Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

Characterization of the paste-aggregate interfacial transition zone surface roughness and its relationship to the fracture toughness of concrete

Notched concrete beams containing varying amounts of pea gravel aggregate were tested under three-point bend, and their fracture toughness determined. The roughness of the region near the interface between the cement paste and the aggregate was evaluated by digitizing images from a confocal tandem scanning microscope. The average roughness of the paste was found to be related to the fracture parametersK _IC (critical stress intensity factor) and a _c (critical crack extension), as determined by the two-parameter fracture model. The roughness in the proximity of the paste-aggregate interface was generally higher than that of the paste far from the aggregate, and it decreased with the distance from the aggregate. This study indicates that aggregate particles increase the toughness of the cement paste portion of concrete, and that this is an important mechanism for toughening concrete.     

Influence of limestone content,fineness, and composition on the properties and microstructure of alkali-activated slag cement

The influence of the fineness, concentration, and chemico-mineralogical composition of limestone on the workability, reaction kinetics, compressive strength, microstructure, and binder gel characteristics of sodium carbonate–based waste-activated waste slag cement pastes was investigated in this work. Alkali-activated slag cements incorporated with limestone, containing 33–100% of calcite, at a content of up to 60% with a 28-day compressive strength of 26.2–48.8 MPa were proposed. The main reaction products of hardened alkali-activated cement pastes and those incorporated with limestone are CSH, CaCO₃, Na₂Ca(CO₃)₂·5H₂O, and Na₂CaSiO₄. “Physically active” limestone does not chemically react with the binder gel but it can improve the physical structure. The higher packing density of mixed cement, without an increase in the water demand, the satisfactory binding strength of limestone with the binder gel lead to the improvement in the physical structure and compressive strength of alkali-activated slag paste.     

Composition effects on the pH of a hydraulic calcium phosphate cement

The pH of a hydraulic calcium phosphate cement (HCPC) made of monocalcium phosphate monohydrate (Ca(H2PO4)2·H2O; MCPM), -tricalcium phosphate (-(Ca3(PO4)2; -TCP) and water was measured as a function of reaction time and composition at room temperature. During setting, the cement pH varies from very acidic pH values, i.e., 2.5, to almost neutral pH values, i.e., 6. The cement pH profile significantly depends on the initial cement composition. However, all profiles are characterized by a sharp initial decrease of the pH due to the dissolution of MCPM crystals and the precipitation of dicalcium phosphate dihydrate (CaHPO4· 2H2O; DCPD) crystals. With an excess of MCPM, the final pH stays low, and its value can be predicted from the initial composition of the cement and solubility data. With an excess of -TCP, the end pH is close to 5, which is much lower than 5.9, the value predicted by calculation. Results suggest that the difference may be due to the presence of impurities in the cement. Replacing MCPM by phosphoric acid renders the cement paste very acidic for the initial 30 s, but then the pH profile follows that obtained with MCPM. Adding pyrophosphate ions into the cement paste postpones the position of the pH minimum. The delay, which is proportional to the concentration of pyrophosphate ions, is thought to be due to the inhibiting action of pyrophosphate ions on the precipitation of DCPD crystals.     

On the development of an apatitic calcium phosphate bone cement

Development of an apatitic calcium phosphate bone cement is reported. 100 μ Particles of tetracalcium phosphate (TTCP) and dicalcium phosphate dihydrate (DCPD) were mixed in equimolar ratio to form the cement powder. The wetting medium used was distilled water with Na₂HPO₄ as accelerator to manipulate the setting time. The cement powder, on wetting with the medium, formed a workable putty. The setting times of the putty were measured using a Vicat type apparatus and the compressive strength was determined with a Universal Testing Machine. The nature of the precipitated cement was analyzed through X-ray diffraction (XRD), fourier transform infrared spectrometry (FTIR) and energy dispersive electron microprobe (EDAX). The results showed the phase to be apatitic with a calcium-to-phosphorous ratio close to that of hydroxyapatite. The microstructure analysis using scanning electron microscopy (SEM) showed hydroxyapatite nano-crystallite growth over particulate matrix surface. The structure has an apparent porosity of ∼ 52%. There were no appreciable dimensional or thermal changes during setting. The cement passed the in vitro toxicological screening (cytotoxicity and haemolysis) tests. Optimization of the cement was done by manipulating the accelerator concentration so that the setting time, hardening time and the compressive strength had clinically relevant values.     

