Fabrication of porous low crystalline calcite block by carbonation of calcium hydroxide compact

Calcium carbonate (CaCO₃) has been widely used as a bone substitute material because of its excellent tissue response and good resorbability. In this experimental study, we propose a new method obtaining porous CaCO₃ monolith for an artificial bone substitute. In the method, calcium hydroxide compacts were exposed to carbon dioxide saturated with water vapor at room temperature. Carbonation completed within 3 days and calcite was the only product. The mechanical strength of CaCO₃ monolith increased with carbonation period and molding pressure. Development of mechanical strength proceeded through two steps; the first rapid increase by bonding with calcite layer formed at the surface of calcium hydroxide particles and the latter increase by the full conversion of calcium hydroxide to calcite. The latter process was thought to be controlled by the diffusion of CO₂ through micropores in the surface calcite layer. Porosity of calcite blocks thus prepared had 36.8–48.1% depending on molding pressure between 1 MPa and 5 MPa. We concluded that the present method may be useful for the preparation of bone substitutes or the preparation of source material for bone substitutes since this method succeeded in fabricating a low-crystalline, and thus a highly reactive, porous calcite block.     

Chemical Interaction of Calcium Oxide and Calcium Hydroxide with CO<Subscript>2</Subscript> during Mechanical Activation

The reactions of CaO and Ca(OH)₂ with CO₂ during mechanical activation are studied by IR spectroscopy, x-ray diffraction, and thermal analysis. The results indicate that the earlier described extensive sorption of CO₂ by Ca-containing silicates during grinding is not related to the formation of CaO or Ca(OH)₂. Mechanical activation in CO₂ converts calcium oxide to amorphous CaCO₃ and calcium hydroxide to calcite, whereas Ca-containing silicates under such conditions homogeneously dissolve CO₂ to form a material similar to analogous carbonate-containing silicate glasses.     

Carbonate binders: Reaction kinetics,strength and microstructure

This study focussed on the synthesis of calcium carbonate binders, in situ, from the reaction between hydrated lime and carbon dioxide (CO₂). The aim was to establish the characteristics of the calcium carbonate binders that are associated with its strength, which was considered as an indicator of binder performance. The role of the parameters that are known to play an important part in the kinetics of hydrated lime carbonation processes, in changing the strength of a binder was examined in detail.The parameters identified were CO₂ gas pressure, exposure time and the initial degree of compaction of raw material. All hydrated lime mixtures were prepared at a constant water/solid ratio of 0.25. The hydrated lime compacts made at a range of compaction water/solid ratio (W/S) of 0.25. The hydrated lime compacts made at a range of compaction pressures (0.65–6.0 MPa) were exposed to different CO₂ gas pressures (ambient to 2 MPa) for different periods of time. The resulting products were tested for the amount of Ca(OH)₂ that had converted to carbonate, and for compressive strength. A microstructural analysis of the products was carried out using scanning electron microscopy.The rate of Ca(OH)₂ conversion to carbonate seemed to be enhanced with increasing gas pressure, but it decreased with increasing compaction of the initial mixture. It was revealed that the crystalline state and the morphology of the carbonate formed, rather than the degree of conversion of calcium hydroxide into carbonate, is highly critical to the strength of the binder. The study concluded that in the development of calcium carbonate binder, it is important to meet the experimental conditions that favour the crystallisation of calcium carbonate.     

Strength development and microstructural characteristics of barium hydroxide-activated ground granulated blast furnace slag

In this study, barium hydroxide was proposed as a main activator for ground granulated blast-furnace slag (GGBFS) to produce a strong binder, and it was compared to calcium hydroxide in terms of strength development, reaction products, and microstructure. The Ba(OH)₂-activated GGBFS (BHAS) achieved a significantly higher compressive strength than Ca(OH)₂-activated GGBFS (CHAS), except at 3 days, mainly due to (1) the more formation of hydration products, leading to a notable reduction in pore sizes and volume, and (2) the higher solubility of Ba(OH)₂, resulting in a higher dissolution of GGBFS than that of Ca(OH)₂. Although calcium silicate hydrate (C-S-H) was a major reaction product in both mixtures, the Ca/Si ratios were much different. In the BHAS, the presence of barium ions prohibited the synthesis of ettringite and monocarboaluminate, which formed in the CHAS mixtures, but it induced Ba-bearing products, strätlingite, and the hydrotalcite-like phase. The removal of ettringite was the cause of the lower strength of the BHAS at 3 days compared to that of the CHAS.     

