness of root end fillingmaterials were all affected over time20. Theconversion process that is occurred during hydrationof CA phase has an adverse effect on physico-mechanical properties of hardened CA pastes,namely, bulk density and micro-hardnessmeasurements (Figs. 5,6). In this process thetransformation of meta-stable hydrated compounds,CAH10, C2AH8 and AH3 to the stable C3AH6 andγ-AH3 is highly dependent on temperature of thecuring medium25. The process of conversion isconsidered very critical for long curing periods dueto the chemical and morphological changes duringhydration of CA. The conversion of hexagonalcrystals (C2AH8) to cubic crystals (C3AH6) increasesthe porosity of the hardened paste while densityand hardness will be adversely affected. The presentof huge amount of AH3 gel makes the measurementsof porosity very difficult as it precipitates inside thepores and encapsulates both of anhydrous andhydrated compounds. The increase of densityvalues from 1 to 3 curing ages of pastes of bothpure CA phase and composite material CA/HA (Fig.5, a&b) is mainly due to the formation of hydratedcompounds during the early hydration period at37°C in addition to the filling effect of the addedbismuth oxide as radio-opaque filler. For pure CAmaterials, the density values showed a littledecrease at curing ages from 3 up to 14 days inboth curing media as a result of conversion process.The presence of 50 wt% nano-size HA in composite

material (CA/HA) will act as a filler material besideits role in improving the biocompatibility of calciumaluminate cement. This nano particles of HA will berearranged in the pore system of the hardenedpaste, that enhances the density values especially atlater ages of hydration14. The continuous decrease of

the micro-hardness data (Fig. 6a,b) for bothinvestigated systems is mainly due to the processof conversion that is responsible for the highlyporous

system of both synthesized materials, in addition tothe poor mechanical properties of the addedhydroxyapatite in CA/HA composite30. Investigation

of density and hardness data of both indicates thatthe use of saliva solution as a curing mediumenhances the hydration reactions of the investigatedmaterials due to the presence of free cations andanions (Na+, K+, Ca++, Cl-) present in saliva solution.These free radicals may accelerate the rate ofhydration reaction of the CA cement31.

Since these materials set and harden invivo, there is always a certain amounts of ion leakageinto surrounding tissues depending upon phasecomposition and setting time. Generally, CA phase

has a low leakage since the material has a short

setting time and good cohesiveness. The gel phase

(AH3) together with the main hydrated compounds

CAH10 and C2AH8 are formed immediately at the

beginning of the hydration reaction. The hydrationprocess starts via dissolution of CA phase in theaqueous curing medium producing Ca2+, Al(OH)4

-

and OH- ions followed by precipitation of thehydrated phases.

Hydration of CA cement can be expressedby the following chemical reactions32:

CA + 10H → CAH10 ...(1)2CA + 11H → C2AH8 + AH3 ...(2)3CA + 12H → C3AH6 + 2AH3 ...(3)

2CAH10 → C2AH8+ AH3 + 9H ...(4)3C2AH8 → 2C3AH6 + AH3 + 9H ...(5)

C3AH6 phase cannot form directly fromsolution but must be preceded by another phasewith a process called conversion (eqs. 4,5) andbecomes dominant over C2AH8 depending on thehydration temperature.


The data of calcium ion concentrations(Fig. 7) of pastes cured under distilled water arehigher after one week curing period than thosecured for one monthfor both investigated materialsbecause during hydration process most of calciumaluminate (CA) reacts to form the less solublehydrated compounds mentioned above. Although,the hydroxyapatite (HA) phase in the compositeCA/HA is considered the hydrated form of calciumphosphate cement and it is slightly soluble in water,it showed a decrease in calcium ion concentrationmore than pure CA at later curing age that isattributed to the encapsulation of HA particles bythe hydrated products in such a porous structure.Although the mechanical performance of syntheticHA is very poor it was used in this study in nano-size to improve the biocompatibility of CA cement30.The very minute phosphorous ion concentration(Fig. 8) of the composite CA/HA confirms the abovementioned encapsulation effect of the hydratedphases of CA around HA particles that may retardits dissolution in the immersing liquid.

Calcium aluminate cement is highlyresistance to many aggressive media including,dilute acid solutions and natural waters that containCO2 as a significant solute. This high resistance tochemical attack is mainly due to restrictedpenetration by aggressive medium. Pastes that hadbeen exposed to hydration medium containing Cl-

and SO42- showed a significant ingress of Cl- but

very limited of SO42- 33. Small amount of calcium

chloroaluminate (Al2O3. CaCl2. 3CaO. 10H2O) maybe formed due to the presence of calcium chloride(CaCl2) in saliva solution that is used in thisinvestigation as curing medium, while most ofchloride ions (Cl-) seemed to be adsorbed on theCAH10. In a porous CA paste where conversion hasoccurred, a part of C3AH6 reacts with atmosphericCO2 in a process called carbonation to give calciteand hydrous alumina (alumina gel) that areprecipitated in the pores and prevent watermolecules needed to promote the reaction. Thisbehavior is a strong evidence that calciumaluminate pastes has high resistance to variousforms of attack due to some effects that impede thecontinuous penetration of aggressive cations andanions in addition to blocking of pores by thehydrated compounds33. The characteristic X-raydiffraction peaks of calcium aluminate chloridehydrate (Al2O3.CaCl2.3CaO.10H2O) and calcite

