Konstantinos Sotiriadis

Microstructural characterization of dental zinc phosphate cements using combined small angle neutron scattering and microfocus X-ray computed tomography

OBJECTIVE To characterize the microstructure of two zinc phosphate cement formulations in order to investigate the role of liquid/solid ratio and composition of powder component, on the developed porosity and, consequently, on compressive strength. METHODS X-ray powder diffraction with the Rietveld method was used to study the phase composition of zinc oxide powder and cements. Powder component and cement microstructure were investigated with scanning electron microscopy. Small angle neutron scattering (SANS) and microfocus X-ray computed tomography (XmCT) were together employed to characterize porosity and microstructure of dental cements. Compressive strength tests were performed to evaluate their mechanical performance. RESULTS The beneficial effects obtained by the addition of Al, Mg and B to modulate powder reactivity were mitigated by the crystallization of a Zn aluminate phase not involved in the cement setting reaction. Both cements showed spherical pores with a bimodal distribution at the micro/nano-scale. Pores, containing a low density gel-like phase, developed through segregation of liquid during setting. Increasing liquid/solid ratio from 0.378 to 0.571, increased both SANS and XmCT-derived specific surface area (by 56% and 22%, respectively), porosity (XmCT-derived porosity increased from 3.8% to 5.2%), the relative fraction of large pores ≥50μm, decreased compressive strength from 50±3MPa to 39±3MPa, and favored microstructural and compositional inhomogeneities. SIGNIFICANCE Explain aspects of powder design affecting the setting reaction and, in turn, cement performance, to help in optimizing cement formulation. The mechanism behind development of porosity and specific surface area explains mechanical performance, and processes such as erosion and fluoride release/uptake.OBJECTIVE To characterize the microstructure of two zinc phosphate cement formulations in order to investigate the role of liquid/solid ratio and composition of powder component, on the developed porosity and, consequently, on compressive strength. METHODS X-ray powder diffraction with the Rietveld method was used to study the phase composition of zinc oxide powder and cements. Powder component and cement microstructure were investigated with scanning electron microscopy. Small angle neutron scattering (SANS) and microfocus X-ray computed tomography (XmCT) were together employed to characterize porosity and microstructure of dental cements. Compressive strength tests were performed to evaluate their mechanical performance. RESULTS The beneficial effects obtained by the addition of Al, Mg and B to modulate powder reactivity were mitigated by the crystallization of a Zn aluminate phase not involved in the cement setting reaction. Both cements showed spherical pores with a bimodal distribution at the micro/nano-scale. Pores, containing a low density gel-like phase, developed through segregation of liquid during setting. Increasing liquid/solid ratio from 0.378 to 0.571, increased both SANS and XmCT-derived specific surface area (by 56% and 22%, respectively), porosity (XmCT-derived porosity increased from 3.8% to 5.2%), the relative fraction of large pores ≥50μm, decreased compressive strength from 50±3MPa to 39±3MPa, and favored microstructural and compositional inhomogeneities. SIGNIFICANCE Explain aspects of powder design affecting the setting reaction and, in turn, cement performance, to help in optimizing cement formulation. The mechanism behind development of porosity and specific surface area explains mechanical performance, and processes such as erosion and fluoride release/uptake.

Study on the Properties of Geocement Based Thermal Insulating Materials for High Temperature Technical Appliances

Three different types ofgeocement thermal insulation materials for energy storage unit were selected after preliminary test. Perlite and vermiculite were added as thermal insulation fillers during mixing process. The produced materials were subjected to heat treatment before determining their properties. The materials produced are characterized by low density (321–376 kg/m3) and thermal conductivity (0.0727 – 0.0766 W/mK). These materials ensure the minimum heat losses of the energy storage unit if the temperature of the medium is 318–923 K and the thickness of thermal insulation layer is 153–160 mm.

New Thermal Insulating Material Based on Geocement

A new thermal insulating material was developed on the basis of a geocement, formulated as Na2OAl2O36SiO220H2O. Ground limestone and aluminosilicate pellets were used as fillers for its production (composition: geocement 64.29 wt. [%]; fillers 35.71 wt. [%]). This material, which is applied having a thickness of 3.0-4.5 mm, swells when it is exposed to an external heat flow of 1273 K average temperature. Swelling is due to the matrix phases and filler dehydration, which include heulandite, ussingite, sodium zeolite and other phases. As a result, a finely porous glassy aluminosilicate frame of jadeite-albite composition is formed, which is characterized by low thermal conductivity (0.09-0.175 Wm-1K-1). The developed material can be used to protect and to insulate wooden, metal and concrete surfaces from an one-sided heat source. The paper is dedicated to the great scientist of the XXI century in the field of alkali-activated cements and materials based on them, Pavlo Kryvenko, in honor of his 75th birthday anniversary.

Influence of Sulfates on Chloride Diffusion and Chloride-Induced Reinforcement Corrosion in Limestone Cement Materials at Low Temperature

AbstractAn ordinary portland cement and two portland limestone cements [15 and 35% weight-to-weight ratio (w/w) limestone content] were used for concrete and reinforced mortar specimens preparation...