Flexural strength reduction of cement pastes exposed to CaCl2 solutions

Calcium chloride (CaCl₂) can react with calcium hydroxide (Ca(OH)₂) to form calcium oxychloride which can reduce flexural strength and damage concrete. This paper aims to characterize the reduction in flexural strength of cement pastes exposed to CaCl₂ solutions using the ball-on-three-balls test. The amounts of Ca(OH)₂ and calcium oxychloride in the cement paste are measured using thermogravimetric analysis and low-temperature differential scanning calorimetry, respectively. The volume change that occurs as a result of the reactions between the cement paste and CaCl₂ is also measured. The reduction in flexural strength increases as the concentration of the CaCl₂ solution increases and the exposure temperature decreases. The flexural strength reduction can be mitigated by increasing the amount of supplementary cementitious materials (fly ash) in the cement pastes. Lowering the water-cementitious materials ratio also reduces the flexural strength reduction. The flexural strength reduction is correlated with the amount of calcium oxychloride and the volume change in the cement pastes exposed to the CaCl₂ solution. While the flexural strength reduction is believed to be primarily due to the formation of calcium oxychloride, the formation of Friedel's salt and Kuzel's salt also contributes to the flexural strength reduction.     

Influence of porosity and relative humidity on consolidation of dolostone with calcium hydroxide nanoparticles: Effectiveness assessment with non-destructive techniques

Slaked lime (Ca(OH)₂) nanoparticles were exposed at 33% and 75% relative humidity (RH) to consolidate dolostone samples used in historical buildings. Non-destructive techniques (NDT) were applied to determine the chemical, morphological, physical and hydric properties of the stone samples, before and after 20 days treatment. Morphological and mineralogical characterisation of the nanoparticles was performed. 75% RH favors the consolidation process studied under Environmental Scanning Electron Microscopy (ESEM-EDS), spectrophotometry, capillarity, water absorption under vacuum, ultrasound velocity, Nuclear Magnetic Resonance (imaging and relaxometry) and Optical Surface Roughness analyses. At 75% RH the nanoparticles fill the pores and inter-crystalline dolomite grain contacts but do not favor calcite re-crystallization as it occurs at 33% RH. The ESEM, XRD and TEM analyses under 75% RH reveal the fast transformation of portlandite (Ca(OH)₂) into vaterite (CaCO₃), monohydrocalcite (CaCO₃ · H₂O) and calcite (CaCO₃), and eventually the physical and hydric properties of the stones significantly improve. New insights are provided for the assessment of consolidation effectiveness of porous carbonate stones with calcium hydroxide nanoparticles under optimum RH conditions combining several NDT.     

Synthesis and characterization of cellulose acetate–calcium carbonate hybrid nanocomposite

A hybrid nanocomposite composed of calcium carbonate (CaCO₃) and cellulose acetate (CA) was fabricated by bubbling CO₂ gas into the mixture of CA and Ca(OH)₂ solution. Cellulose acetate–calcium carbonate (CA–CC) nanocomposite was characterized by spectral, thermal and optical methods. FTIR and XRD analysis confirmed the formation of the hybrid nanocomposite and XRD confirmed the formation of CaCO₃ with calcite polymorph. Thermal analysis showed CA–CC nanocomposite has better thermal stability than pristine CA. The CaCO₃ nanoparticles were in sphere shape with 100–1000 nm diameter.     

Damage in cement pastes exposed to MgCl<Subscript>2</Subscript> solutions

Magnesium chloride (MgCl₂) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl₂ solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)₂ and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl₂ solutions with a (Ca(OH)₂/MgCl₂) molar ratio larger than 1. As the MgCl₂ concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)₂ and the additional reduction due to the calcium leaching from C-S-H (at MgCl₂ concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)₂ is leached, while no additional reduction is observed at high MgCl₂ concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl₂ solutions.     

In-situ observation on the transformation of calcium phosphate cement into hydroxyapatite

In the present study, the in-situ transformation of calcium phosphate cement into hydroxyapatite (HAp) within the first hour is monitored with a synchrotron X-ray beam. A disodium hydrogen phosphate solution is used as cement liquid to activate the reaction between dicalcium phosphate anhydrous (DCPA) and calcium hydroxide (Ca(OH)₂). The XRD analysis indicates that the amounts of DCPA and Ca(OH)₂ first decrease within the first min of the reaction. Then, the intensity of DCPA's XRD peaks starts to increase instead in the period of 5 to 20 min. After 20 min, the DCPA particles are consumed slowly to form fine HAp particles. Large pores are evident upon the completion of reaction.     

