Composition and properties of silver-containing calcium carbonate–calcium phosphate bone cement

The introduction of silver, either in the liquid phase (as silver nitrate solution: Ag(L)) or in the solid phase (as silver phosphate salt: Ag(S)) of calcium carbonate–calcium phosphate (CaCO₃–CaP) bone cement, its influence on the composition of the set cement (C-Ag(L) and C-Ag(S) cements with a Ca/Ag atomic ratio equal to 10.3) and its biological properties were investigated. The fine characterisation of the chemical setting of silver-doped and reference cements was performed using FTIR spectroscopy. We showed that the formation of apatite was enhanced from the first hours of maturation of C-Ag(L) cement in comparison with the reference cement, whereas a longer period of maturation (about 10 h) was required to observe this increase for C-Ag(S) cement, although in both cases, silver was present in the set cements mainly as silver phosphate. The role of silver nitrate on the setting chemical reaction is discussed and a chemical scheme is proposed. Antibacterial activity tests (S. aureus and S. epidermidis) and in vitro cytotoxicity tests (human bone marrow stromal cells (HBMSC)) showed that silver-loaded CaCO₃–CaP cements had antibacterial properties (anti-adhesion and anti-biofilm formation) without a toxic effect on HBMSC cells, making C-Ag(S) cement a promising candidate for the prevention of bone implant-associated infections.     

In vitro studies of calcium phosphate silicate bone cements

A novel calcium phosphate silicate bone cement (CPSC) was synthesized in a process, in which nanocomposite forms in situ between calcium silicate hydrate (C–S–H) gel and hydroxyapatite (HAP). The cement powder consists of tricalcium silicate (C₃S) and calcium phosphate monobasic (CPM). During cement setting, C₃S hydrates to produce C–S–H and calcium hydroxide (CH); CPM reacts with the CH to precipitate HAP in situ within C–S–H. This process, largely removing CH from the set cement, enhances its biocompatibility and bioactivity. The testing results of cell culture confirmed that the biocompatibility of CPSC was improved as compared to pure C₃S. The results of XRD and SEM characterizations showed that CPSC paste induced formation of HAP layer after immersion in simulated body fluid for 7 days, suggesting that CPSC was bioactive in vitro. CPSC cement, which has good biocompatibility and low/no cytotoxicity, could be a promising candidate as biomedical cement.     

Preparation of an ultra fast binding cement from calcium silicate-based mixed oxide nanoparticles

Building construction takes time, in part because the binding process of cement is based on the slow re-crystallization and precipitation of calcium silicate species. Since the material's reactivity is surface area limited, a reduction in particle size of Portland cements has been used to prepare faster binding formulations. The present work investigates a new and direct, one-step preparation of calcium silicate-based nanoparticles of a typical Portland cement composition by flame spray synthesis. Isothermal calorimetry revealed that the hardening of this new nano-cement corroborated a more than tenfold increase of initial reactivity with different reaction kinetics if compared to conventionally prepared cements. At present, the unfavourably high porosity of nano-cements, however, underlines the need for additional improvements of chemical composition and formulation to make these highly reactive materials applicable to modern construction work, where load-bearing strength is of importance.     

Ultra-fine cements for special applications

Ultra-fine cements produced on the basis of Portland cement are much finer than ordinary cement and have a steep grading curve. They are specially suited for rock and soil injection but nowadays also for filling and sealing of cracks in concrete.     

硅酸二钙及以其为主要矿物的低钙水泥的研究进展

硅酸二钙是一种钙含量较低的硅酸盐胶凝矿物,与传统高钙硅酸盐矿物硅酸三钙相比,其具有石灰石消耗少、生成能耗低、水化热低、水化产物中氢氧化钙含量少等显著特点,发展以硅酸二钙为主要矿物的低钙水泥符合节能减排和可持续发展的要求,是水泥工业发展的重要方向。对硅酸二钙的结构、晶型转变及水化活性的研究情况进行了综合评述,并对以硅酸二钙为主要矿物的低钙水泥的组成、性能及存在的主要问题进行了介绍。在此基础上,指出了以硅酸二钙为主要矿物的低钙水泥研发的技术方向。     

Stabilization/solidification of hazardous and radioactive wastes with alkali-activated cements

