Hydration of calcium sulfoaluminate cements — Experimental findings and thermodynamic modelling

Calcium sulfoaluminate cements (CSA) are a promising low-CO₂ alternative to ordinary Portland cements and are as well of interest concerning their use as binder for waste encapsulation. In this study, the hydration of two CSA cements has been investigated experimentally and by thermodynamic modelling between 1 h and 28 days at w/c ratios of 0.72 and 0.80, respectively.The main hydration product of CSA is ettringite, which precipitates together with amorphous Al(OH)₃ until the calcium sulfate is consumed after around 1-2 days of hydration. Afterwards, monosulfate is formed. In the presence of belite, strätlingite occurs as an additional hydration product. The pore solution analysis reveals that strätlingite can bind a part of the potassium ions, which are released by the clinker minerals. The microstructure of both cements is quite dense even after 16 h of hydration, with not much pore space available at a sample age of 28 days.The pore solution of both cements is dominated during the first hours of hydration by potassium, sodium, calcium, aluminium and sulfate; the pH is around 10-11. When the calcium sulfate is depleted, the sulfate concentration drops by a factor of 10. This increases pH to around 12.5-12.8.Based on the experimental data, a thermodynamic hydration model for CSA cements based on cement composition, hydration kinetics of clinker phases and calculations of thermodynamic equilibria by geochemical speciation has been established. The modelled phase development with ongoing hydration agrees well with the experimental findings.     

Calcium silicate/calcium aluminate composite biocement for bone restorative application: synthesis,characterisation and in vitro biocompatibility

Pure β-dicalcium silicate and monocalcium aluminate powder were prepared by Pechini method. A series of calcium silicate/calcium aluminate cements (CSC/CAC) were prepared. The setting time, crystalline phases, microstructures, compressive strength, cells attachment and silicon release of the cements were investigated. The results indicate that the setting time of CSC/CAC was shorter than that of either CSC or CAC. The hydration products in CSC/CAC composite are gehlenite (Ca₂Al₂SiO₇·8H₂O), calcium aluminate hydrate (Ca₃Al₂O_6?×?H₂O), and katoite (Ca₂Al₂O₆·6H₂O). Platelike crystals were found in the microstructure. The liquid to powder ratio has a significant effect on the porosity and the strength of CSC/CAC. The MC3T3 cells attached well to the surfaces of CSC/CAC. However, the cells proliferation on the surface of 7S3A was better than that of 3S7A due to its higher silicon release. In general, CSC/CAC exhibits good biocompatibility and relative high strength, and may be suitable for some non-load bearing bone restorative applications.     

A novel bi-phase Sr-doped magnesium phosphate/calcium silicate composite scaffold and its osteogenesis promoting effect

In order to improve the osteoconductivity and the osteoinductivity of bone tissue engineering scaffold, a novel bi-phase strontium-doped magnesium phosphate/calcium silicate (Sr-MP/CSC) composite scaffold was fabricated by the self-solidifying/particulate leaching method. The bi-phase composition of the well-crystallized struvite grains wrapped by the calcium silicate floccules was propitious to the deformability and toughness of composite scaffold, and the porous structure with interconnected macropores of 100–400?µm was beneficial to supporting the tissue growth and transporting nutrients and metabolites. When the Sr-MP/CSC composite scaffolds were degraded in the simulated physiological environment, the doped strontium could be sustainably released together with Ca²⁺, Mg²⁺, PO₄^3- and silidous ions. The proliferation and osteogenic differentiation of rat bone marrow stromal cells (BMSCs) on these composite scaffolds were obviously promoted. More valuably, the Sr-doped MP/CSC scaffolds exhibited the more obvious promotion to ALP activity, Col I and OCN expression than the un-doped MP/CSC scaffold, especially in the later stage. The results suggested that the strontium combined with calcium, magnesium and silicon could synergically promote osteogenesis, and the Sr-MP/CSC composite might be one of the promising bone tissue engineering scaffold materials.     

Investigation of calcium phosphate formation from calcium propionate and triethyl phosphate

Synthetic calcium phosphates are used in for example bone cements and implant coatings to increase biocompatibility. The common method to produce tricalcium phosphate (TCP) uses high temperatures, which creates large crystals with low specific surface areas. In order to investigate new methods to produce TCP at lower temperatures, the reaction between calcium propionate and triethyl phosphate conducted at 220 °C was studied. The method had a near 100% conversion rate, the main synthesis products were calcium phosphate and ethyl propionate. The formed calcium phosphate polymorph could be controlled depending on the water content of the precursor mixture. Anhydrous conditions created amorphous calcium phosphate. As the concentration of water increased, β-TCP was formed, followed by calcium deficient hydroxyapatite and monetite. The particle size increased with the water content, from 20 to 40 nm for amorphous calcium phosphate to tenths of micrometers for monetite. The specific surface areas varied between 209 m²/g for the amorphous product to 3.6 m²/g for the monetite product.     

