A low-temperature sol-gel route for the synthesis of bioactive calcium silicates

Received 14 August 2012 Received in revised form 19 November 2012 Accepted 28 November 2012 Available online 7 February 2013

Geotechnical investigation of low-plasticity organic soil treated with nano-calcium carbonate

Soil stabilization using nanomaterials is an emerging research area although,to date,its investigation has mostly been laboratory-based and therefore requires extensive study for transfer to practical field ap-plications.The present study advocates nano-calcium carbonate(NCC)material,a relatively unexplored nanomaterial additive,for stabilization of low-plasticity fine-grained soil having moderate organic content.The plasticity index,compaction,unconfined compressive strength(UCS),compressibility and permeability characteristics of the 0.2%,0.4%,0.6%and 0.8%NCC-treated soil,and untreated soil(as control),were determined,including investigations of the effect of up to 90-d curing on the UCS and permeability properties.In terms of UCS improvement,0.4%NCC addition was identified as the optimum dosage,mobilizing a UCS at 90-d curing of almost twice that for the untreated soil.For treated soil,particle aggregation arising from NCC addition initially produced an increase in the permeability coef-ficient,but its magnitude decreased for increased curing owing to calcium silicate hydrate(CSH)gel formation,although still remaining higher compared to the untreated soil for all dosages and curing periods investigated.Compression index decreased for all NCC-treated soil investigated.SEM micro-graphs indicated the presence of gel patches along with particle aggregation.X-ray diffraction(XRD)results showed the presence of hydration products,such as CSH.Significant increases in UCS are initially attributed to void filling and then because of CSH gel formation with increased curing.

Structure, Formation, Properties, and Application of Calcium and Magnesium Silicate Hydrates System—A Review

In order to expand the advantages of strong durability and high compressive strength of calcium silicate hydrates(C-S-H),at the same time to make up for the poor early mechanical strength of magnesium silicate hydrates (M-S-H),we present the features and advantages of C-S-H and M-S-H and a comprehensive review of the progress on CaO-MgO-SiO_(2)-H_(2)O.Moreover,we systematically describe natural calcium and magnesium silicate minerals and thermodynamic properties of CaO-MgO-SiO_(2)-H_(2)O.The effect of magnesium on C-S-H and calcium on M-S-H is summarized deeply;the formation and structural feature of CaO-MgO-SiO_(2)-H_(2)O is also explained in detail.Finally,the development of calcium and magnesium silicate hydrates in the future is pointed out,and the further research is discussed and estimated.

Study on Multi-Scale Tensile Strength and Tensile Strain of Calcium Silicate Hydrate Layered Nanocomposites Under External Physical Field

Calcium silicate hydrate(C-S-H)is the mainly strength source of cement-based materials,but there is little basic research.In this paper,molecular dynamics method is applied to analyze the multi-scale tensile strength and tensile strain of C-S-H layered materials under the condition of external physical fields(temperature and strain rate).The results show that the tensile strength and strain of C-S-H model decrease with temperature raises.The temperature(from 1 K to 600 K)has obvious influence on the tensile strain and strength of C-S-H layered materials.In addition,at(0.00025 ps^(-1)-0.001 ps^(-1)),the tensile strain and strength of C-S-H layered materials are less sensitive to strain rate.The whole model is closer to a 3-dimensional deformation.However,at(0.001 ps^(-1)-0.005 ps^(-1)),the dynamic load effect begins to increase,and the work done by the load per unit time increased.The tensile strain and strength of C-S-H layered materials indicates intensified by the change of strain rate.The energies are randomly distributed in the system,not concentrated in a certain area.

Hydration reactivity difference between dicalcium silicate and tricalcium silicate revealed from structural and Bader charge analysis

