Advances in Applied Ceramics: Structural,Functional and Bioceramics

Alkali activated binders are a new class of binding material with comparable or enhanced performance to Portland cement. These binding materials are obtained by a chemical reaction between an aluminosilicate material and a highly alkaline solution. In most cases, the setting hardening process of this binder is performed at high curing temperatures. In this paper, alkali activated mortars based on vitreous calcium aluminosilicate (VCAS) cured at room temperature are evaluated. Mechanical strength development and microstructural analysis (scanning electron microscopy, thermogravimetric analysis, X-ray diffraction and mercury intrusion porosimetry) of these materials are performed. Mortars yielded compressive strength ∼89 MPa after 360 days. This is the first time that VCAS is used as aluminosilicate source material in the production of alkali activated mortars cured at room temperature.

Among low cost or readily available raw materials, reservoir clay sediments are of interest as potential precursors in geopolymer binder manufacture. These materials come from dredging of reservoirs because periodical sediment removal is necessary in order to keep a satisfactory level of functionality. In this paper, two sediments, coming from reservoirs located in Southern Italy, have undergone preliminary characterisation by X-ray diffraction, differential thermogravimetry and Fourier transformed infrared (FTIR) spectroscopy. Then, the sediments were submitted to 1 and 2 h calcination treatments at 650 and 750°C. The effects of calcination were evaluated by means of ²⁷Al magic angle spinning nuclear magnetic resonance and FTIR. The calcined samples were mixed with 5M NaOH solution, and the obtained mixtures were studied for reactivity by means of differential scanning calorimetry. Finally, cylindrical samples were prepared with the same mixtures and cured for 3 days at 60°C plus 4 and 25 days at room temperature. The obtained samples were subjected to unconfined compressive strength determinations in order to verify the actual occurrence of geopolymerisation. The results show that the calcined clay sediments can be suitable precursors in polycondensation reactions.

The innovative multicomponent matrix for immobilising cadmium and caesium introduced in this paper consists of belite cement, gismondine type NaP zeolite and nanoadditions. With this innovative design, the solid blend was able to absorb a high proportion of simulated radioactive liquid waste. Matrix integrity and effect of the high waste/solid ratio were assessed by X-ray diffraction analysis, while BET-N₂ surface area, pore size distribution and nanoporosity were determined with nitrogen adsorption isotherms. Matrices whose sole component was fly ash belite cement were used as a reference. This innovative material affords substantial social and environmental benefits, inasmuch as the cement used has a low CO₂ content and low heat of hydration. Moreover, both the cement and the zeolite can be synthesised ecoefficiently from an industrial byproduct, thereby eliminating the need to stockpile waste while preserving natural resources and placing valorised byproducts on the market.

This paper reports results on the porosity and pore size distribution (PSD) of cement paste containing simulated desulphurised waste (SDW). The SDW was chosen due to the variability in chemical composition of real desulphurised waste. The SDW is a combination of fly ash and gypsum. The content of fly ash in the SDW changed from 0 to 100% by weight. The water to binder ratio was 0·5. The binder consists of cement and SDW. Cement in the pastes was partially replaced with 25 wt-% SDW. The porosity and PSD of cement pastes at 28 days of curing is reported. Increasing amount of gypsum does not seem to greatly change the pore volume; however, there is tendency of obtaining coarser pore structure in the presence of gypsum. The compressive strength increases with increasing amounts of gypsum. Correlation between strength and PSD is conducted.

The novel low carbon scheme for manufacturing cement and concrete proposed in this paper combines three processes. First, a Portland-like cement is made at reduced temperature and cooled so that it self-pulverises to a powder of cement fineness without the need for grinding. Second, CO₂ enriched gas is extracted from the cement kiln flue, and this may be used without further refinement in a third part of the process to activate this cement (which is only poorly hydraulic) in the production of precast concrete products. Considerable energy savings are anticipated, and additional sustainability gains arise from the closed loop manufacturing process. Proof of principle has been demonstrated and a preliminary estimate made of CO₂ emission reduction: scale-up challenges are still to be addressed. The initial focus is on the production of precast concrete, where there is strong customer demand and regulatory pressure for low embodied carbon cements, together with a need for greater process efficiency. Other potential applications are foreseen in areas such as environmental engineering and non-structural concrete.

Mineral trioxide aggregate (MTA) is a clinical product comprising a mixture of 80 wt-% Portland cement and 20 wt-% bismuth oxide, which is used as a root-filling material in dentistry. The influence of bismuth oxide on the hydration reactions of Portland cements is not well understood. In this study, the impact of 20 wt-% replacement of bismuth oxide on the hydration of white Portland cement was monitored by powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and transmission electron microscopy (TEM). The findings of this research have confirmed that bismuth oxide is an inert additive in white Portland cement, which does not participate in the hydration reactions.

The steel slag based ceramics (SSBC) with different contents of magnesium ions were prepared by conventional sintering process. The variations of linear shrinkage, water absorption, bulk density and apparent porosity of the samples fired at different temperatures from 1130 to 1210°C were measured. The phase formation and microstructure of final ceramics were observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that increasing magnesium ions content in SSBC does not change the crystal phases but increases the sintering temperature. The final phases in SSBC are pyroxenes including augite and diopside. Crystalline phases in SSBC form before densification. Substitution of Fe²⁺ for Mg²⁺ in phases would increase amount of Fe²⁺ in crystal, but decreases the amount in liquid, which is the main factor of the sintering temperature increase. Good degrees of densification and high crystallinity of ceramics show good mechanical properties.

The effect of high energy dry milling on the structural and crystalline state of sintered biphase calcium phosphates was studied. After various periods of grinding, the initial biphase calcium phosphate material alters its crystalline structure and phase composition. The phase transformations achieved during milling were recorded by powder X-ray diffraction, scanning electron microscopy, infrared spectroscopy and chemical analysis. X-ray diffraction analysis of samples milled for 20 h showed that the initial composition of the biphase ceramics changed and part of β-tertiary calcium phosphate was nanocrystalline and partially in amorphous state. Hydroxylapatite fully transformed into nanocrystalline phase. Cytotoxicity tests of samples milled for 20 h clearly present cellular viability with pronounced biological activity.

By close inspection of the well polished cross-sections, two categories of interfaces were classified, namely, adhesive bonding between veneering porcelains and zirconia or alumina cores versus reactive bonding between veneering porcelains and cores of glass infiltrated alumina or lithium disilicate based glass ceramics. Argon ion beam cross-section polishing technique was applied to achieve gentle and fine polishing required for high resolution interfacial characterisation by scanning electron microscopy. The observations suggest that it is desirable to enhance the interfacial reactive bonding in order to avoid delamination in alumina and zirconia based composites.

A microwave heated methyl trichlorosilane based chemical vapour infiltration technique has been used to form SiC_f/SiC composites from SiC fibre preforms preimpregnated with SiC powder using two different fabrication techniques. While infiltration rates obtained from samples loaded with powder were generally higher than for preforms without powder, preferential infiltration occurred in regions where the SiC powder was most concentrated as a result of the initial non-uniform distribution of the powder across the preforms. The consequence was density gradients in the final composites. Nevertheless, average densities as high as 75% could be achieved in a 10 h process.