Drug and ion releasing tetracalcium phosphate based dual action cement for regenerative treatment of infected bone defects

Calcium phosphate cements (CPCs) are ideally suited for the local delivery of antibiotics in infected bone defects as they have multiple binding sites for loading various drugs. CPCs can also be substituted with ions such as Ag⁺, Zn²⁺, Mg²⁺, Sr²⁺, etc., to exhibit extended broad-spectrum antimicrobial activity. Strontium (Sr) in particular is known to enhance the new bone formation and decrease bone resorption. The current work aims to develop a dual action tetracalcium phosphate (TTCP) based cement which releases both the Sr²⁺ ion and ornidazole antibiotic drug for the treatment of bone infections. The TTCP with Sr²⁺ ion substitution was prepared by the solid state reaction method and it was used to form ornidazole loaded CPC. The ornidazole loaded cement prepared using 8?at% Sr substituted TTCP (8SCPC-O) showed complete hydroxyapatite (HA) formation in phosphate buffered solution at the end of 1 week. Fine needle-shaped HA crystals were observed in 8SCPC-O cement. In vitro drug release studies showed an accelerated ornidazole release from the 8SCPC-O sample when compared to samples without Sr substitution. Ornidazole releasing cements were found to be biocompatible with skeletal myoblast (L6) cells. Antibacterial activity of ornidazole releasing cement was evident from day 1 onwards against E. coli. The above results suggest 8SCPC-O as a good candidate for treating local bone infections.     

Investigation of biocompatible nanosized materials for development of strong calcium phosphate bone cement: Comparison of nano-titania,nano-silicon carbide and amorphous nano-silica

The present paper deals with the effect of adding SiC, TiO₂ and SiO₂ nanoparticles on setting time, mechanical strength and hydraulic reactions of calcium phosphate cements (CPCs). The initial and final setting times of CPC increased by adding both nano-SiC and nano-TiO₂ additives but decreased by using nano-silica. Nano-titania and nano-silica had great effect on compressive strength of as-set CPC whereas slight changes were found by using nano-SiC. Although a sharp increase in compressive strength of all cements was observed by soaking them in physiological solution, the soaked additive-free cements and nano-SiO₂-added ones exhibited the greatest strength values. The results showed that adding these nano-additives did not influence on conversion rate of cement reactants to apatite phase during soaking in physiological solution period but the morphology of the formed phase was almost different. Overall, the results determined that nano-SiO₂ and nano-TiO₂ particles were appropriate additives to improve short-term mechanical strength of CPCs a(s-set CPCs), though nano-SiO₂ was found more effective because it improves the long-term mechanical strength of CPC (after soaking) too.     

Study on the new bone cement based on calcium sulfate and Mg,CO3 doped hydroxyapatite

In order to improve some features of bone substitutes the new self-setting composite-type implant material based on Mg²⁺/CO₃^2? co-substituted hydroxyapatite (Mg-CHA) and calcium sulfate hemihydrate (CSH) was developed. Synthetic hydroxyapatites doped with small amounts of additives found in natural bone (e.g. Mg²⁺ and CO₃^2?) are regarded as promising components of calcium phosphate bone cements (CPCs). The CPCs, now available on the market, due to low resorption rate are too stable to permit material degradation and are slowly replaced by the newly formed bone. To improve cement resorption we used calcium sulfate which is a well-known biodegradable and biocompatible bone defect filler. Combining properties of Mg-CHA and CSH allowed developing a new, promising, easy shapeable implant material with high potential for bone regeneration.     

Effects of strontium doping on the degradation and Sr ion release behaviors of α‐tricalcium phosphate bone cement

Strontium plays important physicochemical and biological roles in the applications of bone repair materials. The available methods of Sr doping in bone cements were believed to make a key effect on the biodegradation and Sr ion release behaviors of cements. In this work, Sr‐doped octacalcium phosphate (Sr‐OCP), Sr‐doped α‐tricalcium phosphate (Sr‐α‐TCP), SrCO₃, and SrCl₂ with different actual availability of Sr²⁺ were imported into α‐TCP bone cements, and their effects on the biodegradation and ions release of cements were comparatively investigated. Incorporation of different Sr carriers had led to distinct hydration morphologies, crystal evolutions, degradation rates, and microenvironments of bone cements during their in vitro biodegradation. Compared with other Sr carriers, Sr‐OCP facilitated the hydration reaction of α‐TCP, which induced the enhanced degradation and Sr ion release behaviors. In conclusion, Sr‐OCP was supposed to be a more potential Sr carrier applied in the synthesis of biodegradable Sr‐doped calcium phosphate bone cements.     