Cement hydration is the underlying mechanism for the strength development in cement-based materials.The structural and electronic properties of calcium silicates should be elucidated to reveal their difference in hydration reactivity.Here,we comprehensively comparedβ-C_(2)S and M_(3)-C_(3)S and investigated their structural properties and Bader charge in the unit cell,during surface reconstruction and after single water adsorption via density functional theory.We identified different types of atoms inβ-C_(2)S and M_(3)-C_(3)S by considering the bonding characteristics and Bader charge.We then divided the atoms into the following groups:forβ-C_(2)S,Ca and O atoms divided into two and four groups,respectively;for M_(3)-C_(3)S,Ca,O,and Si atoms divided into four,four,and three groups,respectively.Results revealed that the valence electron distribution on the surface was more uniform than that on the unit cell,indicating that some atoms became more reactive after surface relaxation.During water adsorption,the electrons ofβ-C_(2)S and M_(3)-C_(3)S were transferred from the surface to the adsorbed water molecules through position redistribution and bond formation/breaking.On this basis,we explained whyβ-C_(2)S and M_(3)-C_(3)S had activity differences.A type of O atom with special bond characteristics(no O–Si bonds)and high reactivity existed in the unit cell of M_(3)-C_(3)S.Bader charge analysis showed that the reactivity of Ca and O atoms was generally higher in M_(3)-C_(3)S than inβ-C_(2)S.Ca/O atoms had average valence electron numbers of6.437/7.550 inβ-C_(2)S and 6.481/7.537 in M_(3)-C_(3)S.Moreover,the number of electrons gained by water molecules in M_(3)-C_(3)S at the surface was higher than that inβ-C_(2)S.The average variations in the valence electrons of H_(2)O onβ-C_(2)S and M_(3)-C_(3)S were 0.041 and 0.226,respectively.This study further explains the differences in the hydration reactivity of calcium silicates and would be also useful for the design of highly reactive and environmentally friendly cements.

Effect of C/S Ratio on Morphology and Structure of Hydrothermally Synthesized Calcium Silicate Hydrate

The samples of the C-S-H series were synthesized by hydrothermal reaction of fumed silica, CaO and deionized water at initial C/S ratios between 1.0-1.7. Phase composition and structural and morphology characteristics of C-S-H samples were analyzed by XRD, IR and SEM. The experimental results showed that the d-spacing of （002）, （110） and （020） decreased, the d-spacing of （200） increased, and the d-spacing of （310） varied randomly, the polymerization of silica tetrahedra of C-S-H decreased, and morphology of C-S-H samples varied from sheet shapes to long reticular fibers as C/S ratio increased.

Fractionation and solubility of cadmium in paddy soils amended with porous hydrated calcium silicate

Previous studies have shown that porous hydrated calcium silicate （PS） is very effective in decreasing cadmium （Cd） content in brown rice. However, it is unclear whether the PS influences cadmium transformation in soil. The present study examined the effect of PS on pH, cadmium transformation and cadmium solubility in Andosol and Alluvial soil, and also compared its effects with CaCO3, acidic porous hydrated calcium silicate （APS） and silica gel. Soil cadmium was operationally fractionationed into exchangeable （Exch）, bound to carbonates （Carb）, bound to iron and manganese oxides （FeMnOx）, bound to organic matters （OM） and residual （Res） fraction. Application of PS and CaCO3 at hig rates enhanced soil pH, while APS and silica gel did not obviously change soil pH. PS and CaCO3 also increased the FeMnOx-Cd in Andosol and Carb-Cd in Alluvial soil, thus reducing the Exch-Cd in the tested soils. However, PS was less effective than CaCO3 at the same application rate. Cadmium fractions in the two soils were not changed by the treatments of APS and silica gel. There were no obvious differences in the solubility of cadmium in soils treated with PS, APS, silica gel and CaCO3 except Andosol treated 2.0% CaCO3 at the same pH of soil-CaC12 suspensions. These findings suggested that the decrease of cadmium availability in soil was mainly attributed to the increase of soil pH caused by PS.

Hydrothermal synthesis of zeolites-calcium silicate hydrate composite from coal fly ash with co-activation of Ca(OH)_(2)-NaOH for aqueous heavy metals removal

Coal fly ash is a typical secondary aluminum/silicon resource.The preparation of zeolite-type absorbent is a potential way for its value-added utilization,while the purity and adsorption property of zeolite are limited due to the occurrence of side reactions in the synthesis process.In this study,a designated composite consisted of crystalline zeolites and amorphous calcium silicate hydrate was selected,which was direct synthesized from fly ash under conditions of a Ca/Si molar ratio of 0.8,an initial NaOH concentration of 0.5 mol/L,a hydrothermal temperature of 170℃and a liquid–solid ratio of 15 mL/g.The results indicated that this composite had superior adsorption property for a variety of heavy metals,which was based on the exchange of calcium and sodium ions in zeolites and calcium silicate hydrate.Its adsorption capacities for Pb^(2+),Ni^(2+),Cd^(2+),Zn^(2+),Cu^(2+)and Cr^(3+)attained 409.4,222.4,147.5,93.2,101.1 and 157.0 mg/g,respectively,in single solution with a pH of 4.5.After regulating the synthesis conditions,the transformation of amorphous calcium silicate hydrate into crystallized tobermorite weakened the adsorption capacity of the composite.Besides,due to the competitive adsorption in a multiple ions solution,the adsorption capacities for these heavy metals had a reduction.

