Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Thermal Study of the Effect of Additives on the Hydration of Tricalcium Silicate

The rate of paste hydration of 3C_aO·SiO₂ (C₃S) and the effects of additions of CaCl₂, CdI₂, and CrCl₃, were studied by differential thermal analysis and thermogravimetry. X-ray analyses were used to identify the synthesized C₃S. The salts CaCl₂, CdI₂, and CrCl₃, accelerated the hydration of C₃S. The degree of hydration was estimated by the amount of Ca(OH)₂, formed, as determined by TG.     

Characterization of C-S-H from Highly Reactive β-Dicalcium Silicate Prepared from Hillebrandite

β-dicalcium silicate synthesized by thermal dissociation of hydrothermally prepared hillebrandite (Ca₂(SiO₃)(OH)₂) exhibits extremely high hydration activity. Characterization of the hydrates obtained and investigation of the hydration mechanism was carried out with the aid of trimethylsilylation analysis, ²⁹Si magic angle spinning nuclear magnetic resonance, transmission electron microscopy selected area electron diffraction, and XRD. The silicate anion structure of C-S-H consisted mainly of a dimer and a single-chain polymer. Polymerization advances with increasing curing temperature and curing time. The C-S-H has an oriented fibrous structure and exhibits a 0.73-nm dreierketten in the longitudinal direction. On heating, the C-S-H dissociates to form β-C₂S. The temperature at which βC₂S begins to form decreases with increasing chain length of the C-S-H or as the Ca/Si ratio becomes higher. The high activity of β-C₂S is due to its large specific surface area and the fact that the hydration is chemical-reaction-rate-controlled until its completion. As a result, the hydration progresses in situ and C-S-H with a high Ca/Si ratio is formed.     

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.     

Application of Selective 29Si Isotopic Enrichment to Studies of the Structure of Calcium Silicate Hydrate (C-S-H) Gels

Selective isotopic enrichment of SiO₂ with ²⁹Si in a mixture with tricalcium silicate (C₃S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q₂ signal has been observed in ²⁹SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C₃S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure.     

Highly Reactive β-Dicalcium Silicate: III, Hydration Behavior at 40–80°C

The effect of curing temperature (40°, 60°, 80°C) on the hydration behavior of β-dicalcium silicate (β-C₂S) was investigated. The β-C₂S was obtained by decomposition of hillebrandite, Ca₂(SiO₃)(OH)₂, at 600°C, has a specific surface area of about 7 m²/g, and is in the form of fibrous crystals. The dependence of the hydration reaction on temperature continues until the reaction is completed. The hydration is completed in 1 day at 80°C and in 14 days at 14°C. The hydration mechanism is different above and below 60°C, but at a given temperature, the reaction mechanism and the silicate anion structures of C-S-H do not change significantly from the initial to the late stages of the reaction. High curing temperature and long curing times after completion of reaction promote silicate polymerization. The Ca/Si ratio of C-S-H shows high values, being almost 2.0 above 60°C and 1.95 below 40°C.     

Slurry Consistency and In Situ Synchrotron X-Ray Diffraction During the Early Hydration of Portland Cements With Calcium Chloride

Class A and H oil well cements are compared at 25° and 50°C with 0%, 1%, 2%, and 4% CaCl₂. Up to 4% CaCl₂ accelerated Class A thickening, but 4% led to slower thickening than 2% for Class H. C₃S hydration in the two cements responded differently to CaCl₂. CaCl₂ always accelerated aluminate hydration. For Class A, CaCl₂ accelerated early Ca(OH)₂ precipitation, but sometimes reduced the amount at longer times. This may be coupled to C–S–H gel composition changes. For Class H, Ca(OH)₂ precipitation changes nonlinearly with CaCl₂ concentration. Ettringite to monosulfate conversion and Friedel's salt formation were sometimes seen.     

