See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/7545647 Characterization of Portland Cement for Use as a Dental Restorative Material Article in Dental Materials · June 2006 DOI: 10.1016/j.dental.2005.06.005 · Source: PubMed CITATIONS READS 43 111 4 authors, including: Josette Camilleri Franco E. Montesin 102 PUBLICATIONS 2,698 CITATIONS 8 PUBLICATIONS 638 CITATIONS University of Malta SEE PROFILE University of Malta SEE PROFILE Richard Curtis St Benedict's, Ealing, UK 51 PUBLICATIONS 781 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: European Network of Bioadhesion Expertise (ENBA) View project All content following this page was uploaded by Josette Camilleri on 19 March 2014. The user has requested enhancement of the downloaded file. Dental Materials (2006) 22, 569–575 www.intl.elsevierhealth.com/journals/dema Characterization of Portland cement for use as a dental restorative material Josette Camilleria,b,*, Franco E. Montesina, Richard V. Curtisb, Thomas R. Pitt Fordb a Department of Building and Civil Engineering, Faculty of Architecture and Civil Engineering, University of Malta, Malta b Department of Conservative Dentistry and Dental Biomaterials of the Guy’s, King’s and St Thomas’ Dental Institute, King’s College London, London, UK Received 27 January 2005; received in revised form 2 June 2005; accepted 16 June 2005 KEYWORDS Compressive strength; Dental filling; Portland cement; Setting time Summary Objectives: The aim of this study was to evaluate the suitability of fastsetting cement formulations based on Portland cement as dental core build-up materials using two different methods of testing compressive strength and evaluation of setting times. Methods: Four fast-setting cements based on Portland cement and their four respective densified with small particle (DSP) mortars were tested for setting time, constitution of cement by EDAX, and compressive strength using International and British Standards. Ordinary Portland cement (OPC) was used as a control. Results: All the fast-setting cements had a similar elemental composition to OPC and the setting times were less than 7 min. The compressive strength of OPC was different between the two methods (P!0.001). All the fast-setting cements tested showed no difference in compressive strength regardless of the method of testing at 1 and 7 days (PO0.05), but the cylinders showed a lower compressive strength at 28 days (P!0.05). The OPC DSP mortar showed poorer compressive strength than OPC (P!0.01) at all times for cube testing but not for cylinder testing, where no difference was observed. The fast-setting DSP mortars had a lower compressive strength at 1 day (P!0.005) with both methods. At later times, there was no difference between the cements and DSP mortars for the cubes. Significance: The pure fast-setting cements set in !7 min and were not susceptible to changes in the compressive strength testing procedure at 1 and 7 days but at 28 days all the fast-setting cements had a significantly higher strength with the test using cubes (P!0.05). A reduction in strength was observed at 28 days in cylinder testing. Most of the cements tested did not show encouraging strengths, however, * Corresponding author. Address: Department of Building and Civil Engineering, Faculty of Architecture and Civil Engineering, University of Malta, Malta. Tel.: C356 2340 2894. E-mail address: joz@global.net.mt (J. Camilleri). 0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.06.005 570 J. Camilleri et al. one of the prototype cements tested could be a prospective dental restorative material. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. Introduction With the introduction of mineral trioxide aggregate (MTA) as a root-end filling material, a new example of the class described as ‘water-based cement’ has been introduced in dentistry [1]. As MTA is a dental water-based cement, its strength testing should conform to EN ISO 9917-1-2003 Part 1 [2]. MTA has a similar constitution to ordinary Portland cement (OPC) and is composed primarily of tricalcium silicate and dicalcium silicate [3]. The setting time of MTA is 2 h 45 min [4], which is too long for the material to be used as a conventional dental filling. The long setting time initiated the need to improve the properties of the existing material and produce one that sets faster and has compressive strength at least similar to glass ionomer cement. The improved characteristics of such a prototype material could make it suitable for a wider variety of applications in dentistry. The mechanical properties of Portland cement can be improved by producing mortars, which are densified with small particles (DSP mortars). In the production of DSP mortars, the cement grains are dispersed at low water/cement ratios with the use of high levels of dispersing agent and the inter-grain hydrate gel is densified by the addition of very fine silica (mean diameter 0.1 mm) [5]. Superplasticizers are very effective dispersing agents used to reduce the water content of the mix and retain its workability. Water can be reduced at an equivalent flow with an increase in compressive strength [6]. The aim of this study was to evaluate the suitability of fast-setting cement formulations based on OPC as core build-up material using two different methods of testing compressive strength and evaluation of setting times. Materials and methods Four accelerated cements were used for this study. 