Effect of Calcium Silicate Hydrate Seeds on Hydration and Mechanical Properties of Cement

C-S-H series are synthesized at different temperatures and ages by pozzolanic reaction.The change of particle size distribution,phase composition,morphology and nanostructure of C-S-H with temperatures and ages,and the effects of C-S-H seeds and seeds parameters on the hydration behavior and mechanical properties development of cement were investigated by DLS,XRD,SEM,^(29)Si NMR,TAM-air isothermal calorimeter and mechanical properties test.The results show that the particle size,crystallinity,basal spacing and Q^(2)/Q^(1) ratio of C-S-H increases with the increase of synthesis temperature and age.The addition of synthesized C-S-H seeds to cement pastes results in the strong acceleration effect on cement hydration and significant improvement of the early strength of cement paste and mortar.The 1 day-C-S-H seeds synthesized at room temperature can increase the strength of cement paste by about 30 MPa at 12 h.The effect does not show a very regular change with the increase of the temperature and age of seeds synthesis.Considering the effect of C-S-H seeds on the hydration and mechanical properties of cement,and economy and short cycle of seeds synthesis,the C-S-H seeds synthesized at room temperature for 1 day or 55 ℃ water bath for 12 hours is recommended..

An evaluation of the inflammatory response of lipopolysaccharide-treated primary dental pulp cells with regard to calcium silicate-based cements

This study compared the biological changes of lipopolysaccharide （LPS）-treated dental pulp （DP） cells directly cultured on mineral trioxide aggregate （MTA） and calcium silicate （CS） cements. DP cells were treated with LPS for 24 h. Then, the LPS-treated DP cells were cultured on MTA or CS cements. Cell viability, cell death mechanism and interleukin （IL）-1β expressions were analysed. A one-way analysis of variance was used to evaluate the significance of the differences between the means. A significantly higher IL-1β expression （2.9-fold） was found for LPS-treated cells （P〈0.05） compared with DP cells without LPS treatment at 24 h. Absorbance values of LPS-treated cells cultured on CS cement were higher than a tissue culture plate. A significant difference （P〈0.05） in cell viability was observed between cells on CS and MTA cements 24 h after seeding. At 48 h, a high concentration of Si （5 mM） was released from MTA, which induced LPS-treated DP cell apoptosis. The present study demonstrates that CS cement is biocompatible with cultured LPS-treated DP cells. MTA stimulates inflammation in LPS-treated DP cells, which leads to greater IL-1β expression and apoptosis.

Thermal conductivity measurements on xonotlite-type calcium silicate by the transient hot-strip method

The experimental results of the thermal conductivities of xonotlite-type calcium silicate insulation materials were presented at different temperatures and pressures. Two appropriative surroundings, i.e. an elevated temperature surrounding from ambient temperature to 1450 K and a vacuum surrounding from atmosphere pressure to 10-3 Pa, were designed for the transient hot-strip （THS） method. The thermal conductivities of xonotlite-type calcium silicate with four densities from ambient temperature to 1000 K and 0.045 Pa to atmospheric pressure were measured. The results show that the thermal conductivity of xonotlite-type calcium silicate decreases apparently with the fall of density, and decreases apparently with the drop of pressure, and reaches the least value at about 100 Pa. The thermal conductivity of xonotlite-type calcium silicate increases almost linearly with T0, and increases more abundantly with low density than with high density. The thermal conductivity measurement uncertainty is estimated to be approximately 3% at ambient temperature, and 6% at 800 K.

Removal of Chromium(Ⅲ) from Monoammonium Phosphate Solutions by a Porous Adsorbent of Fluor(calcium silicate) Composites

The products of monoammonium phosphate containing Cr^3+resulted in disqualification,and further posed a serious threat to ecological environment and human beings.Herein,the porous adsorbent of fluor(calcium silicate)composites(FCSc)was prepared by hydrothermal method using diatomaceous earth,hydrated lime and additive(NaF)as raw materials,which was characterized and used for the removal of Cr^3+from monoammonium phosphate solutions.The effects of different parameters,such as solution pH,initial Cr^3+concentration,temperature and contact time on the adsorption of Cr^3+onto FCSc were investigated in details.The results indicated that the adsorption process was in agreement with the pseudo-second-order kinetic model and Freundlich isotherm.The spontaneous and endothermic nature of the adsorption process was obtained by analyzing various thermodynamic parameters(△G0,△H0,and△S0).In addition,computational monte carlo simulations between Cr3+ions and FCSc were conducted to elucidate the adsorption mechanism.Such kind of porous adsorbent provided a potential application in the removal of impurities from monoammonium phosphate industry.