纳米二氧化硅/磷酸钙复合骨水泥的力学强度和水化过程

在磷酸钙骨水泥（CPC）中添加纳米二氧化硅（nSiO2）获得了nSiO2/CPC复合骨水泥。利用维卡仪、万能压力试验机和热导式等温量热仪研究了nSiO2的添加量对nSiO2/CPC复合骨水泥的凝结时间、抗压强度和水化行为的影响,利用X射线衍射和扫描电子显微镜等技术研究添加的nSiO2对nSiO2/CPC复合骨水泥固化产物的相组成和断面形貌的影响。研究结果表明：nSiO2添加量为5%的nSiO2/CPC复合骨水泥凝结时间由16 min缩短到10 min,抗压强度由原来的（24 2）MPa增加到（33 4）MPa,提高了38%,但添加超过5%的nSiO2会影响水化产物磷灰石的生成从而使复合骨水泥的力学强度下降。在水化反应过程中,一方面nSiO2作为填料,吸附了固化液中的水导致CPC的实际固液比增大,减弱了CPC的水化进程,由于实际固化液下降也减少了水化产物的孔隙大小和数目;另一方面,nSiO2与水化产生的Ca（OH）2发生化学反应形成CSH凝胶,改善了nSiO2和CPC基体之间的界面结合。这两方面的作用结果使得添加适量的nSiO2可以提高nSiO2/CPC复合骨水泥的抗压强度,缩短其凝结时间。     

Direct transformation of calcium sulfite to α-calcium sulfate hemihydrate in a concentrated Ca-Mg-Mn chloride solution under atmospheric pressure

Massive quantities of sulfite-rich flue gas desulfurization (FGD) scrubber sludge have been generated by coal burning power plants. Utilization of the sulfite-rich sludge for preparing α-calcium sulfate hemihydrate (α-HH), an important kind of cementitious material, is of particular interest to electric utilities and environmental preservation. In the experiment, calcium sulfite hemihydrate was directly transformed to α-HH without the occurrence of calcium sulfate dihydrate (DH). The transformation was performed in a concentrated CaCl2 solution containing Mg2+ and Mn²⁺ at 95 °C, atmospheric pressure and low pH. The oxidation of calcium sulfite and the subsequent crystallization of α-HH constitute the whole conversion, during which the oxidation turns out to be the rate controlling step. Solid solution comprised of calcium sulfite hemihydrate and calcium sulfate was found to coexist with α-HH in the suspension. Calcium sulfate increases and calcium sulfite decreases spontaneously until the solid solution disappears. Thus, it is a potential alternative to utilize sulfite-rich FGD scrubber sludge for the direct preparation of α-HH.     

Influence of magnesium and silver ions on rheological properties of hydroxyapatite/chitosan/calcium sulphate based bone cements

Rheology of bioceramic bone cements is usually described as properties of ceramic slurries, neglecting the self-setting character of these materials. In our studies calcium sulphate based bone cements with Ag⁺, Mg²⁺ and Mg²⁺/CO₃^2- modified hydroxyapatite were investigated. Despite of expectations, it has been proven that the presence of magnesium ions significantly influence the rheological properties of cement pastes. Changes in rheological properties were connected with (I) chemical interactions between Mg²⁺ and sulphate ions (II) chemical interaction between Mg²⁺ and chitosan. These effects were not observed for silver additive. Most of the developed calcium sulphate based pastes, except material containing MgHA and chitosan, have been categorized as thick pastes applicable with the spatula. It has been found that the chitosan present around and at the calcium sulphate grains acted as a lubricant and prolong the period of quasi-constant viscosity of the pastes.     

Setting behavior,mechanical property and biocompatibility of anti-washout wollastonite/calcium phosphate composite cement

In this study, a calcium phosphate-based composite cement was fabricated by incorporating wollastonite (WS) into calcium phosphate cement (CPC). The setting behavior, microstructure, injectability, porosity, compressive strength, anti-washout property, in vitro degradation, and cell behavior of the WS/CPC composite cement were systematically investigated. The results revealed that the addition of WS promoted the hydration reaction but without affecting the hydration product of CPC. The injectability of the WS/CPC composite cement declined with the incorporation of WS to a certain extent, especially when the content of WS was higher than 20 wt%. By incorporating appropriate amount of WS into CPC, the composite cement obtained feasible setting time, enhanced compressive strength, improved anti-washout performance, and favorable biocompatibility. On the basis of its improved comprehensive application-relevant properties, the WS/CPC composite cement is prospective to be a promising biomaterial for bone defect repairing.     