1. A mixture of 80% OPC (Central Cement, Italy) and 20% high alumina cement (Ciment Fondu Lafarge, Chilton, UK): OPC/HAC; 2. Emaco Ultra-rapid (MBT-FEB, Manchester, UK), which was passed through a 300 mm sieve prior use to eliminate the larger particles and fibers; 3. Sika Rapid (Sika, Switzerland), which was passed through a 90 mm sieve prior use to eliminate the larger particles; 4. Exocem R1 rapid (Ruredil spa, Milan, Italy). The control material was OPC. The Emaco and Sika were sieved prior use to eliminate the larger particles. In addition, all these cements were modified to become densified with small particles (DSP) mortars to improve potentially their mechanical properties; DSP mortars were produced by replacing 10% of the cement with microsilica (Meyco 610, MBT-FEB, Manchester, UK), and addition of 1% water reducing admixture (Rheobuild 1000, MBT-FEB, Manchester, UK). The cements were mixed with water for testing of setting time and compressive strength in ratios recommended by the manufacturer for the proprietary brands and according to the standard consistence test for the other materials (Table 1). Chemical constitution The chemical constitution of the fast-setting cements was determined using Energy Dispersive Analysis by X-rays (EDAX) in the scanning electron microscope (SEM) (Hitachi S3500, Hitachi, Wokingham, UK) and compared with that of OPC. A thin Table 1 Water to cement percentage and mean setting times of cements (SD), nZ4. Cement type %Water to cement Initial set minutes Final set minutes OPC OPC/HAC Emaco (300 mm) Sika rapid (90 mm) Exocem OPC DSP OPC/HAC DSP Emaco DSP (300 mm) Sika DSP (90 mm) Exocem DSP 32.5 35 25 262.0 (13.6) 3.6 (0.14) 4.2 (0.3) 479.5 (12.8) 6.4 (0.5) 5.8 (0.5) 32.5 2.5 (0.4) 6.2 (1.9) 32.5 30 33.5 4.3 (1.4) 301.3 (18.4) 4.4 (0.5) 6.9 (1.8) 497.5 (9.6) 6.8 (0.4) 35 4.1 (1.2) 5.8 (0.1) 35 3.3 (0.2) 7.0 (0.4) 27.5 6.0 (0.4) 8.9 (0.3) Use of Portland cement as a dental restorative material layer of powder was dispersed over a polymethylmethacrylate slab mounted on an aluminum stub (Agar Scientific, Stansted, UK). The stubs were carbon-coated (Emitech K250, Ashford, UK) for electrical conductivity. The specimens were then viewed under the SEM and the energy dispersive analysis by X-ray (EDAX) was carried out to determine the constituent elements of the powders. Setting time The setting time of the cements was tested using the Vicat apparatus according to BS EN 196-3 (1995) [7]; 400 g of the cement were mixed with water on a moistened aluminum tray with a small trowel. The amount of water required was calculated with the standard consistency needle [7] for Portland cement and mixtures of Portland and high alumina cement, according to manufacturers’ instructions for the fast-setting cements. The cement was then compacted in a two-part brass mold and initial and final setting times were determined. The Vicat assembly was placed in an incubator at 37 8C. The test was repeated four times. Compressive strength The compressive strength of the cements was determined according to BS 4550 Part 3 Section 3.4 1978 [8] for testing Portland cement and according to BS EN ISO 9917-1: 2003 for testing dental cements [2]. For the first part of the experiment, 740 g of cement without standardized sand were mixed on a moistened aluminum tray with a trowel for 2 min at the established water/cement ratio. After mixing, the cement was loaded into cubes of side length 70.7 mm held together in a vice. The metal mold was previously coated with mold oil (Separol, Sika, Switzerland). The mix was vibrated for 1 min to allow all the air bubbles to come to the surface. The surface of the cube was then made smooth with the flat end of a trowel. The mixes were allowed to cure for 24 h at room temperature. They were then removed from the molds and stored in an incubator at 37 8C and 100% humidity. Three cubes were prepared for each time point for different materials. For the second part of the test, 10 g of cement was mixed on a glass slab with water at the established water/cement ratio. The mix was then compacted in brass molds of 6 mm in diameter