Synthesis of Calcium Silicate Hydrate based on Steel Slag with Various Alkalinities

This study aimed to improve the hydraulic potential properties of the slag. Therefore, a method of dynamic hydrothermal synthesis was applied to synthesize calcium silicate hydrate. The phases and nanostructures were characterized by XRD, FTIR, TEM, and BET nitrogen adsorption. The infl uence of alkalinity of steel slag on its structures and properties was discussed. The experimental results show that, the main product is amorphous calcium silicate hydrate gel with fl occulent or fi brous pattern with a BET specifi c surface area up to 77 m2/g and pore volume of 0.34 mL/g. Compared with low alkalinity steel slag, calcium silicate hydrate synthesized from higher alkalinity steel slag is prone to transform to tobermorite structure.

Heterogeneous Nature of Calcium Silicate Hydrate(C-S-H) Gel:A Molecular Dynamics Study

Structure and mechanical properties of Calcium silicate hydrate (C-S-H) at a molecular level act as "DNA" of cement-based construction materials.In order to understand loading resistance capability of C-S-H gel,research on molecular dynamics (MD) was carried out to simulate the uniaxial tension test on C-S-H model along x,y,and z directions.Due to the structure and dynamic differences of the layered structure,the C-S-H model demonstrates heterogeneous mechanical behavior.On an XY plane,the cohesive force can reach 4 GPa,which is mainly provided by the Ca-O and Si-O ionic-covalent bonds.The good plasticity of calcium silicate sheet is attributed to the silicate branch structure formation and the recovery role of interlayer calcium atoms.However,in z direction,C-S-H layers connected by the unstable H-bonds network,have the weakest tensile strength 2.2 GPa.This results in the brittle failure mode in z direction.The relatively low tensile strength and poor plasticity in z direction provides molecular insights into the tensile weakness of cement materials at macro-level.

Influence of Hydrothermal Synthesis Conditions on the Formation of Calcium Silicate Hydrates:from Amorphous to Crystalline Phases

Hydrothermal treatment has been widely applied in the synthesis of well crystalline calcium silicate hydrate（CSH）, such as tobermorite and xonotlite. However, both morphology and crystallinity of CSH are greatly affected by the conditions of hydrothermal treatment including siliceous materials, temperature increase rate and isothermal periods. In this study, the influence of hydrothermal conditions on the growth of nano-crystalline CSH was investigated based on XRD analysis. Results showed that siliceous materials with amorphous nature（i e, nano silica powder） are beneficial to synthesize pure amorphous CSH, while the use of more crystallized siliceous materials（i e, diatomite and quartz powder） leads to producing crystalline CSH. Results also indicate that the formation of tobermorite and xonotlite is greatly affected by the temperature rise rate during hydrothermal treatment.

Hydrothermal Synthesis of Xonotlite-type Calcium Silicate Insulation Material Using Industrial Zirconium Waste Residue

Xonotlite-type insulation material was prepared by hydrothermal synthesis technology using industrial zirconium waste residue in this paper,and the phase analysis together with the observation of micro–morphology were also carried out by XRD,SEM and TEM.The density and thermal conductivity were measured finally.The results indicate,chlorine ion impurity contained in zirconium waste residue can be removed effectively via water washed process,and the reactive activity of silicon dioxide is almost not affected,which make it be a good substitution of silicon material for the preparation of calcium silicate insulation material by hydrothermal synthesis technique.The density and thermal conductivity of xonotlite-type calcium silicate insulation material obtained by hydrothermal synthesis technique can reach 159 kg/m3,0.049 W/（m·° C）,respectively,meeting with National Standard well,when synthesis conditions are selected as follows：Ca/Si molar ratio equal to 1,synthesis temperature at 210°C and kept for 8 hrs.It provides a new approach to realize lightweight and low thermal conductivity of calcium silicate insulation material.