Effect of several additives and their admixtures on the physico-chemical properties of a calcium phosphate cement

Combinations of citrate (C₆H₅O ₇ ^3-– ), pyrophosphate (P₂O ₇ ^4– ) and sulfate (SO ₄ ^2– ) ions were used to modify the physico-chemical properties of a calcium phosphate cement (CPC) composed of -tricalcium phosphate (-TCP) and phosphoric acid (PA) solution. The results obtained with only one additive at a time are similar to those previously published. New facts are: the positive effect of C₆H₅O ₇ ^3– ions on cement failure strain and their negative effect on cement pH. The position of the setting time maximum measured at an SO ₄ ^2– concentration of 0.09 M was not displaced by the addition of C₆H₅O ₇ ^3– and P₂O ₇ ^4– ions. However, the effect of SO ₄ ^2– ions on the setting time was depressed by C₆H₅O ₇ ^3– ions. Moreover, no increase in tensile strength was observed when increasing amounts of SO ₄ ^2– were added into a C₆H₅O ₇ ^3– -containing cement. The latter results suggest a competitive effect of C₆H₅O ₇ ^3– and SO ₄ ^2– on setting time and tensile strength. Anhydrous dicalcium phosphate (DCP; CaHPO₄) appeared in cement samples dried just after setting, but not in cement samples incubated for 24 h in deionized water before the drying step. It is believed that the setting reaction is stopped by the drying step, leaving a low internal pH in the sample, hence providing favorable conditions for the transformation of dicalcium phosphate dihydrate (DCPD) into DCP. Interestingly, even though C₆H₅O ₇ ^3– ions dramatically lowered the equilibrium pH of the cement with 5 ml of deionized water, they still prevented the occurrence of the transformation of DCPD into DCP.     

Rheological behaviour of fresh cement paste as measured by squeeze flow

A method is proposed for measuring the rheology of cement paste under conditions that suppress shear flow, i.e. squeezing. This method is based on squeezing samples in a servohydraulic compression-tension testing machine, and is different from the commonly used shear flow experiments. Possible artefacts such as the buoyancy of the piston that penetrates the paste, sedimentation of cement paste, geometry of the container, and friction at the interface between the top plate (or piston) and sample are investigated. Plots of stress versus apparent strain were obtained and compared with results from standard shear flow experiments. Because cement paste has both viscoelastic and viscoplastic characteristics, results are analysed in terms of both solid-like deformation and liquid-like flow behaviour. A first-approximation theoretical analysis is developed, based on the assumption that cement paste behaves as a non-Newtonian liquid, and results are compared with the experimental results.Nomenclature Shear strain rate in power law fluid model - _zr Shear strain converted from _zr - Shear strain rate - Normal strain rate - _zr Component of shear strain - _zr Component of shear strain rate - _zz Component of normal strain - Viscosity - Density of cement paste (3.2 g cm^–3) - _Cav Calculated average normal stress of cement paste - _Nav Calculated average normal stress of power law fluid - _m Measured normal stress of cement paste - _zz Normal stress in z direction - _eq Equivalent shear stress converted from normal stress - _rz Shear stress in momentum equation - a _i Coefficients in polynomial function of geometric factor for cement paste - B Buoyancy force - CGF Geometric factor for cement paste - d _o Amplitude of squeeze motion - F _N Load in normal direction - g Gravitational constant - h Sample height - h _o Initial sample height - Velocity of platen - k Order of polynomial function of geometric factor for cement paste - m Consistency in power-law fluid model - n Power index in power-law fluid model - P Pressure - P _a Atmospheric pressure - PGF Geometric factor for power-law fluid model - r Radial direction in cylindrical coordinates - R Radius of sample - s 1/n - V Volume of the top platen submerged into cement paste - v _r Velocity inr direction - v _z Velocity in z direction - z Vertical direction in cylindrical coordinates     

