Β-dicalcium Silicate Research Articles

Accelerated Formation of β-Dicalcium Silicate by Solid-state Reaction in Water Vapor Atmosphere

Abstract β-Ca2SiO4 powders were prepared from commercially available raw materials, CaCO3 and amorphous SiO2 (a-SiO2), by solid-state reactions in water vapor atmosphere. The formation of β-Ca2SiO4 was accelerated by water vapor. Water vapor is considered to enhance the thermal decomposition of CaCO3 and to attack surface Si–O–Si bonds in a-SiO2 to accelerate the formation of β-Ca2SiO4. The effect of water vapor atmosphere on the formation of β-Ca2SiO4 from a mixture of CaCO3 and a-SiO2 was stronger than that of mechanochemical processing for 30 min.

Low-temperature synthesis of nanocrystalline β-dicalcium silicate with high specific surface area

β-Dicalcium silicate (β-Ca2SiO4) was synthesized for the first time by a simple solution combustion method using citric acid as fuel. The influence of calcination temperature on the average crystallite size, specific surface area and morphology of the powders were investigated by X-ray diffraction technique (XRD), scanning electron microscopy (SEM) and N2 adsorption measurements (BET). The results showed that the increase of calcination temperature from 650°C to 1100°C resulted in larger crystallite size and lower specific surface area of β-Ca2SiO4. The highest specific surface area could reach as high as 26.7 m2/g when the as-burnt powders were calcined at 650°C.

Charge states of Ca atoms in β-dicalcium silicate

In order to study the crystal structure of β ‐ Ca 2 SiO 4 , time-of-flight neutron powder diffraction experiments were carried out at temperatures between room temperature (RT) and 600 °C. Rietveld refinement at RT has shown that β ‐ Ca 2 SiO 4 is monoclinic based on P2 1/ n symmetry and two different types of Ca sites, Ca(1) and Ca(2). All interatomic distances within 3 Å were calculated, with the valences of Ca(1) with seven Ca O bonds and Ca(2) with eight were estimated to be 1.87+ and 2+ by the Zachariasen–Brown–Altermatt formula (bond valence sum). Applying charge neutrality the two charge states of Ca in β ‐ Ca 2 SiO 4 are [Ca(1)SiO 4] 2− and Ca(2) 2+, respectively. Furthermore, the [Ca(1)SiO 4] 2− unit has the shortest Ca–O distance, and its length kept constant at 2.23 Å at all temperatures. In the short-range structure analysis at RT, the shortest Ca–O bond was also observed in a radial distribution function. These results imply that the [Ca(1)SiO 4] 2− unit has covalency on the shortest Ca–O in addition to Si–O.

The effect of carbon dioxide on β-dicalcium silicate and Portland cement

Carbonation of silicate-based minerals and industrial residues can help to reduce CO2 emissions as well as produce useful materials. However, until a full understanding of the chemistry and microstructural development of carbonation products is obtained, their utilization in engineering applications may remain limited. In respect of this, the present work examines microstructural properties of accelerated carbonated dicalcium silicate and Portland cement by using the complementary analytical techniques of XRD, SEM, TG-DTA, and NMR MAS. It was found that carbon dioxide reacts with calcium silicates to form calcite and aragonite and a polymerized silicate product comprised of cross-linked Q3 co-ordinated silicon and fully polymerized Q4 co-ordinated silicon. The extent of silicate polymerization was higher in carbonated dicalcium silicate, however, in the Portland cement-derived product, Al substitution in the Si-framework was detected. The amount of CO2 that reacted with dicalcium silicate and Portland cement was 48 and 37% by mass, respectively.

Hydrothermal synthesis of β-dicalcium silicate (β-Ca 2SiO 4)

β-Dicalcium silicate (β-Ca 2SiO 4) has been prepared by hydrothermal method. Aerosil, rice husk ash and aerosol containing 10 wt% fly ash were used as sources of silica for reaction with freshly prepared CaO obtained from the decomposition of CaCO 3. β-Dicalcium silicates obtained by hydrothermal method were characterized by X-ray diffraction, DSC and SEM techniques. Hydration reactions were monitored by measuring non-evaporable water contents, free lime and compressive strength values. The extent of hydrations was found to be much higher as compared to that of β-dicalcium silicate prepared in a conventional manner using B 2O 3 as a stabilizer. Possible causes of high reactivity have been discussed.

