Insights on chemical and physical chloride binding in blended cement pastes

This study investigates chloride binding in blended cement pastes exposed to 0.5 M NaCl solutions (with and without pH adjustment) using X-ray diffraction and energy-dispersive X-ray spectroscopy image analysis (edxia). The aim is to better understand the effects of the binder type, the water-to-binder ratio and the pH on the chemical binding in AFm phases and the physical binding on C-A-S-H. Results show that the binding cannot be predicted from AFm and C-A-S-H contents alone because competing ions in the system affect both the Friedel's salt solid solution chemistry and the C-A-S-H binding capacity. Notably, the high content of aluminous hydrates in LC3 systems leads to a high chemical binding even if Friedel's salt solid solutions have relatively low chloride contents (particularly at a higher pH). On the contrary, the CEMIII/A paste showed low binding because of relatively high sulfate and magnesium contents which compete for incorporation/adsorption in aluminous hydrates (AFm, ettringite and hydrotalcite).

Development and characterisation of injectable calcium phosphate cements for use in vertebroplasty

The global objective of this thesis was to understand how the starting components of brushite cements influence the cement properties relevant for its use in vertebroplasty. Therefore, this work focussed on the following cement properties : mechanical strength, X-ray opacity and heat release upon setting. To carry out the global objective, a test protocol was first developed to characterise the mechanical properties of calcium phosphate cements. The Mohr's circles representation allowed to understand that for a material like our cement, ultimate stress measured with the Brazilian test can only underestimate tensile strength because the compressive/tensile strength ratio is lower than 8. Nevertheless, the Brazilian test remains a test that is easy to use with brittle materials and requires simple sample shape. Therefore, it was considered as an appropriate test for our cements, and was used as a way to measure the indirect tensile strength of the cements tested in the run of this work. Refining two of the starting powders, MCPM and CSH, allowed to increase significantly both compressive and diametral tensile strengths. Milling of these powders provided a five-fold increase in axial compression, and a three-fold increase in diametral compression. The mechanical strengths of cements made with milled MCPM and CSH were the highest measured in this thesis work : 4.1 MPa diametral tensile strength and 22.1 MPa compressive strength. The mechanical properties of the cement were found to be strongly dependent on both CSH granulometry and chemical purity. A computer model was developed to calculate the linear X-ray attenuation coefficient of any material of known composition. It was then validated by experimental measurements. The standard linear X-ray attenuation coefficient a cement should exhibit to be used in vertebroplasty was found to amount 2.47 cm-1 for the conditions simulated. It was calculated from an acrylic cement composition considered a standard in vertebroplasty. Calculation of the attenuation coefficient of a brushite cement revealed that this material is not radiopaque enough for safe use in vertebroplasty. Therefore, we added iodinated molecules to a brushite cement formulation so as to produce the required X-ray opacification. We found that incorporation of Iopentol, a contrast media commonly used in X-ray imaging applications, does not impair the mechanical properties of the cement and does not alter the setting time of the reaction either. Finally, a custom-made calorimeter was developed for measurement of kinetic and thermometric parameters of cements. One single measurement allowed calculation of the maximum reaction rate of the setting reactions, the working and setting times of the cement pastes, the degree of conversion of the cements, and the temperature increase induced by the setting reaction. The degree of hydration of the calcium sulfate was found to have a significant influence on the kinetics of the setting reaction. The reaction is accelerated by calcium sulfate dihydrate because it provides nuclei for heterogeneous precipitation of brushite. In contrast, calcium sulfate hemihydrate was found to slow down the reaction by releasing sulfate ions in the mixing liquid. As the reaction is faster when CSD is used, heat production occurs faster and accumulates in the system. As a consequence, the temperature elevation during setting is more important.