This paper reviews progresses on the use of alkali-activated cements for stabilization/solidification of hazardous and radioactive wastes. Alkali-activated cements consist of an alkaline activator and cementing components, such as blast furnace slag, coal fly ash, phosphorus slag, steel slag, metakaolin, etc., or a combination of two or more of them. Properly designed alkali-activated cements can exhibit both higher early and later strengths than conventional portland cement. The main hydration product of alkali-activated cements is calcium silicate hydrate (CSH) with low Ca/Si ratios or aluminosilicate gel at room temperature; CSH, tobmorite, xonotlite and/or zeolites under hydrothermal condition, no metastable crystalline compounds such as Ca(OH)(2) and calcium sulphoaluminates exist. Alkali-activated cements also exhibit excellent resistance to corrosive environments. The leachability of contaminants from alkali-activated cement stabilized hazardous and radioactive wastes is lower than that from hardened portland cement stabilized wastes. From all these aspects, it is concluded that alkali-activated cements are better matrix for solidification/stabilization of hazardous and radioactive wastes than Portland cement.     

Evaluation of colloidal silica suspension as efficient additive for improving physicochemical and in vitro biological properties of calcium sulfate-based nanocomposite bone cement

In the present study new calcium sulfate-based nanocomposite bone cement with improved physicochemical and biological properties was developed. The powder component of the cement consists of 60 wt% α-calcium sulfate hemihydrate and 40 wt% biomimetically synthesized apatite, while the liquid component consists of an aqueous colloidal silica suspension (20 wt%). In this study, the above mentioned powder phase was mixed with distilled water to prepare a calcium sulfate/nanoapatite composite without any additive. Structural properties, setting time, compressive strength, in vitro bioactivity and cellular properties of the cements were investigated by appropriate techniques. From X-ray diffractometer analysis, except gypsum and apatite, no further phases were found in both silica-containing and silica-free cements. The results showed that both setting time and compressive strength of the calcium sulfate/nanoapatite cement improved by using colloidal silica suspension as cement liquid. Meanwhile, the condensed phase produced from the polymerization process of colloidal silica filled the micropores of the microstructure and covered rodlike gypsum crystals and thus controlled cement disintegration in simulated body fluid. Additionally, formation of apatite layer was favored on the surfaces of the new cement while no apatite precipitation was observed for the cement prepared by distilled water. In this study, it was also revealed that the number of viable osteosarcoma cells cultured with extracts of both cements were comparable, while silica-containing cement increased alkaline phosphatase activity of the cells. These results suggest that the developed cement may be a suitable bone filling material after well passing of the corresponding in vivo tests.     

Influence of EDTA on Demineralization Rate of Dentine： Calcification Treatment in Root Canal Therapy

The aim of this study was to investigate the influence of ethylenediaminetetraacetic acid （EDTA） irrigation on demineralization rate of dentine located in the apical third of root canal walls. Teeth were divided into A and B two groups. In group A, all of the teeth was irrigated with EDTA and NaOCI （sodium hypochlorite）, followed by cutting the apical third into slices longitudinally to examine the influence of EDTA on different portions of apical third of root canal. In group B, the apical third of a tooth was firstly cut into slices longitudinally, followed by coating the root canal walls with EDTA to in-situ observe the demineralization of dentine with different time. It was found that the influence of EDTA on root-canal was gradually increased from the apical to the upper end of the apical third for group A. In addition, the demineralization rate of dentine was remarkable in the first 25 min for group B. The diffusion of EDTA into root dentine would lead to potential damage to the dentine. Furthermore, demineralization rate curve was calculated.     

Effects of tobermorite and calcium silicate hydrate (I) crystals formed within polymer concretes

In the development of hydrothermally stable vinyl-type polymer concretes (PC), the effect of calcium silicate hydrates produced by the hydrothermal reaction of an anhydrous cement-silica flour system used as a filler in the PC was determined. Results from measurements of the mechanical properties of PC specimens after exposure to a 25% brine solution at a temperature of 240° C for 10, 30 and 90 days were used to quantify these effects. In addition, X-ray diffraction, differential thermal analysis, and scanning electron microscopy were used to perform quantitiative and morphological analyses of hydrated calcium silicate compounds synthesized during exposure to hot brine of PC samples containing cements having molar ratios of CaO/SiO₂ of 1.33, 0.99 and 0.54. The data indicated that 11.3 Å tobermorite and calcium silicate hydrate-(I)-type [C-S-H(I)] crystals produced from the hydrothermal reactions of cement having the lowest CaO/SiO₂ ratio of 0.54 significantly affect the long-term hydrothermal stability of the composite. Exposure for approximately 30 days to a 240° C hydrothermal environment was required for synthesis of a highly crystalline tobermorite to occur on the amorphous polymer surfaces. Morphological examination of tobermorite revealed circular radiating crystals of diameter about 20m.     