Highly Reactive β-Dicalcium Silicate: V, Influence of Specific Surface Area on Hydration

The hydration of β-C₂S prepared from hillebrandite [Ca₂(SiO₃)(OH)₂] and having specific surface areas of 6.8, 5.5, and 3.1 m²/g was investigated. Different specific areas were obtained by varying the dissociation temperature of hillebrandite. In addition, the hydration of β-C₂S synthesized from high-temperature solid-state reaction was also studied as a comparison. The specific surface area exerts a strong influence on the hydration rate, which increases as the surface area increases. The degree of influence changes with the reaction, becoming greater as hydration progresses. There is initially a linear relationship between specific area and the time required to complete a specific reaction. The specific surface area also affects the reaction mechanism. In the case of specific areas of 5.5 m²/g or less, the reaction changes from a chemical reaction to a diffusion-controlled one, and the degree of reaction comes almost to a halt at 80% to 85%. The Ca/Si ratios of hydrate and the silicate anion structures were also investigated in this study.     

Highly Reactive β-Dicalcium Silicate: I, Hydration Behavior at Room Temperature

The hydration behavior at 25°C of β-dicalcium silicate synthesized from hillebrandite (Ca₂,(SiO₃)(OH)₂) at 600°C was studied over a period of 224 d. The hydration rate of the β-dicalcium silicate having fibrous crystals with specific surface area of 7 m²/g is extremely rapid. For water/solids ratios of 0.5 and 1.0, the hydration reaction is completed in 28 and 14 d, respectively. The hydrate contains almost no Ca(OH)₂, and its Ca/Si ratio is close to 2. SEM observations indicate that the hydrate forms an outer shell on the surface of β-dicalcium silicate and grows inwards. The silicate anion structure is considered to consist of dimers and single-chain structures from ²⁹Si MAS NMR. Variations of physical properties of press-formed bodies have also been discussed.     

Synthesis and Hydration Characteristics of Alinite Cement

A chlorine-bearing alinite cement was synthesized using reagent-grade chemicals, and the phase evolution and hydration behavior of the alinite clinker were examined. The effects of the MgO content on alinite formation and hydration also were investigated. Alinite began to appear at 1000°C from β-C₂S, C₁₁A₇CaCl₂, and unreacted raw materials, and an almost single-phase alinite was obtained at 1300°C. The alinite phase also was produced without MgO addition. However, CaO, β-C₂S, and C₁₁A₇CaCl₂ phases were present. Alinite cements hydrated rapidly after a short incubation period, and the hydration products were C-S-H gels, Ca(OH)₂, and a Fridel's saltlike phase. The local environmental changes of silicon and aluminum during the formation and hydration of alinite were determined using magic-angle-spinning nuclear magnetic resonance spectroscopy. The Cl⁻-ion exsolution from the alinite paste during hydration was measured using ion chromatography.     

Approach by zeta-potential measurement on the surface change of hydrating C3S

The adsorption of Ca²⁺ on C₃S surface and the formation of C-S-H in the induction period of C₃S hydration were investigated by measuring zeta-potential, heat evolution change and by analyzing dissolved ions. Within 20 minutes, the dissolution of Ca²⁺ and silicate ions and the adsorption of Ca²⁺ on the surface of C₃S were alternately repeated. In the induction period (20 50 minutes), Ca²⁺ was adsorbed fairly strongly on C₃S surface giving positive zeta-potentials and the dissolution stopped for a time. In the hydration between 60 and 140 minutes, the C-S-H gel proceeded to crystallize and Ca²⁺ began to dissolve again, giving negative zeta-potentials by Si-O⁻. After 140 minutes, the second evolution was caused by the crystallization of C-S-H and Ca(OH)₂.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Chemical Composition of C-S-H Gel Formed in the Hydration of Calcium Silicate Pastes

The chemical and physical compositions of C-S-H formed in paste hydration of C₃S cured at 4°, 25°, and 65°C and of C₂S and a C₃S/C₂S blend cured at 25°C were studied using quantitative X-ray diffraction analysis (QXDA), extraction, and thermal effluent gas analysis (TEGA) to determine the free CH content. The amount of CH determined by extraction is greater than or equal to that determined by QXDA. The difference between the two results increased with the H₂O surface area of the paste, indicating the existence of adsorbed CH in the form of Ca²⁺ and (OH)⁻ ions which cannot be measured by QXDA and suggesting that the extraction method is better for estimating the composition of C-S-H.     