EPFL2002

Properties and phase transitions in lead free piezoelectrics

Naama Klein

Piezoelectric materials are used in a wide range of applications, with lead zirconate titanate (PZT) ceramics being the dominant family of materials, due to high piezoelectric coefficients, dielectric permittivity and coupling factors. The high properties have been found in compositions in the proximity of a morphotropic phase boundary (MPB). The MPB exhibits a weak temperature dependence so that the stability of the properties is attained over a large temperature range. However, environmental problems rising from the toxicity of lead have driven scientists around the world to search for lead free piezoelectric materials. These new materials should exhibit properties comparable to those of PZT and the efforts are at the present concentrated on solid solutions with an MPB. In this thesis an emphasis was made on constructing the phase diagrams and detecting an MPB in two of the most promising families of lead-free solid solutions: bismuth sodium titanate based and potassium sodium niobate based ceramics and single crystals. By investigating the structure of the phases, using X-ray and neutron diffractions and Raman spectroscopy, and measuring the dielectric and piezoelectric properties at temperatures from -266 °C to 550 °C, the phase transitions were assigned and phase diagrams constructed. Both systems (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (0 ≤ x ≤ 0.09) (BNBT) and (1-x)(K0.5Na0.5)NbO3-xLiNbO3 (0.02 ≤ x ≤ 0.1) (KNLN), reveal similar MPBs to the one found for PZT. However, in PZT the MPB is pseudovertical up to about 350 °C whereas the MPB in BNBT is vertical only till 75 °C and in KNLN is till -50 °C. This difference in the temperature stability range is crucial in most applications. Other interesting findings in the BNBT phase diagram correspond to the phase sequence at the temperature range from 200 to 300 °C, where ferroelectric phase transforms into a mixed phase (antiferroelectric or paraelectric) with polar regions. As temperature rises, the polar regions gradually disappear. Whereas dielectric and piezoelectric properties of BNBT could compete with those of PZT in some compositions, and fairly simple processing of the ceramics is used, the fabrication of KNLN ceramics has shown to be complex and reproducible properties difficult to obtain. Even after the critical processing conditions were identified in this thesis, and reproducibility was achieved, properties were significantly lower than for PZT. Exceptional values of the thickness coupling coefficients were measured on potassium niobate (KN) and KNLN single crystals rotated away from the Z axis by θ = 45° about the orthorhombic Y axis, [001]C-cut. Monodomain and domain-engineered single crystals cut at this angle showed very high coefficients (kt ≤ 70%), which are extremely stable over a large temperature range and after thinning to thickness below 60 µm. Together with low permittivity, these properties make KN single crystals a good candidate for high-frequency applications. The higher permittivity of KNLN makes it an attractive crystal for applications at lower frequencies. Another possible application of lead free piezoelectrics could be in biological systems. For this reason, preliminary in-vitro biocompatibility study of KNLN and PZT ceramics was conducted, showing similar microscopic attachment of cells to the two ceramics and slightly lower viability values when compared to non coated tissue culture plates. Laboratory attempts to introduce KNLN ceramics as an implant in the mineral of the bone, hydroxyapatite (HA), ended with a strong chemical reaction between the two. However, some piezoelectric response could be measured from the composite of HA and KNLN.

EPFL2009

Quantification methods for chloride binding in Portland cement and limestone systems

Fabien Jacques Michel Georget, Hamed Maraghechi, Karen Scrivener, Shiyu Sui, Wei Sun, William Wilson

This paper presents a comparison of methods to quantify the chloride binding capacity for cementitious materials composed of Ordinary Portland Cement (OPC) and Limestone (LS, 0 to 55% replacement). Physical and chemical chloride bindings were investigated to better understand the binding capacity of the different systems and to provide basic information for modelling. Cement pastes samples were exposed to a 0.5 M NaCl solution and their chloride contents were quantified as free ions in pore solution, chemically bound ions in AFm phases, and physically bound ions on Calcium-Aluminum-Silicate-Hydrates (C-A-S--H). Notably, the study showed that (1) considering a solid solution between Friedel's salt and Hemicarboaluminate (He) was necessary for the quantification of chemically bound chlorides in limestone blended system and (2) an accurate estimation of the C-A-S-H content was critical for quantifying physically bound chlorides.

PERGAMON-ELSEVIER SCIENCE LTD2019

Properties and phase transitions in lead free piezoelectrics

Naama Klein

EPFL2009

Development and characterisation of injectable calcium phosphate cements for use in vertebroplasty

EPFL2002