Hydration of β-dicalcium silicate at high temperatures under hydrothermal conditions

Hydration of β-dicalcium silicate was carried out under hydrothermal conditions at different temperatures from 50 °C to 400 °C up to 5 days by using two methods to start the reactions at room temperature or at a desired reaction temperature. 9 C-S-H phases with the same Ca/Si ratio as precursor (γ-dicalcium silicate hydrate and α-dicalcium silicate hydrate and dellaite), Ca-rich compositions (jaffeite and reinhardbraunsite), Si-rich compositions (Ca 8Si 5O 18, kilchoanite and foshagite), and C-S-H gel were obtained at the initial stage of the hydration of β-dicalcium silicate. The reaction products were different in dependence in the hydrothermal processes. It was found that α-dicalcium silicate hydrate was directly formed from β-dicalcium silicate at low temperatures below 220 °C. The products obtained at above 240 °C were different in dependence in the hydrothermal processes, due to the different decomposition route of γ-dicalcium silicate hydrate, the initial product from β-dicalcium silicate. The room temperature mixing method gave reinhardbraunsite and kilchoanite through Ca 8Si 5O 18. In the case of the high temperature mixing method, γ-dicalcium silicate hydrate decomposed to from Ca 8Si 5O 18 and reinhardbraunsite with jaffeite, then Ca 8Si 5O 18 decomposed to from jaffeite and kilchoanite, and final products at 400 °C were reinhardbraunsite and foshagite which was formed from kilchoanite.

Sorption of Co 2+ and Sr 2+ by waste-derived 11 Å tobermorite

Newsprint recycling gives rise to significant volumes of waste sludge which can be de-watered and combusted for energy-recovery. The residual combustion ash, whose primary crystalline constituents are; gehlenite (Ca 2Al 2SiO 7), åkermanite (Ca 2MgSi 2O 7), β-dicalcium silicate (Ca 2SiO 4) and anorthite (CaAl 2Si 2O 8), is currently consigned to landfill disposal. It is demonstrated herein that a mixed product of Al-substituted 11 Å tobermorite (Ca 5Si 6O 18H 2 · 4H 2O) and katoite (Ca 3Al 2SiO 12H 8) can be synthesised from newsprint recycling combustion ash via a hydrothermal route. Batch sorption studies confirm that this mixed product is an effective sorbent for the exclusion of Co 2+ and Sr 2+ from acidic aqueous media. Kinetic sorption data are analysed in accordance with the pseudo-first- and pseudo-second-order models, and steady-state data is fitted to the Langmuir and Freundlich isotherms. The Langmuir and pseudo-second-order models are found to provide the most appropriate descriptions of the sorption processes. The maximum uptake capacities for Co 2+ and Sr 2+ at 20 °C are 10.47 and 1.52 mg g −1, respectively, and the respective apparent pseudo-second-order rate constants are estimated to be 5.08 × 10 −3 and 6.96 × 10 −3 g mg −1 min −1.

Fabrication and characterization of β-dicalcium silicate/poly( d,l-lactic acid) composite scaffolds

In this paper, poly ( d,l-lactic acid) (PDLLA) incorporated with bioactive β-dicalcium silicate (β-Ca 2SiO 4) powders to prepare composite scaffolds through conventional solvent casting-particle leaching method using sodium chloride as porogen. The pore structures and morphology of the scaffolds were determined by Scanning Electron Microscopy (SEM) and the porosity of the scaffolds was measured by Archimedes's method. The bioactivity of the composite scaffolds were evaluated by soaking in simulated body fluid (SBF) for 7 days and the formation of hydroxyapatite (HAp) on the surface was determined by SEM and Electron Dispersive Spectrometry (EDS). The results showed that the composite scaffolds could induce the formation of HAp on their surface after immersion in SBF for 7 days. In addition the hydrophilicity was characterized by measuring the water contact angles of the composite and the results indicated that the addition of β-Ca 2SiO 4 particles could improve the hydrophilicity and the increase of hydrophilicity could be obtained by the increase of the content of β-Ca 2SiO 4 particles. Our studies suggested that the combination of PDLLA with bioactive β-Ca 2SiO 4 could take advantage of the bioactivity and hydrophilicity of β-Ca 2SiO 4 and might be a promising approach to obtain composite scaffolds with improved properties.