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO₂-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO₂ curing was consumed during the further hydration of C₃A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)₂ was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO₂ cured mortar decreased further after water curing.     

Hydrothermal Synthesis of nanocrystalline powders of alkaline-earth hydroxyapatites, A10(PO4)6(OH)2 (A = Ca, Sr and Ba)

In the present work, we report a direct precipitation of nanocrystalline powders of alkaline-earth hydroxyapatites of the compositions, A₁₀(PO₄)₆(OH)₂ (A = Ca, Sr or Ba) from aqueous solutions containing Na₃PO₄ and MCl₂ (M = Ca, Sr or Ba) at 150°C for 2 days and autogeneous pressure under hydrothermal conditions. The products were characterized by X-ray powder diffraction, transmission electron microscopy and scanning electron microscopy. The paper also discusses a convenient and economical hydrothermal route for the extraction of nanocrystalline calcium hydroxyapatite, from fish bone waste.     

Polymer-assisted synthesis of hydroxyapatite nanoparticle

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) nanoparticles were synthesized using calcining calcium dihydrogenphosphate (Ca(H₂PO₄)₂ · H₂O), calcium hydroxide (Ca(OH)₂), and polyethylene glycol (PEG) at 900 °C in an oxygen atmosphere. This one-step process yields HA nanoparticles with similar particle sizes (e.g., 50–80 nm) that are well-crystallized and non-aggregated. PEG is an important factor in controlling the particle size, crystal phase, and degree of aggregation in these HA particles. This conclusion is supported by results from a field-emission scanning electron microscope (FE-SEM), X-ray diffractometry (XRD), energy dispersive X-ray analysis (EDS), a high-resolution transmission electron microscope (HR-TEM), and dynamic light scattering (DLS).     

Thermal conductivity improvement of copper–carbon fiber composite by addition of an insulator: calcium hydroxide

The effects of adding calcium hydroxide (Ca(OH)₂) to a copper–CF (30 %) composite (Cu–CF(30 %)) were studied. After sintering at 700 °C, precipitates of calcium oxide (CaO) were included in the copper matrix. When less than 10 % of Ca(OH)₂ was added, the thermal conductivity was similar to or higher than the reference composite Cu–CF(30 %). A thermal conductivity of 322 W m^?1 K^?1 was measured for the Cu–Ca(OH)₂(3 %)–CF(30 %) composite. The effects of heat treatment (400, 600, and 1000 °C during 24 h) on the composite Cu–Ca(OH)₂(3 %)–CF(30 %) were studied. At the lower annealing temperature, CaO inside the matrix migrated to the interface of the copper matrix and the CF. At 1000 °C, the formation of the interphase calcium carbide (CaC₂) at the interface of the copper and CFs was highlighted by TEM observations. Carbide formation at the interface led to a decrease in both thermal conductivity (around 270 W m^?1 K^?1) and the coefficient of thermal expansion (CTE (10.1 × 10^?6 K^?1)).     

Silver and copper addition enhances the antimicrobial activity of calcium hydroxide coatings on titanium

Electrochemically assisted deposition of Ca(OH)₂ (Portlandite) coatings on titanium surfaces has been proven as a promising method to provide the substrate with a most desirable combination of significant bacterial growth reduction on one hand and good biocompatibility on the other. Due to the rapid in vivo transformation of Ca(OH)₂ to hydroxyapatite, the antimicrobial activity will be an ephemeral property of the coating when implanted into the human body. In this study, the ability to reduce bacterial growth of such portlandite coatings was significantly enhanced by an ionic modification with copper and silver ions. Antibacterial tests revealed a noticeably elevated reduction of bacterial growth, especially for copper and even at a relatively low copper content of about 0.3 wt.%. In addition, the cytocompatibility, a crucial prerequisite for potential in vivo biocompatibility, of the copper-modified coating was comparable to pure calcium hydroxide coatings.     

In situ preparation of Calcium hydroxide films

The in situ preparation of Calcium hydroxide films in an ultra high vacuum (UHV) is constrained by the decomposition of species at the surface and the absence of OH bulk diffusion. Therefore, it is not possible to prepare such films simply by water exposure to a Calcium layer.We present four different approaches for the preparation of Ca(OH)₂ films in an UHV. Two of these methods are found to be ineffective for the preparation, the other two are shown to produce Calcium hydroxide films. Both of the two effective procedures make use of H₂ gas exposure. Metastable Induced Electron Spectroscopy, Ultraviolet Photoelectron Spectroscopy, and X-ray Photoelectron Spectroscopy are employed to verify quality and purity of the films.     