Scaling analysis of nanofiltration systems fed with saturated calcium sulfate solutions in the presence of carbonate ions

Nanofiltration of saturated calcium sulfate solution of 0.02 mol/L calcium content, with molar ratio ranging from 0.0500 to 2.8 × 10⁻³ was carried out in 6.5 × 10⁻⁴ m² active membrane area laboratory module at 1.2 × 10⁶ Pa transmembrane pressure using the total retentate recycle mode. Permeate mass flow and retentate calcium concentration vs. time and concentration factor (CF) curves allowed identification of calcium sulfate crystallization mechanisms. Though both bulk and surface crystallization mechanisms were identified, they were, however, strongly affected by water quality. A non-fouling CF range up to 2, which is probably due to the existence of metastable supersaturated CaSO₄ solution, was also observed in the case of molar ratio equal to 2.8 × 10^−3. Inorganic scales at the end of each experiment were removed from a NF module, dried at room temperature during 24 h and then examined using the X-ray diffraction method. Gypsum and aragonite were identified as the most common calcium sulfate and calcium carbonate precipitates, respectively. A mixed salt Ca₂(CO₃)(SO₄)·4H₂O (so-called rapidcreekite) was also identified as a result of carbonate and sulfate co-precipitation under low carbonate content conditions.     

β—TCP／MCPM体系磷酸钙骨水泥的可注射性研究

本文以β-磷酸三钙和一水合磷酸二氢钙作为磷酸钙骨水泥的固相粉料。以焦磷酸钠溶液作为液相,制备出具有良好固化性能和力学性能并可快速固化的可注射型磷酸钙骨水泥,并考察了该体系液固比、原料粒径及添加剂等制备条件对其注射性能及固化性能、力学性能的影响。实验发现适当提高液固比,增大粒径尺寸以及添加低含量硫酸钙有利于增加磷酸钙骨水泥材料的可注射性能。关键词：磷酸钙骨水泥：注射性能;β-磷酸三钙     

Soluble phosphate salts as setting aids for premixed calcium phosphate bone cement pastes

Recently, premixed calcium phosphate cement pastes have been proposed as biomaterials for bone tissue repair and regeneration. Use of premixed pastes saves the time and removes an extra step during a medical operation. α-Tricalcium phosphate (α-TCP) based cements set to form calcium deficient hydroxyapatite which has a moderate bioresorbtion speed. α-TCP cements require a setting aid, usually a sodium or potassium phosphate salt, to speed up the setting process. Within the current research we investigated which setting aid has significant advantage, if α-TCP is used in form of non-aqueous premixed paste. This approach offers the application of simple ingredients to produce a premixed calcium phosphate cement. The following properties of cement formulations were evaluated: cohesion, phase composition, microstructure, pH value of the liquid surrounding the cement, and compressive strength.Compositions using mixture of basic and acidic potassium phosphate salts (KH₂PO₄ and K₂HPO₄) in sufficient amounts give the best overall results (adequate cohesion and pH of the surrounding liquid, hydrolysis of starting materials within 48 h, and compressive strength of 12 ± 3 MPa). Cement prepared with basic sodium phosphate salt (Na₂HPO₄) as setting aid had considerably higher compressive strength 22 ± 1 MPa, but the pH of the surrounding liquid was basic (9.0).     

Multiphase zinc and magnesium mono-substituted calcium phosphates derived from cuttlefish bone: A multifunctional biomaterials