Kinetics and Mechanism of Adsorption of Phosphate on Fluorine-containing Calcium Silicate

The nanowires-reticulated calcium silicate with a specific surface area more than 100 m^2/g was prepared by a hydrothermalprocess using hydrated lime（Ca（OH）_2,HL）and silica containing soluble fluoride,which was a by-product of fluorine industry,and the soluble fluoride in raw silica was fixed as CaSiF_6 at the same time.The kinetic characteristics and mechanism of adsorbing phosphate by fluorine-containing calcium silicate were investigated in the experiments of phosphorus（P）removalfrom aqueous solution.The results show that the prepared fluorine-containing calcium silicate has excellent performance for adsorbing phosphate,the adsorption process appears to follow pseudo-second-order reaction kinetics and the process is mainly controlled by chemisorption.The product resulted from P adsorption is mainly composed of hydroxyapatite（HAP）and fluorapatite（FAP）,which are further used as adsorbents of heavy metalion Cd^（2＋） in aqueous solution and display excellent performance.

Development of Extended-Release Formulation of Ibuprofen Using Blends of Calcium Silicate and Polyvinyl Pyrrolidone as Tablet Matrix

A tablet matrix system was developed for ibuprofen and the influence of the polymer blend and concentration on the release rate of the drug was evaluated.Tablets containing different concentrations of calcium silicate and PVP(polyvinyl pyrrolidone)were prepared using direct compression and the weight uniformity,crushing strength,friability,drug content uniformity,dissolution profile,and in vitro release kinetics were examined.Formulated tablets were found to be within the official acceptable limits of physical and chemical parameters except for the thickness test that was below the conformation of extended-release tablets.The crushing strength of the tablets was in the range of 2.5 to 5.6 kg/f,the weight variations of the tablets of all the formulation was less than±5%.The friability of all the formulations was in the range of 0.6%to 1.83%.Tablet thickness and diameter was in range of 3.18 mm to 4.48 mm and 12.53 mm to 12.64 mm respectively.Absolute drug contents of all the formulations were found to be in range of 83.50%to 98%.The release kinetic of F3 containing 20 mg of calcium silicate,40 mg of PVP as matrix formers showed the best linearity(r^2=0.6975)with%drug release of 96 showing that combination of the two polymers(20 mg calcium silicate and 40 mg PVP)for use as a matrix former is best for extended-release formulation of ibuprofen.

Unsaturated transport properties of water molecules and ions in graphene oxide/hydrated calcium silicate nanochannels:from basic principles to complex environmental performance effects

The problems of traditional concrete such as brittleness,poor toughness and short service life of concrete engineering under acid rain or marine environment need to be solved urgently.Hydrated calcium silicate(C-S-H)is a key component to improve the mechanical properties and durability of concrete.However,the traditional method of concrete material design based on empirical models or comparative tests has become a bottleneck restricting the sustainable development of concrete.The synthesis method,molecular structure and properties of C-S-H were systematically described in this paper;The interface structure and interaction of graphene oxide/calcium silicate hydrate(C-S-H/GO)were discussed.On this basis,the saturated and unsaturated transport characteristics of ions and water molecules in C-S-H/GO nanochannels under the environment of ocean and acid rain were introduced.The contents of this review provide the basis for improving the multi-scale transmission theory and microstructure design of concrete.It has important guiding significance for analyzing and improving the service life of concrete in complex environment.

Expansive Soil Stabilization by Bagasse Ash in Partial Replacement of Cement

This study examined the effects of using bagasse ash in replacement of ordinary Portland cement(OPC)in the treatment of expansive soils.The study concentrated on the compaction characteristics,volume change,compressive strength,splitting tensile strength,microstructure,California bearing ratio(CBR)value,and shear wave velocity of expansive soils treated with cement.Different bagasse ash replacement ratios were used to create soil samples.At varying curing times of 7,14,and 28 days,standard compaction tests,unconfined compressive strength tests,CBR tests,Brazilian split tensile testing,and bender element(BE)tests were carried out.According to X-ray diffraction(XRD)investigations,quartz and crystobalite make up the majority of the minerals in bagasse ash.Bagasse ash contains a variety of grain sizes,including numerous fiber-shaped particles,according to a scanning electronic microscope(SEM)test.For all of the treated specimens with various replacement ratios,the overall additive content has not changed.The results of the Brazilian split tensile tests demonstrate improved tensile strength for all specimens with various replacement proportions.A lower maximum dry density and a greater optimal water content would result from the substitution of bagasse ash.When the replacement ratio is not more than 20%,the CBR values of the parts replaced specimens are even higher than the cement treatments.The results of BE testing on the treated soils show that there is significant stiffness anisotropy but that it steadily diminishes with curing time and replacement ratio.According to the study,bagasse ash is a useful mineral additive,and the best replacement ratio(CBA20)is 20%.