and 16 mm high with 2 mm caps at both ends to produce 571 cylinders of 6 mm in diameter and 12 mm high. A vibrator was used for 1 min during the filling and compaction of the cements into the cylinder to avoid entrapped air. The cylinders were stored in an incubator at 37 8C and 100% humidity. Six cylinders were prepared for each time point for different materials. Compressive strength testing was performed after 1 day, 7 and 28 days. The cubes were compressed using a compression machine (Avery Denison Impact Laboratory and Engineering Supplies, Leeds, UK) with a loading rate of 0.6 N mmK2 sK1 until they failed. The cylinders were compressed using a Universal testing machine (Instron 1195, High Wycombe, UK) with a cross head speed of 1 mm/min until they failed. The maximum load required to fracture the samples was noted. Compressive strength was calculated using the formula: Compressive strength Z Applied load at failure ðNewtonsÞ Original cross sectional area ðmm2 Þ Effect of specimen size on compressive strength. The effect of specimen size on compressive strength was evaluated by casting cubes of OPC. Thirty cubes were cast in molds of side length 70.7 mm while another 30 cubes were cast in 50 mm side length molds. The specimens were left at room temperature covered with a sheet of plastic for 24 h after which they were demolded and cured at 100% humidity at 37 8C for 28 days. The cubes were crushed using a compression machine (Avery Denison) with a loading rate of 0.6 N mmK2 sK1 until they failed. Statistical analysis The data were analyzed statistically using SPSS. The distribution was first evaluated to determine the statistical test that would be performed. The data was plotted and the distribution curve was analyzed together with the Kolmogorov–Zmirnov test with PZ0.05. This signified a normal distribution and thus parametric tests could be performed accordingly. With normally distributed data, Analysis of Variance (ANOVA) with PZ0.05 was first performed to determine any variation between the means. Then two-tailed independent sample t-test at 95% 572 J. Camilleri et al. confidence level with PZ0.05 was used to analyze the data. Results Chemical constitution The three fast-setting cements had a similar elemental constitution to OPC (Fig. 1). They were composed of calcium, silicon, aluminum and a small proportion of sulphur. A small iron peak was visible in all samples but was not labeled due to the small proportion; the iron peak was only labeled in Sika rapid. Setting time The setting times of the cements and their respective DSP mortars are shown in Table 1. All the cements tested except for OPC had adequate setting time (!9 min) and thus could be potential dental restorative materials. There was no statistically significant difference between the setting time of cements and the DSP mortars with PO0.05 (independent sample t-test at 95% confidence level). Figure 1 Compressive strength testing The results of compressive strength tests performed on cubes cast according to BS 4550 Section 3 Part 3.4 1970 are shown in Table 2. The cubes failed in normal mode (Fig. 3a). The standard deviation was small for all the cubes tested. The results of compressive strength testing according to BS EN ISO 9917: 2003 are shown in Table 3. The cylinders failed in shear mode (Fig. 3b). The standard deviation for the cylinder testing was larger than for the cube testing. For the cylinder testing, the strength values did not rise incrementally with material aging in contrast to the cubes. The data tested had a normal distribution. The OPC DSP mortar showed poorer compressive strength than the OPC (P!0.01) at all times for cube testing but not for cylinder testing, where no difference was observed. The fast-setting DSP mortars had lower compressive strength at 1 day (P!0.005) when testing both cubes and cylinders. At later ages, there was no difference between these cements and their DSP mortars for the cube testing. The OPC/HAC and Emaco DSP cylinders showed poor compressive strength values at EDAX of (a) OPC, (b) Emaco ultra-rapid, (c) Sika rapid and (d) Exocem, respectively. Use of Portland cement as a dental restorative material 573 Table 2 Mean compressive strength of 70.7! 