Design of strain hardening cement-based composites with alkali treated natural curauá fiber

The work in hand presents the design process of Strain Hardening Cement-based Composite reinforced with natural curauá fiber. The matrix fracture energy and matrix-fiber bond were studied and implemented into the theoretical model for critical fiber volume prediction, which was verified by mechanical tests for tensile, bending and compression strength. The influence of matrix-fiber bond on critical fiber volume is presented. The fiber properties are improved by two-stage treatment. Firstly, hot water (80 °C) washing in water changed every 3 h. Secondly, immersion in 1% solution of calcium hydroxide Ca(OH)₂ with water for 60 min for calcium deposition on fiber surface to increase bond properties. The critical fiber volume predicted by the model was 4% for 20 mm treated curauá fiber and was verified on the composites, which presented strain-hardening behavior under tensile test and deflection-hardening under bending.     

Sonochemical synthesis of calcium phosphate powders

β-tricalcium phosphate (β-TCP) and biphasic calcium phosphate powders (BCP), consisting of hydroxyapatite (HA) and β-TCP, were synthesized by thermal decomposition of precursor powders obtained from neutralization method. The precursor powders with a Ca/P molar ratio of 1.5 were prepared by adding an orthophosphoric acid (H₃PO₄) solution to an aqueous suspension containing calcium hydroxide (Ca(OH)₂). Mixing was carried out by vigorous stirring and under sonochemical irradiation at 50 kHz, respectively. Glycerol and D-glucose were added to evaluate their influence on the precipitation of the resulting calcium phosphate powders. After calcination at 1000°C for 3 h BCP nanopowders of various HA/β-TCP ratio were obtained.     

Further studies of calcium phosphate growth on phosphorylated cotton fibres

Further studies using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX), micro-Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and solid state magic angle spinning nuclear magnetic resonance (MAS NMR) techniques of calcium phosphate growth on Ca(OH)₂-treated urea/H₃PO₃- and urea/H₃PO₄-modified cotton fibres are reported. In the case of the Ca(OH)₂-treated urea/H₃PO₃-modified fibres which have been reported in an earlier paper, further experiments subjecting the urea/H₃PO₃-modified cotton to alternative soaking treatment procedures to Ca(OH)₂ as well as different calcium phosphate growth media such as the alkaline phosphatase-catalysed hydrolysis of disodium p-nitrophenylphosphate to free phosphate have reaffirmed the importance of the Ca(OH)₂ treatment step for the stimulus and growth of calcium phosphate growth on the fibres. Studies on cotton phosphorylated by a slightly different method using urea/H₃PO₄ instead of urea/H₃PO₃ show that a phosphorylated cotton with similar properties to the urea/H₃PO₃-modified fibres can be produced. Soaking of these fibres in saturated Ca(OH)₂ solution leads to cotton coated with thin layers of calcium phosphate formed by partial hydrolysis of the PO₄ functionalities in the phosphorylated cotton which are believed to act as nucleation layers for further calcium phosphate deposition when the fibres are subsequently soaked in 1.5×SBF solution. SEM/EDX studies of the calcium phosphate coatings formed on the Ca(OH)₂-treated urea-H₃PO₄ fibres as a function of soaking time in 1.5 × SBF show that coatings deposit and become noticeably thick after approximately 9 days. XPS studies indicated the presence of carbonate species in the calcium phosphate coating deposited. In common with the calcium phosphate coated Ca(OH)₂-treated urea/H₃PO₃-modified fibres studied earlier, the average EDX-measured Ca: P ratios of the coatings formed on the Ca(OH)₂-treated urea/H₃PO₄ fibres are 1.60 and give very similar micro-FTIR spectra with evidence of carbonate which suggests that amorphous calcium deficient apatite has deposited.     

Structural stability of calcium phosphate cement during aging in water

A calcium phosphate cement (CPC) has been prepared by mixing dicalcium phosphate anhydrate (DCPA, CaHPO₄) and calcium hydroxide (Ca(OH)₂) with a sodium phosphate (Na₂HPO₄) solution. After setting and hardening, the cement is aged in water. High resolution structural and microstructure analyses are carried out to evaluate the stability of the CPC in water over a period of 150 days. The lattice parameters of the apatite crystal remain the same throughout the aging process. The size of apatite crystallites is not changed either; nevertheless, the shape of the particles changes from equiaxed to rod-like.