Injectability evaluation of tricalcium phosphate bone cement

Calcium phosphate cements are biomaterials made from a mixture of calcium phosphate powder in aqueous solutions that forms a paste that reacts at the body temperature and hardens as a result of precipitation reactions. These cements are commonly used in dentistry and orthopedic bone filling surgeries, which require extremely invasive procedures. The challenge consists in formulating an injectable paste by additives incorporation. In this work, three different additives (carboxymethylcellulose, agar polymer and sodium alginate) were incorporated to tricalcium phosphate, in concentrations of 0.4, 0.8, 1.6, 3.2 and 6.4 wt.%. Injectability was evaluated through a new method developed for this purpose. Results showed that it was possible to obtain injectable compositions of α-tricalcium phosphate cement. It was verified that the injectability depends on the rheological behavior of the pastes and injection time. In this study, pastes with viscosity suitable for good homogenization and injection were obtained.     

Surface treatmet of sillimanite-based microspheres for strength enhancement of high-temperature lightweight cement composites

The compressive strength of high-temperature lightweight cementing materials containing sillimanite-based hollow microspheres as a filler can be improved by treating the surfaces of the microspheres with a calcium hydroxide-saturated solution at temperatures up to 200° C. The precipitation of an epitaxial layer formed by an interaction between the hot calcium hydroxide solution end the surface of the sphere played an essential role in developing favourable bonding characteristics at the interfaces and in promoting the hydration of the cement matrix. Bonding is associated with the formation of an intermediate layer of aluminium-rich calcium silicate hydrate, produced by an interfacial reaction of the cement paste with the epitaxy under the hydrothermal environment at 300° C. A dense intermediate layer consisting of a rim structure of 4m thickness acts in cross-linking and coupling functions that serve to connect the cement matrix and spheres, thereby improving the interfacial bond strength. The presence of the epitaxial layer on the treated sphere surfaces leads to the formation of a well-crystallized tobermorite matrix phase, which is responsible for the development of strength in autoclaved lightweight cements.     

Effective formulations for the preparation of calcium phosphate bone cements

In the system CaO–P₂O₅–H₂O 13 different solids with varying Ca/P ratios are known. In addition calcium phosphates containing other biocompatible constituents like Na, or K, or Mg or Cl or carbonate, are known. Therefore, a large number of combinations of such compounds is possible which might result in the formation of calcium phosphate cements upon mixing with water. However, the number of calcium phosphates possibly formed by precipitation at room or body temperatures is limited to 12, which should limit the number of suitable combinations. In this study more than 450 different combinations of reactants have been investigated. The results were evaluated on the basis of the following criteria: (a) was the intended reaction product formed? (b) was the final setting time shorter than 60 min? (c) was the compressive strength after soaking for 1 day in Ringer's solution at 37°C higher than 2 MPa? We found that 15 formulations satisfied all of these criteria. The distribution of cements synthesized in this way was 3 DCPD type, 3 CMP type, 6 OCP type and 3 CDHA type cements. The DCPD type cements were acidic during setting and remained that for a long time afterwards. CDHA type cements were neutral or basic during setting, and remained neutral after completion of the reaction. The OCP type cements were neutral both during and after setting. Two CMP type cements were basic both during and after setting. In this study compressive strengths were found up to 90 MPa. Also, in the literature values up to 90 MPa have been reported for this type of cement. Taking into account the excellent biocompatibility and the good osteoconductivity of calcium phosphates and the fact that these calcium phosphate cements can be injected into the site of operation, it may be expected that these materials will become the materials of choice for bone replacement and augmentation. Their suitability for the fixation of metal endoprostheses for joint replacement should be investigated as well.     