Exposure of Mortars Modified with Rubber Aggregates and Polymer Admixtures to Acid Environments and Elevated Temperature Conditions

AbstractThe aim of the present paper was to study the effect of incorporating rubber aggregates and acrylic or EVA polymers on the properties of cement mortar exposed to acid (HCl, H2SO4, HNO3) and elevated temperature conditions. Compressive and bending strength tests, water absorption test, mass measurements, and visual inspection were performed. Impact-echo and impedance spectroscopy methods were used to investigate the effect of heat exposure. The use of rubber aggregates led to reduction of strength values and water absorption. The use of polymeric admixtures further decreased water absorption and inhibited compressive strength loss and delayed mass loss when the specimens were stored in acid solutions. The acoustic and electric measurements showed that the use of rubber aggregates or both rubber aggregates and polymers led to reduction of absorption frequency or relative permittivity, respectively. The drop of frequencies was attributed to the formation of microcracks, as well as to decomposition an...

High-Temperature Degradation of Mortar Containing Rubber Aggregates and EVA Binder Evaluated by Impact-Echo Method

The application of Impact-echo acoustic method to evaluate the degradation and the composition of mortar composites modified with rubber aggregates and EVA polymer binder degraded at high temperatures was studied. Specimens were prepared by using a type CEM I Portland cement and siliceous sand, as well as by substituting 25% of sand with shredded automobile tires and by adding of EVA polymer binder (10% w/w to cement mass). The samples were subjected to high-temperature treatment in the temperature range of 200-400 °C. The results of non-destructive testing of such samples by acoustic methods confirmed the differences in the structure of mortar specimens. Addition of rubber aggregates in samples caused absorption of lower frequency as compared to reference specimens. A significant decrease of the absorbed frequencies was observed depending on the temperature. The largest decrease happened after exposure of samples at 200-300 °C. It indicates that the effect of heat treatment was reduced when the EVA binder was added.

The Effect of Firing Temperature on the Composition and Microstructure of a Geocement-Based Binder of Sodium Water-Glass

The fire performance of a geocement-based binder was investigated with a combination of analytical techniques, in terms of changes in composition and microstructure. Geocement, formulated as Na2O∙Al2O3∙6SiO2∙20H2O, was prepared using metakaolin, sodium water-glass, rotten stone and sodium hydroxide. The mixture was homogenized by passing through a hydrodynamic cavitator. Cubes of 20 mm were prepared, hardened at laboratory conditions for 28 days, and subsequently burnt at 600, 800 and 1200 °C in a laboratory furnace. Cavitation treatment resulted in a highly amorphous binder; amorphous fraction decreased upon firing up to 800 °C due to crystallization, and increased above 1000 °C because of melt formation. Porosity increased with firing temperature and pores larger than 1 mm in diameter prevailed at 1200 °C. The material remained stable up to 1200 °C. The results indicate the adequacy of this geocement-based binder for preparing fire-protecting materials.

Use of Impact-echo Method to Test High-temperature Degraded Cementitious Composite Materials Containing Rubber Aggregates and Acrylic Polymer Binder

The present paper deals with the use of Impact-echo acoustic method to test cement-based mortar composites degraded at high temperatures. The specimens were prepared by using a type CEM I Portland cement and siliceous sand, as well as by substituting 25% of sand with shredded automobile tires and by adding acrylic polymer binder (20% w/w to cement mass). The samples were were subjected to high-temperature treatment in the temperature range of 200-400 °C. The acoustic results confirmed the differences in the structure of mortar specimens. The addition of rubber aggregates in H2 and H3 specimens resulted in lower frequencies compared to H1 specimen. It was also observed the dropping down of the frequencies for the temperatures between 200-300°C, indicating that the effect of heat treatment was mitigated when the acrylic binder was added.

Chloride Diffusion in Limestone Cement Concrete Exposed to Combined Chloride and Sulfate Solutions at Low Temperature

The chloride diffusion in limestone cement concrete exposed to combined chloride and sulfate solutions at low temperature was studied. For this purpose, a normal Portland cement and two Portland limestone cements (15% and 35% w/w limestone content) were used for concrete preparation. The specimens were immersed in two combined chloride-sulfate solutions of different sulfate content, and stored at 5°C. The total and free chloride contents, as well as the chloride diffusion coefficients were determined for each concrete composition. The results show that the total chloride content and free to total chloride ratio are increased with time. The sulfate content of the corrosive solutions has not a clear effect on total chloride content and chloride diffusion coefficient. It seems that the lower sulfate content results, in general, in higher free to total chloride ratio values. The use of limestone in cement results in higher chloride concentrations in concrete and free to total chloride ratio values. In general, these phenomena are intensified for higher limestone content.

Use of Thermal Insulating Perlite Composite Materials Based on Geocement to Protect Technological Equipment

Two different thermal insulating materials based on geocement (formulated as Na2OAl2O36SiO220H2O) were produced as possible environmentally friendly materials, to substitute existing thermal insulation of technological equipment. Expanded perlite was added as filler during the mixing process. The two thermal insulating perlite composite materials were produced in the form of two-component mixture with a ratio of liquid phase (geocement dispersion) to solid phase (perlite) of 3:1.8 and 3:1.0. The developed materials are characterized by: compressive strength of 0.11-0.14 MPa; average density of 296-320 kg/m3; thermal conductivity coefficient of 0.068-0.078 W/mK; adhesion on metal surface of 1.15-1.32 MPa. The measured temperatures on the thermal insulation coatings of technological equipment confirm the effectiveness of the materials developed. These materials, having a thickness not exceeding 3.5 cm, are recommended for application to technological equipment of complex geometric shape at an operating temperature until 1073 K.