Penetration resistance and electrical resistivity of cement paste with superplasticizer

The purpose of this paper is to investigate the setting process and evolution of electrical resistivity of Portland cement pastes with constant water to cement ratio (w/c) of 0.3 and with different dosages of naphthalene superplasticizer (SP) from 0 to 1.2 %. The setting process of cement paste was monitored by the Vicat needle test. The depth of penetration was recorded and used to calculate the shear resistance generated by the cement paste. Electrical resistivity was measured by a non-contacting electrical resistivity apparatus. The hyperbolic curve of electrical resistivity versus time was plotted to determine the ultimate electrical resistivity. The results show that the addition of SP to the pastes with a fixed w/c can cause longer setting time and delay the evolution of electrical resistivity. The final setting time (t _f) and the occurring time of maximum rate of electrical resistivity (t _r) were both delayed when the dosage of SP was increased. This may indicates that the electrical resistivity measurement can be used to monitor the setting process of cement. The compressive strength at 28 days and the ultimate electrical resistivity show a same tendency for the cement pastes with different dosages of SP. Thus, it would be possible to predict the compressive strength of hardened cement paste by its ultimate electrical resistivity.     

In vitro fluoride uptake by bovine enamel in using cyanoacrylate adhesives containing fluoride compounds

To study thein vitro uptake of fluoride from 3-year-old bovine enamel, enamel biopsies for fluoride analysis were performed using five successive etchings. At 10 µm etch depth the uptake of fluoride was maximum for all methyl--cyanoacrylates containing six kinds of fluoride compounds. The fluoride concentration was pronounced by 1 month of immersion, rather than by 1 week of immersion in distilled water. In particular, fluoride compounds such as BiF₃, NaF, SnF₂ and ZnF₂ had a significant difference (p<0.01) from KF and Na₂FPO₄ at 1 month of immersion.     

Production and characterization of new calcium phosphate bone cements in the CaHPOv4–α-Ca3(PO4)2 system: pH, workability and setting times

The initial setting properties of calcium phosphate cements in the CaHPOv₄–-Ca₃(PO₄)₂ (DCP–-TCP) system have been investigated. Interest was focused on the pH, workability, cohesion time and initial and final setting times. The addition of CaCO₃ modified the structure of the cement reaction product such that it became more similar to the apatite phase in bone mineral. The addition of 10% w/w of CaCO₃ reduced the viscosity of the cement pastes resulting in an increase in initial and final setting times and improved injectability. © 1999 Kluwer Academic Publishers     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

The d.c. conductivity, , and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.     

Injectable magnesium-doped brushite cement for controlled drug release application

Dicalcium phosphate dihydrate (CaHPO₄·2H₂O), also known as brushite, is one of the important bioceramics for bone regeneration. However, fast setting of the brushite cement under physiological conditions has limited its clinical use. Furthermore, brushite cement without any additives normally has poor injectability due to the liquid–solid phase separation. In the present study, magnesium-doped β-tricalcium phosphate (Mg-β-TCP) with chemical formula of β-Ca_2.96?x Mg _x (PO₄)₂ was used to prepare injectable brushite cements with improved physicochemical properties. β-TCP containing different amounts of Mg²⁺ ions were reacted with monocalcium phosphate monohydrate [Ca(H₂PO₄)₂·H₂O, MCPM] in the presence of water to furnish corresponding brushite cement. The samples were characterized using X-ray diffractometry, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The effect of magnesium ions on the structural, mechanical, and setting properties of the cements is reported. Our results indicate that the presence of Mg²⁺ ions increases the degree of injectability, setting time, and mechanical properties of the brushite cement. The compressive strength of brushite cement was substantially increased upon incorporation of Mg²⁺ ions. Furthermore, the setting times of the brushite cement were significantly improved. Gentamicin sulfate, amoxicillin and ampicillin trihydrate were incorporated into the Mg-brushite cement, and their release profiles showed a sustained drug release over 14 days. Cumulative releases of 99.3, 87, and 79 % were observed for gentamicin sulfate, amoxicillin, and ampicillin trihydrate, respectively.     