Apatite bone cement reinforced with calcium silicate fibers

Several research efforts have been made in the attempt to reinforce calcium phosphate cements (CPCs) with polymeric and carbon fibers. Due to their low compatibility with the cement matrix, results were not satisfactory. In this context, calcium silicate fibers (CaSiO₃) may be an alternative material to overcome the main drawback of reinforced CPCs since, despite of their good mechanical properties, they may interact chemically with the CPC matrix. In this work CaSiO₃ fibers, with aspect ratio of 9.6, were synthesized by a reactive molten salt synthesis and used as reinforcement in apatite cement. 5 wt.% of reinforcement addition has increased the compressive strength of the CPC by 250 % (from 14.5 to 50.4 MPa) without preventing the cement to set. Ca and Si release in samples containing fibers could be explained by CaSiO₃ partial hydrolysis which leads to a quick increase in Ca concentration and in silica gel precipitation. The latter may be responsible for apatite precipitation in needle like form during cement setting reaction. The material developed presents potential properties to be employed in bone repair treatment.     

Mechanical property and in vitro biocompatibility of brushite cement modified by polyethylene glycol

Brushite (dicalcium phosphate dihydrate, DCPD) cement, owing to its high solubility in physiological condition and ability to guide new bone formation, is widely used to treat bone defects. In the present study, we have evaluated the effects of poly ethylene glycol (PEG) addition on the setting time, compressive strength and in vitro biocompatibility of brushite cement. The brushite cements were prepared by mixing β-tricalcium phosphate [β-TCP, Ca₃(PO₄)₂] and monocalcium phosphate monohydrate [MCPM, Ca(H₂PO₄)₂ ? H₂O]. PEG was introduced at 2.0 and 5.0 wt% with the liquid. Introduction of PEG resulted in marginal increase in both initial and final setting time, however, significantly affected the compressive strength. Effects of PEG incorporation on in vitro biocompatibility of brushite cements were studied by using human fetal osteoblast cells (hFOB) cells. Field emission scanning electron microscope (FESEM) images and immunohistochemical analysis indicated that pure and PEG incorporated brushite cement facilitates cell adhesion, proliferation and differentiation. Fewer cells expressed vinculin protein with increased PEG content in the cement. Cell proliferation was found to decrease with increased PEG concentration while the cell differentiation increased with PEG content. Our results provide a better understanding of in vitro biocompatibility of PEG added brushite cements that can be used to customize the cement compositions based on application need.     

Hydraulic cement-type fillers for hydrothermally stable polymer concretes

Hydraulic cement fillers, which can prevent the hydrothermal decomposition of vinyl-type polymer concrete (PC) containing carboxylate groups in the polymer molecule, have been developed. Experiments were performed in which a co-polymer was used in combination with an aggregate containing various commercial cements such as Portland type I, III, and V cements, slag cement (IS), and a Class H oil-well cement. It was observed for Class H cement-filled PC that a strong interaction occurred between the cement grains and the CH₂ groups in the main chain of the co-polymer. The other cements exhibited lesser interactions. It was observed that after exposure to a 25% brine at 240° C, ionic bonding of carboxylate anions in the co-polymer with Ca²⁺ ions from the Class H cement and the hydration products of Class H cement had a significant effect on the strength. This improvement is believed to result from a reductive effect on the molecular mobility of the co-polymer chains. Fillers having a-dicalcium silicate (C₂S)/Class H cement ratio from 0.7 to 1.5 made the greatest contribution to the hydrothermal stability of the vinyl-type PCs.     

骨损伤修复用硫酸钙及其无机复合材料的研究进展

创伤、骨肿瘤、关节置换术等引起骨缺损的修复是目前临床治疗的难点和研究热点领域, 寻找理想的骨修复材料已经成为该领域的重点研究方向。硫酸钙骨水泥作为骨修复材料已有百余年历史, 有着显著的优势。但其降解过快的缺点影响了治疗效果, 限制了应用范围。本文对硫酸钙的理化特性、晶粒形貌与晶型控制、合成方法等进行了系统介绍, 总结了硫酸钙与羟基磷灰石、生物玻璃、磷酸钙和硅酸钙复合材料及其性能研究的新成果, 并提出了克服硫酸钙作为骨修复材料的缺点的若干方法。     