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite–jennite (T/J) and tobermorite–‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C 3S or alite appears to consist of small globular particles, which are ≈4–8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3–4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C 3S or β-C 2S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q 0(H) species. This would also explain why there is more Q 0(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.

Influence of Minor Ions on the Stability and Hydration Rates of β‐Dicalcium Silicate

The effects of Al3+, B3+, P5+, Fe3+, S6+, and K+ ions on the stability of the β‐phase and its hydration rate were studied in reactive dicalcium silicate (C2S, Ca2SiO4) synthesized using the Pechini process. In particular, the dependences of the phase stability and degree of hydration on the calcination temperature (i.e., particle size) and the concentration of the stabilizing ions were investigated. The phase evolution in doped C2S was determined using XRD, and the degree of hydration was estimated by the peak intensity ratio of the hydrates to the nonhydrates in 29Si MAS NMR spectra. The stabilizing ability of the ions varied significantly, and the B3+ ions were quite effective in stabilizing the β‐phase over a wide range of doping concentrations. The hydration results indicated that differently stabilized β‐C2S hydrated at different rates, and Al3+‐ and B3+‐doped C2S exhibited increased degree of hydration for all doping concentration ranges investigated. The effect of the doping concentration on degree of hydration was strongly dependent on the stabilizing ions.

In vitro bioactivity and dissolution of Ca 2(SiO 3)(OH) 2 and β-Ca 2SiO 4 fibers

In this paper, needlelike and dispersible hillebrandite (Ca 2(SiO 3)(OH) 2) and β-dicalcium silicate (β-Ca 2SiO 4) fibers, 1.0–1.5 μm in diameter and 15–25 μm in length were prepared by a modified hydrothermal method. In vitro bioactivity and dissolution of the two types of fibers were evaluated by soaking in the simulated body fluid (SBF) for various periods. Hydroxyapatite-like aggregates appeared on the fibers after soaking for 7 days, and uniform lathlike agglomerates with typical morphology of hydroxyapatite (HAp) formed after 14 days. It was observed that the fibers after soaking still kept stability in fibrous morphology with an aspect ratio in the range of 6–9. The Ca 2(SiO 3)(OH) 2 and β-Ca 2SiO 4 fibers dissolved about 30.91 and 30.42%, respectively, within 14 days. The two types of fibers exhibited certain desirable characteristics including bioactivity, degradability and stability in morphology, and they are promising candidates as reinforcement reagents to develop novel bioactive composites for biomedical applications.

Use of Synthetic Calcium Aluminosilicate for Immobilization of Radioactive Wastes

Leaching rate, phase composition, specific surface area, pore volume, and pore size distribution of synthetic calcium aluminosilicate (SCAS) promising for immobilization of radioactive wastes was studied. The initial SCAS powder for forming monoliths was prepared by superadiabatic combustion of the charge containing fly ash of thermoelectric power stations and limestone. The monoliths were prepared from SCAS by two procedures: (1) solidification with water under common conditions with subsequent prolonged carbonation in air and (2) cold pressing of air-dry powder material. In the first case, a high-strength monolith was formed by filling of the pore volume with a mixture of calcium carbonate in various crystalline polymorphic modifications, and in the second case, by pozzolanic reaction of active silica with the products of β-dicalcium silicate hydration. It was found that, at contact with solution, the pores in the monolithic samples are filled with a calcium hydrosilicate gel decreasing their porosity and, in turn, leaching rate. SCAS matrices exceed special glasses and are close to some ceramic compositions in the safety of immobilization of alkali and alkaline-earth elements.