70.7 mm cement cubes, nZ3 (SD). Cement type Compressive strength (N/mm2) 1 day 7 days 28 days OPC OPC/HAC Emaco (300 mm) Sika rapid (90 mm) Exocem OPC DSP OPC/HAC DSP Emaco DSP (300 mm) Sika DSP (90 mm) Exocem DSP 48.4 (2.6) 24.1 (1.4) 43.0 (0.8) 68.6 (2.9) 39.2 (3.3) 57.0 (6.0) 83.3 (4.4) 40.5 (2.0) 64.8 (1.1) 25.8 (0.8) 26.4 (2.2) 29.9 (1.3) 14.7 38.9 17.0 34.0 25.1 47.6 37.3 48.1 31.5 58.5 42.0 61.0 (0.5) (2.5) (0.7) (3.4) (1.0) (3.9) (0.9) (1.1) (1.9) (4.3) (1.1) (1.7) 16.7 (1.5) 22.4 (2.2) 34.0 (5.6) 12.0 (0.6) 21.3 (2.5) 29.0 (1.5) 7 days (P!0.02) but improved at 28 days (P! 0.05). The Sika and Exocem showed the opposite with no difference at 7 days but poorer strengths at 28 days (P!0.001). Comparison of cylinder to cube compressive strength showed that that there was a statistically significant difference between the tests for OPC at all times (P!0.001). The mixture of OPC and HAC also showed a statistically significant difference between the tested cubes and cylinders at 1 day with PZ0.017. The fast-setting cements were not affected by the different method of testing at 1 and 7 days, but there was a statistically significant difference at 28 days (P!0.05), with the cylinders having lower compressive strength. Table 3 Mean compressive strength of 6!12 mm cement cylinders, nZ6 (SD). Cement type Compressive strength (N/mm2) 1 day 7 days 28 days OPC OPC/HAC Emaco (300 mm) Sika rapid (90 mm) Exocem OPC DSP OPC/HAC DSP Emaco DSP (300 mm) Sika DSP (90 mm) Exocem DSP 21.2 (3.3) 19.2 (0.6) 48.5 (9.6) 21.2 (6.0) 31.7 (7.9) 56.1 (9.5) 26.8 (4.8) 38.8 (8.1) 47.5 (3.0) 22.7 (2.4) 36.5 (14.5) 40.2 (4.3) 17.5 (2.9) 21.5 (5.2) 9.4 (2.1) 29.1 (4.8) 24.7 (7.2) 16.3 (2.2) 18.0 (5.8) 40.4 (11.4) 24.9 20.5 31.1 52.4 9.4 (3.8) 29.0 (2.0) 26.6 (4.1) 17.4 (2.7) 22.9 (5.0) 11.7 (3.8) (4.2) (5.5) (3.2) (9.0) Figure 2 Means and standard deviation of compressive strength on different sized cubes for OPC (nZ30). Effect of specimen size on compressive strength Fig. 2 shows the means and standard deviation of compressive strength of different cubes. The data distribution is shown in Table 4. The data were normally distributed. The larger cubes had a higher compressive strength (P!0.001, t-test). Discussion Although OPC has been developed as a dental material, only limited information has been published on its physical and chemical properties [3,4]. Since the cement is mixed with water in order to set, it can be regarded as dental water-based cement. Thus, its testing could conform to BS EN ISO 9917 2003 [2]. Normally OPC is used as a binder in concrete for construction purposes; it is thus not used on its own. For construction purposes, there are standards that stipulate the way the material should be tested. The testing for setting time and compressive strength of OPC is defined by BS EN 196-3 (1995) [7] and BS 4550 Section 3, Part 3.4 (1978) [8], respectively. Like all water-based Table 4 One-Sample Kolmogorov–Smirnov Test for comparison of cubes of different sizes. Cube 2 Surface area (mm ) N Mean Std dev. Minimum Maximum Kolmogorov–Smirnov Z P (two-tailed) 70.7 mm2 5000 30 65.2 3.9 55.04 72.6 0.424 0.994 50 mm2 2500 29 51.5 5.3 40 64.2 0.637 0.812 574 cements, OPC is very susceptible to additions of excess water. Thus, the quantity of water added to the mix is important, as excess water may lead to a marked decrease in strength [9], while too little water may result in a dry mix with no water available for the hydration reaction [10]. The right amount of water added to the powder can be calculated using the Vicat and the standard consistence needle [7]. For the fast-setting cements, this procedure was difficult to perform as the working time of the cement was limited; thus, manufacturer’s instructions were used. In this experiment, cylinders of 6 mm in diameter and 12 mm in height were used. This was done to have a height to diameter ratio of 2. The EN ISO 9917 2003 [2] advises the use of molds producing cylinders of 4 mm in diameter and 6 mm in height. The strength of cylinders for height to diameter ratio 2 is not influenced by the restraining effects of the loading plates. Values higher than 2 may lead to buckling of the specimens and lower values require the use of a correction factor when calculating the compressive strength. No packing material was used to minimize friction between the loading plates and the specimen. The ends were made plane by the use of capping ends and very fine grit sandpaper. Lack of planarity can reduce the effective contact area between the specimen and the bearing plate [11]. The cubes were cast in steel molds and were tested on surfaces cast against the mold rather than the open cast surface. Cubes were crushed at a loading rate of 0.6 N mmK2 sK1 [8] while cylinders were crushed at a cross head speed of 0.75 mm minK1 (loading rate of 50 N minK1) [2]. The observed fracture mode of cements and mortars was in the form of a pyramid the base of which coincided with the top and bottom of the cube and the apex pointing inwards (Fig. 3a). Cylinders failed in shear mode (Fig. 3b). All the cements tested had similar elemental constitution, which was verified by EDAX. The high alumina cement added to the Portland cement reacts with the calcium sulphate thus allowing the free calcium aluminate to hydrate leading to rapid setting of the cement. Portland