Cements from nanocrystalline hydroxyapatite

Calcium phosphate cements are used as bone substitute materials because they may be moulded to fill a void or defect in bone and are osteoconductive. Although apatite cements are stronger than brushite cements, they are potentially less resorbable in vivo. Brushite cements are three-component systems whereby phosphate ions and water react with a soluble calcium phosphate to form brushite (CaHPO4 x 2H2O). Previously reported brushite cement formulations set following the mixture of a calcium phosphate, such as beta-tricalcium phosphate (beta-TCP), with an acidic component such as H3PO4 or monocalcium phosphate monohydrate (MCPM). Due to its low solubility, hydroxyapatite (HA) is yet to be reported as a reactive component in calcium phosphate cement systems. Here we report a new cement system setting to form a matrix consisting predominantly of brushite following the mixture of phosphoric acid with nanocrystalline HA. As a result of the relative ease with which ionic substitutions may be made in apatite this route may offer a novel way to control cement composition or setting characteristics. Since kinetic solubility is dependent on particle size and precipitation temperature is known to affect precipitated HA crystal size, the phase composition and mechanical properties of cements made from HA precipitated at temperatures between 4 and 60 degrees C were investigated.     

Calcium phosphate cement scaffolds with PLGA fibers

The use of calcium phosphate-based biomaterials has revolutionized current orthopedics and dentistry in repairing damaged parts of the skeletal system. Among those biomaterials, the cement made of hydraulic grip calcium phosphate has attracted great interest due to its biocompatibility and hardening “in situ”. However, these cements have low mechanical strength compared with the bones of the human body. In the present work, we have studied the attainment of calcium phosphate cement powders and their addition to poly (co-glycolide) (PLGA) fibers to increase mechanical properties of those cements. We have used a new method that obtains fibers by dripping different reagents. PLGA fibers were frozen after lyophilized. With this new method, which was patented, it was possible to obtain fibers and reinforcing matrix which furthered the increase of mechanical properties, thus allowing the attainment of more resistant materials. The obtained materials were used in the construction of composites and scaffolds for tissue growth, keeping a higher mechanical integrity.     

The nature of the hydration products in hardened cement pastes

An understanding of the performance of portland cement-based materials requires knowledge at the microstructural level. Developments in the instrumentation of several techniques have led to improved understanding of the composition, morphology, and spatial distribution of the various products of cement hydration. In particular, our understanding of the nature of the nearly amorphous calcium silicate hydrate (C–S–H) phases – which are the principal binding phases in all portland cement-based systems – has been advanced by developments in solid-state NMR spectroscopy and analytical TEM. This paper presents an overview of the nature of the hydration products formed in hardened portland cement-based systems. It starts with the most straightforward cementitious calcium silicate systems, C₃S and β-C₂S, and then considers ordinary portland cement and blends of portland cement with silica fume, ground granulated iron blast-furnace slag, and finally alkali hydroxide-activated slag cements.     

Effect of carboxylic and hydroxycarboxylic acids on cement hydration: experimental and molecular modeling study

Most concrete produced includes chemical admixtures such as air entrainers, set modifiers, water reducers, etc., many of which include organic molecules. Hydroxycarboxylic acids, in particular, retard portland cement hydration. The interaction of such acids with hydrating cement phases is a complex, multi-parameter problem. To elucidate the interaction of hydroxycarboxylic and carboxylic acid retarders on hydration of cement, a combined experimental and molecular-computational approach was used. Glycolic acid, acetic acid, calcium glycolate and calcium acetate were used as model compounds. Molecular dynamics simulations were performed to simulate the interactions of select test compounds with the (001) surface of the portlandite crystal (calcium hydroxide) and the (040) surface of the tricalcium silicate crystal. Hydrogen bond density profiles and binding energies were evaluated. The adsorption isotherm for chelate complexes was determined experimentally by equilibrating aqueous solutions of the agents in the presence of various amounts of solid-phase calcium hydroxide. Finally, isothermal calorimetry experiments were used to quantify effects on hydration rate. The glycolic acid shows significant cement retardation, whereas acetic acid does not retard. Glycolic acid was found to retard hydration via calcium chelation and surface adsorption that involves the adsorption of the calcium chelate complex preferentially on tricalcium silicate. Simulation results reveal that calcium glycolate forms a strong hydrogen bonding network near to calcium hydroxide and hydrated tricalcium silicate surfaces and are responsible for its strong adsorption on these surfaces. While acetic acid forms a strong calcium chelate, it does not associate with calcium hydroxide or unhydrated or hydrated tricalcium silicate surfaces.