Synthesis and in vitro bioactivity of dicalcium silicate powders

Abstract In the present investigation, β-dicalcium silicate (β-Ca 2 SiO 4 ) and γ-dicalcium silicate (γ-Ca 2 SiO 4 ) powders were synthesized by sol-gel process and hydrothermal synthesis, respectively. The in vitro bioactivity of both β-, and γ-Ca 2 SiO 4 was investigated by soaking the powders in simulated body fluid (SBF) for various time periods to analyze the nucleation and growth of hydroxyapatite (HAp) on the surface of the powders. After soaked in SBF for 6 h, HAp appeared on the surface of β-, and γ-Ca 2 SiO 4 particles, and uniform lathlike aggregates with typical morphology of HAp crystals formed after 5 days. The β-Ca 2 SiO 4 showed strong hydration when soaked in SBF and the hydration was favorable for formation of carbonate-containing hydroxyapatite on the surface of the materials. The γ-Ca 2 SiO 4 was not hydrated in SBF solution and showed a slower formation of carbonate-containing hydroxyapatite on the surface when compared with β-Ca 2 SiO 4 . In addition, the Si concentration in the SBF increased quickly up to 5 days and remained invariable with increased soaking time. The results obtained indicate that hydroxyapatite nuclei can form and grow on the β-, and γ-Ca 2 SiO 4 particles, and these two dicalcium silicates are potential candidates as biomaterials for hard tissue repair.

X-ray photoelectron spectroscopy of the cement clinker phases tricalcium silicate and β-dicalcium silicate

We have used X-ray photoelectron spectroscopy (XPS) to investigate both tricalcium silicate (Ca 3SiO 5, C 3S) and β-dicalcium silicate (Ca 2SiO 4, β-C 2S), the principal components of cement clinkers. In addition to showing how the two phases may be characterised and differentiated, we show how the sensitivity of these phases to atmospheric carbon dioxide and moisture may, as a result of improper sample preparation, lead to erroneous results. The observed alteration processes of the clinker minerals shed light upon the aging process of cement clinker during storage.

Synthesis of Al-substituted 11 Å tobermorite from newsprint recycling residue: a feasibility study

Newsprint recycling is responsible for significant volumes of secondary waste material for which further reprocessing and market development would be beneficial. In response to this problem, a layer lattice, ion exchange material, Al-substituted 11 Å tobermorite, has been synthesised from newsprint recycling residue comprising gehlenite (Ca 2Al 2SiO 7), akermanite (Ca 2MgSi 2O 7), β-dicalcium silicate (Ca 2SiO 4) and anorthite (CaAl 2Si 2O 8) under hydrothermal conditions at 100 °C in the presence of NaOH. The hydrogarnet phase, katoite (Ca 3Al 2SiO 12H 8), was also formed. Similar treatment regimes in the presence of LiOH and KOH did not yield significant quantities of Al-substituted 11 Å tobermorite. A batch sorption study confirmed that the Al-substituted 11 Å tobermorite-bearing product was effective in the exclusion of Cd 2+, Pb 2+ and Zn 2+ from acidified aqueous media. The potential to enhance the yield of Al-substituted 11 Å tobermorite relative to that of katoite and thus optimise the ion exchange efficiency of the product is discussed with respect to its application to heavy metal-contaminated wastewater treatment.

Phase Transformation and Hydration of Dicalcium Silicate Containing Stabilizers.

β-dicalcium silicate was stabilized at room temperature and synthesized from reagent grade chemicals using B2O3, K2O or SO3 as stabilizers. B2O3 acts as the most effective stabilizer for dicalcium silicate. Phase transformation from β- to γ-dicalcium silicate depends on the start temperature of quenching. Hydration heat of β-C2S shows that SO3 delayed hydration at an early hydration time, but higher heat liberation rate at a later hydration time than the other stabilizers. 29Si MAS NMR analysis shows that the hydrate of dicalcium silicate stabilized by B2O3 is polymerized more than those of dicalcium silicates stabilized by K2O and SO3. Dicalcium silicate presents different shapes of calcium silicate hydrate according to stabilizers: dicalcium silicate stabilized by B2O3 shows crumbled foil shape and K2O needle shape of C-S-H.

Effects of Alkalies on Hydration of .BETA.-Dicalcium Silicate and Its Resultant Hydrates.