Biomimetic calcium phosphate (CaP) systems mono-substituted with zinc (Zn²⁺) and magnesium (Mg²⁺) ions were prepared from a biogenic source (cuttlefish bone) by wet precipitation method. The results revealed that the as-prepared powders were composed of calcium-deficient carbonated hydroxyapatite (HAp), octacalcium phosphate (OCP), and amorphous calcium phosphate (ACP), while the heat-treated powders consisted of HAp, α-tricalcium phosphate (α-TCP), and β-tricalcium phosphate (β-TCP). In addition to Zn²⁺ and Mg²⁺ ions, the presence of CO₃^2?, Sr²⁺ and Na ⁺ ions was detected with elemental analysis, which can be attributed to the use of cuttlefish bone as a natural precursor of Ca²⁺ ions. The data obtained by XRD study demonstrated the decrease in lattice parameters in the OCP and β-TCP phases for Zn-substitution and Mg-substitution in the HAp, OCP, and β-TCP phases. Zn²⁺ occupies the Ca(1,3,4,6,7,8) sites in OCP and Ca(1,2,3,4) sites in β-TCP, while Mg²⁺ occupies the Ca(2) sites in HAp and the Ca(4,5) sites in β-TCP. Phase transformation study under simulated physiological conditions for 7 days showed the transformation of OCP and ACP into the thermodynamically more stable HAp. Characterization of the zeta-potential showed positively charged populations for all prepared CaP powders, while all samples showed high bovine serum albumin adsorption capacity. The culture of human embryonic kidney cells showed that the prepared CaPs are non-cytotoxic and that viability of the cells increases during the culture period. All powders obtained showed antibacterial activity towards Gram-negative Escherichia coli and low antibacterial effect against Gram-positive Staphylococcus aureus, as determined by viability analysis during 48 h. Inhibition zone analysis and observation of the morphology after 24 h showed no antibacterial properties.     

In-vitro formation and growth kinetics of apatite on a new light-cured composite calcium phosphate cement

Calcium phosphate cements are used as synthetic bone grafts with several advantages such as biocompatibility, osteoconductivity, and moldability. In this study, the synthesis of a biocement starting from calcium hydroxide (Ca(OH)₂) and Monocalcium Phosphate Monohydrate (MCPM) was investigated. A 6?wt% Na₂HPO₄ aqueous solution along with a modified polymeric resin (RIVA(SDI)^®) were adopted as the variable liquid phase in self- and light-cure cement groups. XRD analysis and FTIR spectroscopy were used to study the phase composition. The composite microstructure was characterized by scanning electron microscopy (SEM) and the degradation rates were measured by atomic absorption spectroscopy (AAS) analysis. In addition, the effect of soaking time of the cement in simulated body fluid (SBF) on the final phase and morphology was studied. The results showed that soaking the composite in SBF has a significant influence in phase transformation into hydroxyapatite, but following a slower kinetic in light-cured composite cements. Evidences of crosslinking reactions in light-cured cements were observable, which at the same time can legitimize slower apatite formation and faster biodegradation of these composite cements.     

Inorganic cements for biomedical application: calcium phosphate,calcium sulphate and calcium silicate

Inorganic cements have found utility in tissue replacement since the late nineteenth century, one of the first examples being calcium sulphates in the augmentation of bone defects. In the intervening period of time countless formulations of calcium phosphate, sulphate and silicate cement have been researched and as a result, many are now commercially available for a variety of biomedical applications. This review summarises the applications, formulations, advantages and drawbacks of such inorganic cements, suggesting future work that will drive progress in this area into the future of biomaterials research.     

The ionic substituted octacalcium phosphate for biomedical applications: A new pathway to follow?

Numerous studies have found that octacalcium phosphate possesses promising biological properties applicable to bone tissue regeneration. To further improve the osteogenic and regenerative properties of octacalcium phosphate, substitutions with Sr²⁺, Zn²⁺, Mg²⁺, Fe³⁺, Na⁺, F^? and CO₃^2? ions have been investigated in recent years. Despite that, hydroxyapatite is still considered the most promising calcium phosphate for bioactive bone grafts due to its biocompatibility, physicochemical similarity to biological apatite, osteoconductivity and strong bonding with the surrounding tissue. However, better biological properties of octacalcium phosphate in vivo as well as a larger volume of regenerated bone tissue, compared to hydroxyapatite, were confirmed by many studies. This review summarizes recent and relevant studies on cationic and anionic substitutions in the crystal lattice of octacalcium phosphate and its in vitro biological performance. It also discusses future challenges and prospects for the use of substituted octacalacium phosphate.     

Studies on impurity ion and radiofrequency electric field effects on calcium carbonate deposition

The influences of impurity ions (Mg²⁺, Zn²⁺, Fe²⁺, Fe³⁺, and Al³⁺ ) and radiofrequency (44 MHz) electric field (RF) on calcium carbonate deposition in glass capillaries were studied. Calcium carbonate was precipitated from 0.1 M CaCl₂ and Na₂CO₃ solutions (100+ 100 ml) just before the inlet to the capillary. It was found that except for Al ³⁺ ions at 0.001 M concentration, the rest of the ions studied decreased CaC0₃ deposition. A minor influence of the RF field on the deposition was found in some of the systems tested, but it was in the range of the standard deviation of the results. An increase in deposition was observed in systems without impurity ions when equimolar volumes (500 + 500 ml) of 0.0025 M solutions of CaCl₂ and Na₂CO₃ were used for the experiment.