The effects of temperature on the behaviour of an apatitic calcium phosphate cement

An apatitic calcium phosphate cement is obtained by mixing -tricalcium phosphate (-TCP) and precipitated hydroxyapatite into a cement powder, and by then mixing this powder with an aqueous solution of Na₂HPO₄ as an accelerator. Setting times were reduced by about 30% by increasing the temperature from 22 to 37°C. Compressive strength reached higher intermediate and final values at 37 °C. Degrees of transformation of the -TCP in the resulting calcium-deficient hydroxyapatite (CDHA) were much higher at 37 °C after 24 h of storage in Ringer's solution according to X-ray diffraction. Differential scanning calorimetry indicated that the rate of reaction increased by a factor of about 5 when the temperature was increased from 25 to 37 °C. Scanning electron microscopy showed that the microstructure was more homogeneous and that a more tight entanglement of the precipitated CDHA crystals occurred after storage at 37 °C than at room temperature.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

Monitoring the setting of calcium-based bone cements using pulse-echo ultrasound

We present a new technique, based on pulse-echo ultrasound, for monitoring the entire setting process of injectable bone cement. This research has been motivated by the lack of satisfying standards. The main problem with existing standards is the subjectivity, which leads to poor reproducibility. Because of this the results are not comparable between different research groups. A strong advantage with the proposed technique is that if low-intensity ultrasound is used, it provides a non-destructive analysis method. Once the cement paste has been applied to the measurement cell, no manipulation is needed throughout the entire setting process. The problem of the ultrasound affecting the setting of certain cement materials has been investigated, and solutions are discussed. The propagation of ultrasound is temperature-dependent, and therefore a technique for automatic compensation for temperature variations is discussed briefly. The testing was performed on -calcium sulfate hemihydrate (CSH) and mixtures of CSH and -tricalcium phosphate (-TCP). The results show that the acoustic properties of the cement are strongly correlated with the setting time, the density, and the adiabatic bulk modulus. The measured initial and final setting times agree well with the Gillmore needles standard. An important difference compared to the standards, is that the technique presented here allows the user to follow the entire setting process on-line.     

含锶磷酸钙骨水泥的制备及性能研究

通过混合磷酸四钙、磷酸氢钙、磷酸氢锶及稀磷酸制备了一种含锶磷酸钙骨水泥(Sr-CPC),研究了水泥固/液比、含锶量及仿生浸泡时间对其结构组态及抗压强度的影响.结果表明,不同组分的CPC在调和浆体时均存在最佳固/液比,且对应的凝结时间适合临床手术要求;Sr-CPC试样在模拟体液(SBF)中浸泡24h后固化产物为含锶缺钙羟基磷灰石;适量锶的加入及较短时间仿生浸泡均可显著改善Sr-CPC固化体的抗压强度,并且浸泡过程对固化体抗压强度的影响主要体现在其微观结构的变化.     

Whether do nano-particles act as nucleation sites for C-S-H gel growth during cement hydration?