β-dicalcium silicate was synthesized and hydrated with/without alkalies at different temperatures. Alkalies accelerated the hydration of β-C2S and influenced the polymerization of calcium silicate hydrate. KOH and NaHCO3 caused excessive and early liberation heat of the β-C2S. The effect of NaHCO3 was more significant than that of KOH on the acceleration of hydration. The curing temperature was also an important factor in the acceleration of the hydration of β-C2S; the hydration was occurred much more actively at 40°C than at 20°C. The controlled β-C2S showed a much greater degree of polymerization of calcium silicate hydrate than did the alkali-added β-C2S. The hydrates of β-C2S were influenced by alkalies and temperature. The controlled β-C2S showed needle-like and wide needle-like shapes of calcium silicate hydrates, while the alkalie-added β-C2S presented long and wide needle-like shapes and crumbled foil-like shapes of calcium silicate hydrates. The calcium silicate hydrates formed at high temperature exhibited also crumbled foil-like shapes.

Neutron powder diffraction investigation of model cement compounds

A series of synthetic cement compounds: mono- and triclinic-tricalcium silicate, β-dicalcium silicate, cubic- and orthorhombic-tricalcium aluminate and tetracalcium alumino-ferrite have been analyzed with neutron powder diffraction followed by Rietveld refinement. Generally good fits have been obtained and the results form the basis for analysis of cement hydration using neutron diffraction.

高活性セメント鉱物β‐Ｃａ２ＳｉＯ４の低温合成と水和反応ならびに生成ジェル

This paper reviews the fields indicated by the title, which emphasizes the recent work by authors. It is possible to prepare pure β-dicalcium silicate (Ca2SiO4) without adding a stabilizer having a remarkably large specific surface area of about 7 m2 /g by decomposition of hillebrandite(Ca2(SiO3)(OH)2) at 600°C. The hillebrandite is hydrothermally synthe-sized using lime and quartz with Ca/Si=2.0 at 200-250°C. The hydration rate of the β-dicalcium silicate is extremely rapid. For water/solids ratio of 0.5, the hydration reaction is completed in 28 d cured at 25°C and 24h at 80°C. The silicate anion structure of C-S-H (Calcium Silicate Hydrate Gels) consists mainly of a dimer and a single-chain polymer. The C-S-H has an oriented fibrous structure and exhibits a 0.73-nm dreierketten in the longitudinal direction.

An Exploration of the Removal of Iron (Fe3+) from Aqueous Solutions Using β-Dicalcium Silicate and α-Monocalcium Silicate as Alternative Precipitants to Calcium Hydroxide

The present work has studied the removal of iron (Fe3+) ions from solution using β-dicalcium silicate (C2S), α-monocalcium silicate (CS) and calcium hydroxide. The merits of these precipitants were analyzed by studying their effectiveness for removing iron (Fe3+) from solutions of ferric chloride and the properties of the flocs formed. The initial concentration of iron in solution was varied between 10 and 1000 mg l−1 Fe3+ for different additions of precipitant. The residual silicate particles upon hydrolysis appear to enhance the overall removal process in a manner different from the coagulating effect of calcium ions. Considerable enhancement in the rate of removal of iron resulted from use of more finely ground silicates. It is suggested that the reaction can proceed either by the specific adsorption of polynuclear ferric hydroxo complexes on to the surface of negatively charged silica rich layers of C2S, formed by loss of Ca2+ upon hydrolysis or by direct exchange of Fe3+ with Ca2+ in the charged layer. At the end of the second minutes of the settling period, remaining iron concentrations were 12.0 mg l−1 and 525.0 mg l−1, respectively when 3.90 g l−1 of C2S and 2.10 g l−1 of calcium hydroxide were applied to solutions containing 1000 mg l−1 Fe3+. By the end of second minutes of settling period, the remaining iron concentrations were 40 mg l−1 and 78 mg l−1, respectively when 0.39 g l−1 of C2S and 0.21 g l−1 of calcium hydroxide were applied to solutions containing 100 mg l−1Fe3+ in a sedimentation tank designed on the basis of a surface loading rate of 48 m3m−2d−1. In the case of C2S, the specific resistance to filtration and the Capillary Suction Time (CST) values of the sludge produced were lower than that when calcium hydroxide was used thus indicating better dewaterability characteristics. The filter yields of the sludges from C2S and calcium hydroxide treatment of 10, 100 and 1000 mg l−1 iron solutions were estimated as 27.0, 15.8, 23.2 kg m−2h−1 and 12.8, 6.5, 7.8 kg m−2h−1, respectively.