Advances in Applied Ceramics: Structural,Functional and Bioceramics

The paper characterises the phases formed in a solidification/stabilisation (s/s) matrix resulting from a blend of air pollution control residue and co-fired pulverised fuel ashes, activated with a waste, aluminium containing, NaOH solution. Characterisation was performed by X-ray diffraction, simultaneous thermal analysis (thermogravimetric analysis and differential thermal analysis) and SEM. The effects of varying air pollution control/pulverised fuel ashes ratio, curing temperature and time and liquid phase (NaOH or deionised water) on the hydration products in the s/s matrix were studied, as were the changes in phase composition during regulatory leach testing. The principle phases in the s/s matrix, dependent on the process conditions, included Ca(OH)₂, CaCO₃, NaCl, C–S–H (I), Friedel's salt, katoite, zeolitic phases Na–P1 and sodalite, CaClOH and vishnevite. Some retention of chlorides within Friedel's salt and sodalite during leach testing was observed; however, most was released due to its presence as NaCl. Sulphates were retained as vishnevite during leach tests.

This paper examines a series of fly ash and ground granulated blast furnace slag binders formulated to examine the effects of composition and mix design on performance and durability when exposed to aggressive environments found in carbon capture facilities (monoethanolamine or concentrated potassium carbonate, as well as distilled water). Ordinary Portland cement and ordinary Portland cement fly ash blends were examined for comparison. The samples are exposed to solvents by immersion under static conditions. Leaching and mechanical strength tests show a wide range of geopolymer performance, depending on the initial mix design and the nature of the aggressive agent. Alkali carbonate solutions are found to be the most aggressive agent among those tested in terms of geopolymer mass and strength loss; nonetheless, the resistance to structural degradation upon carbonation of the geopolymer binder structure is mitigated to a significant extent by the low calcium content of the geopolymer binder. Less porous materials, or materials with smaller and more tortuous pores, show in many cases a markedly higher durability; porosity is shown to hold the key to durability. In traditional cement based binders, strength is lost through chemical attack upon exposure to solvents.

Sugar cane bagasse ash is an important byproduct of sugar and alcohol production. This residual material requires only grinding and classification in order to achieve adequate fineness and homogeneity to be used as pozzolanic admixture in concrete. In this study, submicrometre ashes were produced from wet grinding using a planetary ball mill. Measurements of the pozzolanic activity of the ashes with cement and lime were carried using mechanical methods based on the compressive strength of mortars. It was observed that the pozzolanic activity of submicrometre ashes is inversely correlated to its particle size. Moreover, it was demonstrated that the grinding intensity clearly increased the sugar cane bagasse ash reactivity.

Calcium phosphate based cements can be moulded to irregular contours often encountered in medicine. Brushite cements are more soluble than hydroxyapatite (HA) in physiological conditions and as such can be more rapidly resorbed. The metastable nature of brushite means that it converts to HA following immersion in physiological conditions. This phase change reduces the resorption rate, resulting in long term stability. In this study, macropores were incorporated into brushite cement to accelerate degradation, preventing hydrolysis and long term stability. The incorporation of beads into the cement had no effect on the composition of the cement. By varying the proportion of beads in the cement, it was possible to control degradation and prevent the formation of HA within the cement. Interestingly, in the macroporous cement, after 14 days of aging, the only crystalline component was β-tricalcium phosphate, suggesting that an amorphous binding phase was formed following brushite dissolution; this phase may maintain the integrity of the cement.

In this study, we report the formation and characterisation of a calcium aluminate cement for potential application as a hard tissue replacement. Phase pure monocalcium aluminate powder was synthesised using the Pechini technique, using a relatively low temperature (1000°C) and sintering time (3 h). Characterisation of the hardened material showed a strong relationship between the liquid/powder ratio to which the cement was mixed and the mechanical performance of the hardened materials with little influence on cement microstructure. Interestingly, following immersion in phosphate buffered saline, there was extensive precipitation on the surface of the cement sample. X-ray diffraction showed that the precipitate was hydroxyapatite, which on evaluation using scanning electron microscopy was shown to be of porous structure.