could be detected in Fig. (9. a) that represents theXRD patterns of CA pastes cured in saliva for 14days showed the characteristic XRD peaks ofhydroxyapatite compound that may be precipitatedin pores from saliva solution. The XRD patterns ofcomposite CA/HA pastes, Fig. (9.b), also shows themain XRD peaks of calcium aluminate chloridehydrate and calcite for samples cured for 14 daysin saliva solution. The XRD patterns in Fig. (9.a)indicate that curing of CA pastes in saliva solutionenhances the hydration reaction that is clear fromthe diminishing trend of peak height of theanhydrous CA particles and the appearance of newpeaks characteristic for the hydrated phases, whilein case of CA/HA pastes, Fig. (9.b), an overlappingof main XRD peaks of HA with those of hydratedcalcium aluminate is predominant. The bandscorresponding to the v3 -CO3 at 1500-1400 cm-1

and the duplets at 500-400 cm-1 that are given in IRspectrums (Fig. 10&11 (a,b)), indicate the partialcarbonation of the hydration products of the twosynthesized materials. Fig. 10 (a) shows themaximum typical band of CAH10 at 3500 cm-1 dueto the OH-valence vibration and the band in therange 1650-1600 cm-1 for the H-O-H bendingvibration at 3 days for CA pastes cured in distilledwater that is due to the encapsulation of theincreasing amount of the hydration productsresulting in little delaying in water penetration toanhydrous powder that is clear in bands diminishat 7 days followed by an increase at 14 days curingperiod. In case of pastes cured in saliva solutionthere is a steady decrease in that IR bands that ismainly attributed to highly ionized medium of salivasolution.

The SEM studies showed that the CA/HAspecimens had formed a homogenous, tiny gapafter 1 month. interface, Fig. 13(a&b).Nevertheless,adhesive separation and gap formation at theinterface were commonly found in CA groups inboth tested periods. In addition a mesh-like apatitelayer was observed in CA/HA Fig. 13, however, thislayer was not observed in CA Fig. 12, indicatingthat the increasing of HA content to 50wt% facilitatedthe precipitation of a hydroxyapatite-like activelayer, suggesting that the HA feature favorable invitro bioactivity20,27-29. On the other hand,CA groupspecimens adhesive separation had shownbetween the CA cement and the tooth surfaceamong all samples.It is obvious that the CA/HA


contains a rather large proportion of entrained air.The source of the air is mainly the mixing methodused on the material. Porosity is an intrinsiccharacteristic of calcium aluminate based cementsand occurs as a result of the spaces between theanhydrous cement grains. These spaces are filledwith water once the material hydrates. As thehydration reaction progresses, the hydrationproducts fill these gaps and the porosity decreases.However the conversion process in which the meta-stable hydrated compounds, CAH10, C2AH8 and AH3

is transformed to the stable C3AH6 and γ-AH3 at therelatively high temperature of curing medium(37 °C) especially at later curing ages will increaseporosity and results in a highly porous system34-36,Although pores have a significant effect on themechanical properties of the cement but it alsoaffects its physical properties and, subsequently, itsbehavior within its environment. Porosity has beenshown to act as reservoir for precipitated HA phaseduring the hydration reaction which have an impactupon bio-remineralization36,38. The results indicatedthat the CA/HA facilitated the precipitation of ahydroxyapatite-like active layer, suggesting thatfilling the hair cracks due to the increasing porosityat later ages, that is favorable in vitro bioactivity. CA/HA materials exhibit dissolution and precipitationbehaviors during the hydration reaction.Hydroxyapatite plays a crucial role in tissueregeneration and maintaining function because ofits bioactive surface.

CONCLUSION

The calcium aluminate cement isconsidered suitable for use as dental material as it

has some distinctive hydration, physical andmechanical characteristics. The CA material setsand hardens in the humid environment withoutadditional curing aids. The mechanism of settingand hardening is a dissolution followed by formationof the hydration phases that results in tight sealbetween the hardened cement and the tooth. Curingin hot and humid conditions results in more poroussystem due to the conversion process in which the

compounds CAH10 and C2AH8 (hexagonal crystals)are converted into the more stable compoundC3AH6 (cubic crystals). The little retardation of settingtime upon substitution of 50% of CA byhydroxyapatite (HA) may be acceptable in dentalbiomedical applications due to the relatively fastcuring process during the first 24 hours. Thepresence of 50 wt% nano-size HA in the compositematerial (CA/HA) has superior filling effect besideits role in improving the biocompatibility of CAcement. The XRD and IR data reveal that the curingof both synthesized materials in saliva solutionenhances the hydration reaction rate. The SEM

studies showed homogenous, almost gap free,interface for CA/HA composite, while, adhesiveseparation and gap formation at the interface werecommonly found in CA samples. The nano-particles

of hydroxyapatite in the composite material will be

rearrangement in the highly porous system and this

may facilitate the precipitation of additional HAcompound in the pores. The results showed thatCA/HA materials exhibit dissolution andprecipitation behaviors during the hydrationreaction. Hydroxyapatite plays an important role intissue regeneration and maintaining functionbecause of its bioactive surface.

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