In this study, three modelling experiments were designed to investigate whether nano-particles incorporated in the cement paste act as nucleation sites for CSH gel growth during cement hydration. The nano-particles with (nano-SiO₂) and without (nano-TiO₂) pozzolanic reactivity were used. In the first experiment, both the cement and nano-particles were dispersed in water to prepare dilute cement paste, in which the cement and nano-particles can contact each other. In the second one, the cement particles were laid inside a filter paper funnel and immersed in tap and ultrapure water with nano-particles dispersed, in order to separate the cement particles with nano-particles by using the filter paper. In the third one, large clinker particle was embedded in resin, surface-polished and then exposed upside down in ultrapure water with and without nano-particles dispersed. After hydration for 7 days, the hydration products in the paste or the nano-particle dispersion were observed by using TEM and the hydrated surface of the embedded clinkers were detected by using SEM. Based on the experimental results and the detailed discussions by using the classic nucleation theory, it was found that there may have no nucleus function of the nano-particles for the CSH gel precipitation during cement hydration, at least in the hydrating system with nano-silica and nano-TiO₂ addition. It was proposed to more reasonably explain the observations in the three modelling experiments by using the topochemical reaction instead of the through-solution mechanism for the CSH gel formation.     

Synthesis and hydration study of Portland cement components prepared by the organic steric entrapment method

The four components of Portland cement; dicalcium silicate (Ca₂SiO₄), tricalcium silicate (Ca₃SiO₅), tricalcium aluminate (Ca₃Al₂O₆), and tetracalcium aluminate iron oxide (Ca₄Al₂Fe₃O₁₀), were made by the PVA complexation process. Powders prepared by this new method can make relatively high yields of pure, synthetic, cement components of nano or sub-micron crystallite dimensions, high specific surface area, and extremely high reactivity at relatively low calcining temperatures in comparison with conventional methods. The above advantages can enhance setting speed, increase strength, and lead to other desirable characteristics of Portland cement. Optimum synthesis conditions, such as PVA content, degree of polymerization of the PVA, and calcination temperature, were determined for each component. Hydration speed and strength of the synthesized, mixed cement paste were also studied at room temperature (25°C). The powders and hydration behavior were characterized by microstructural examination (XRD, SEM) and specific surface areas were measured by nitrogen gas adsorption BET. DSC and Instron were used to study setting speed and compression strength.     

In vitro aging of a calcium phosphate cement

Cement samples made of -tricalcium phoshate (-TCP), phosphoric acid (PA) and water mixtures were incubated in several aqueous solutions to determine their stability over time. The effects of the cement composition and the incubating temperature were investigated in more detail. The cement samples contained mostly dicalcium phosphate dihydrate (DCPD) and remnants of -TCP crystals. Depending on the initial cement composition, a certain amount of dicalcium phosphate (DCP) crystals were formed. The larger the initial PA concentration, the larger the DCP amount. After setting, the cement composition was stable for at least 16 days up to 60 °C. Above that temperature, the DCPD crystals decomposed into DCP crystals. The latter reaction provoked a decrease of the pH of the incubation solution, phenomenon expected for a cement sample containing an excess of PA. As the cement samples contained an excess of -TCP, it was postulated that -TCP crystals became so covered by DCP or DCPD crystals during setting that the setting reaction was stopped prematurely. The latter phenomenon gave a good explanation for the low pH values measured in the incubation solutions. ©©2000 Kluwer Academic Publishers     

Limited compliance of some apatitic calcium phosphate bone cements with clinical requirements

Clinical requirements for calcium phosphate bone cements were formulated in terms of the initial setting time, the final setting time, the cohesion time and the ultimate compressive strength. Two cements were tested. Biocement H was made of a powder containing -tertiary calcium phosphate and precipitated hydroxyapatite. Biocement F was made of a powder containing, in addition, some monetite. The liquid/powder (L/P) ratio was varied over the range 0.30–0.40 ml g-1, whereas the accelerator concentration in the liquid was varied from 0%–4% Na2HPO4 in water. For Biocement H there was no combination L/P ratio and % Na2HPO4 for which all clincal requirements were satisfied. However, Biocement F had a certain area where this was the case. Therefore, it is expected that Biocement F can be applied in clinical situations such as orthopaedics, plastic and reconstructive surgery and oral and maxillofacial surgery, even when early contact with blood is inevitable. © 1998 Kluwer Academic Publishers