The aim of this study was to carry out a mechanical and microstructural characterisation of calcium aluminate cement [EndoBinder (EB)] by compressive and diametral tensile strength tests, Vickers microhardness test and scanning electron microscopy analysis. Test specimens were made according to the type of cement used [EB, grey mineral trioxide aggregate (GMTA) and white mineral trioxide aggregate (WMTA)] and the setting time tested (24 h, 7 days and 21 days). The mean values obtained after mechanical tests showed that EB presented higher compressive strength, with statistically significant difference to WMTA (7/21 days), and higher diametral tensile strength, with statistically significant difference to GMTA (7/21 days) and WMTA (21 days). EndoBinder presented higher microhardness values, differing statistically from GMTA (21 days) and WMTA (7/21 days). The SEM analysis showed a more homogeneous microstructural arrangement for EB, with particles of uniform size and shape. EndoBinder presented adequate mechanical properties, frequently superior to mineral trioxide aggregate, making it a feasible option for endodontic treatment.

Three-dimensional powder printing was used in this study to fabricate custom made magnesium phosphate based structures. In a first step, farringtonite [Mg₃(PO₄)₂] powders were synthesised by sintering and subsequent grinding. Optimised powders suitable for printing were obtained after 60 min of grinding, which resulted in a medium particle size of 16 μm. These powders were reacted with binder liquids consisting of 2M K₂HPO₄, 0·5M (NH₄)₂HPO₄or 20%H₃PO₄ to form a matrix of either struvite-(K) (MgKPO₄.6H₂O), struvite (MgNH₄PO₄.6H₂O) or newberyite (MgHPO₄.3H₂O) by a hydraulic setting reaction. While printing of struvite-(K) was not possible due to the long setting time of this cement system, both newberyite and struvite samples could be processed with good dimensional accuracy. Strength was initially low after printing, namely in the range of 1·3–2·8 MPa; however, additional post-hardening in the binders increased the compressive strength to a maximum of 10 MPa for struvite and 36 MPa for newberyite. This was accompanied by a higher degree of conversion to the setting products, as evidenced by X-ray diffraction analysis. Magnesium phosphate based structures prepared by three-dimensional printing may find an application as biodegradable bone substitutes or can be used for the rapid manufacturing of moulds for metal casting, as demonstrated in this study.

In this present research, calcium phosphate cement (CPC) paste was prepared by combining cement liquids comprised of 4 wt-% disodium hydrogen orthophosphate with cement powders that consisted of β-tricalcium phosphate and monocalcium phosphate monohydrate. In the in vitrostudy, the release of gentamicin sulphate (GS) from standard cylindrical samples of the prepared CPC immersion in phosphate buffered saline solution was measured by high performance liquid chromatography test over a period of time. In addition, as a representative for biomedical properties, setting time, injectability and compressive strength were measured in order to detect changes when mixing CPC with GS. In addition, the pH values of the phosphate buffered saline solution containing samples descended gradually until reaching equilibrium pH. Overall, the results from in vitro release study showed that it reached its maximum level, which was 35% of the initial value of the GS on day 15, suggesting no irreversible binding occurred between the cement paste and the antibiotic.

A near morphotropic phase boundary lead–magnesium–niobium titanate material system was synthesised by the sol–gel method; its thin films were deposited with or without nanoparticle (np) seeding and processed by the rapid thermal annealing. Influences of some process parameters and the np seeding on various properties of the resultant films were systematically investigated. Although no reduction in annealing temperature was achieved upon np seeding, the results indicated that nps might indeed play a role as nucleation centres during the crystallisation. All in all, the lead–magnesium–niobium titanate thin film system described here (particularly those annealed at higher temperatures) appeared to be suitable for its potential applications in wireless communication systems as ultrahigh dielectric constant ϵvalue capacitors; the unexposed film annealed at 750°C, for instance, exhibited the highest dielectric constant ϵ (1425) with a low loss (loss tangent, ∼0·04) along with a strongly pronounced perovskite phase (96·5%).

Net shape fabrication of magnesium aluminate (MgAl₂O₄) spinel (MAS) components like domes and radomes is of great importance because they are employed in certain important strategic applications. Environmentally and economically benign net-shape forming techniques such as aqueous gelcasting (GC) and hydrolysis induced aqueous gelcasting (GCHAS) cannot be employed to MAS powder as it reacts with water during processing and makes the process cumbersome. The processes reported in the literature to overcome this problem and for successful fabrication of net shape MAS components like thin wall radomes/crucibles following aqueous colloidal processing routes such as GC and GCHAS are reviewed in this paper. This article also reviews the effect of surface treatment of MAS powder against hydrolysis and of consolidation route on net shape forming capability and the sintering ability of MAS powder. Furthermore, various other net shaping techniques reported for MAS are also reviewed in this article.