cement was used as the major constituent as this enabled control over the setting time [11]. The DSP mortars reduced the compressive strengths especially at early ages in contrast to what was reported by Bache [5] who reported improvement in compressive strengths of cements by addition of superplasticizers and fine particles. Some of the cements tested did not show an incremental increase in measured compressive strength particularly Emaco ultra-rapid which J. Camilleri et al. Figure 3 (a) Failed cube, and (b) failed cylinder. even exhibited a reduction in compressive strength between 7 and 28 days. There is little published research on testing small Portland cement samples. The measured strength of cement is not unique, as it is known to be dependent on several factors among which are specimen size and testing conditions. The effect of size on measured strength is well documented [11]. It is also well documented that as hydration proceeds, micro-cracks tend to form within the cement. These micro-cracks have a larger influence when the sample is smaller [12]. Besides size, the samples were not cured under water as stipulated by standards for normal, larger cement/concrete samples. The loss in compressive strength could have been caused by internal cracking due to shrinkage/thermal cracks which, given the size of the sample, would affect the measured strength to a greater degree. The effect of the method of curing on the measured compressive strength of cements should be investigated further. The reduction in compressive strength at later curing times could present a problem if Use of Portland cement as a dental restorative material the material is used clinically, as failure of the restoration could result due to a material weakness. The data obtained for changes in compressive strength produced by altering the surface area of the specimen verified that compressive strength testing depends on the specific test conditions. In the present study, Portland cement was used on its own. The use of sand was omitted as the sand particles were too large, and the sand was not added to the potential restorative material. The material was cured at 37 8C as these are the conditions usually present in the mouth. The Portland cement was susceptible to the testing method unlike fast-setting cements. Smaller samples were shown to produce lower compressive strength values when tested. The fast-setting cements might have less porosities; thus, the flaws created, when the material is subjected to compression, would not be as critical as those of Portland cement. Conclusions The pure fast-setting cements set in !7 min were not susceptible to changes in the compressive strength testing procedure at 1 and 7 days but at 28 days, all the fast-setting cements had a significantly higher strength with the test using cubes (P!0.05). Acknowledgements The Commonwealth Scholarship Commission for funding. Mr Philip Tabone, and Attrans Commercials Limited for providing the materials. Ms Mahtab View publication stats 575 Behrouzi for her assistance with procuring the standards. Mr Pio Saliba and Mr Richard Mallett for the technical assistance, and Mr Ken Brady for assistance with the chemical analysis. References [1] Torabinejad M, Watson TF, Pitt Ford TR. Sealing ability of a mineral trioxide aggregate when used as a root end filling material. J Endod 1993;19:591–5. [2] British Standard Institution: Dentistry: Water-based cements. Part 1. Powder/liquid acid-base cements. BS EN ISO 9917-1; 2003. [3] Camilleri J, Montesin FE, Brady K, Sweeney R, Curtis RV, Pitt Ford TR. The constitution of mineral trioxide aggregate. Dent Mater 2005;21:297–03. [4] Torabinejad M, Hong CU, McDonald F, Pitt Ford TR. Physical and chemical properties of a new root-end filling material. J Endod 1995;21:349–53. [5] Bache HH. Dense cement ultrafine particle based material. Proceedings of the 2nd symposium on superplasticizers in concrete, Ottawa Canada, 1981. [6] Hattori K. Superplasticizers in concrete. In: proceedngs from the internatoinal symposium Ottawa, Canada; 1978. p. 49–86. [7] British Standard Institution: Methods of testing cement. Determination of setting time and soundness. BS EN 196-3; 1995. [8] British Standard Institution: Specification for physical testing of Portland cement: compressive strength. BS 4550 Section 3 Part 3.4; 1978. [9] Abrams DA. Design of concrete mixtures Bulletin 1. Structural materials research laboratory. Chicago: Lewis Institute; 1918. [10] Klieger P. Effect of mixing and curing temperature on concrete strength. ACI J Proc 1958;54:1063–81. [11] Neville AM. Properties of concrete Longman scientific and technical essex UK. 3rd ed 1981. [12] Griffith AA. The phenomena of rapture and flow in solids. Philos Trans R Soc A 1920;221:163–98.