MAS NMR Studies of Partially Carbonated Portland Cement and Tricalcium Silicate Pastes

²⁹Si, ²⁷Al, and ¹H MAS NMR studies of partially carbonated mature ordinary Portland cement (OPC) and tricalcium silicate (C₃S) pastes have been carried out. The water-to-solid ratios ( W/S ) have been varied between 0 and 1 at hydration temperatures of 23^o and 90^oC. Various Q ⁿi units with n =0, 1,2,3, and 4, and a Q³ (1Al) group have been identified using ²⁹Si NMR. Cross-polarization experiments, in addition, have made it possible to assign the OH groups. Two types of fourfold- and one type sixfold-coordinated aluminum have been distinguished using ²⁷Al NMR. In C₃S pastes for w/s >0.7, progressive carbonation leads to a nearly perfect three-dimensional network consisting of Q³ and Q⁴only. In contrast, in OPC pasted only about 40% of the highly polymerized silicate units are formed, partially copolymerized with AlO₄ tetrahedra.     

Chemically Modified Water-Glass Binders for Acid-Resistant Mortars

Water-glass as a binder is used in acid and fire-proof applications. A chemical modification influences the curing rate or characteristics like the flexural and compressive strength. The aim of this work is to investigate the influence of the addition of C₃S, C₃A, and Na₃(PO₃)₃ on the chemical durability of cured sodium silicate mortars. The results show that the addition of clinker minerals as accelerator reduces chemical durability and makes these binders unusable for acid-proof applications. The addition of Na₃(PO₃)₃ makes the sodium silicate binder durable against water and acid attacks and triples the compressive strength.     

Si NMR Study of the Hydration of Ca3SiO5 and ß-Ca2SiO4 in the Presence of Silica Fume

²⁹Si magic-angle spinning nuclear magnetic resonance (MASNMR) was used to study the room-temperature hydration of C₃S, ß-C₂S, and reactive ß-C₂S mixed with different amounts of silica fume (SF) that had been hydrated up to nine months and longer. The overall CaO:SiO₂ molar ratios of the mixes were 0.12, 0.20, 0.35, 0.50, and 0.80. NMR spectroscopy was used to quantify the remaining starting materials and the resulting hydration products of different species. A broad peak assigned to Q³, appearing in both the fourier transform (FT) and the cross-polarization (CP) modes, increased in intensity with increased SF content and with age. This Q³ species was attributed to two sources: (1) the surface hydroxylation of SF and (2) the cross-linking of dreierketten (chains of silicate tetrahedra arranged in a repeating three-unit conformation) in the calcium silicate hydrate (C-S-H) structure. A Q⁴ species also appeared in the CP spectra of samples with large SF additions after extended hydration and was attributed to cross-polarization by adjacent hydroxylated Q³ species at the surface of amorphous SiO₂.     

Effect of Temperature and Water-Solid Ratio on Growth of Ca(OH)2 Crystals Formed During Hydration of Ca3SiO5

The growth behavior, time of nucleation, and morphology of Ca(OH)₂ crystals formed during the hydration of Ca₃SiO₅, at 15°, 25°, and 35°C at water-solid ratios ( w/s ) from 0.3 to 5.0 were studied by optical microscopy. In samples with w/s >0.5 growth of Ca(OH)₂ in the c -axis direction is initially dominant. Growth in this direction ends after a few hours, but growth perpendicular to the c axis continues for several days and produces a dendritic morphology. Growth behavior is not so well defined for w/s <0.5, in part because of the large number of unhydrated particles engulfed. Increasing temperature resulted in an increase in the number of Ca(OH)₂ nuclei and a decrease in nucleation time and crystal size. Increasing the w/s ratio improved the euhedral character of the Ca(OH)₂ crystals, decreased the number of engulfed Ca₃SiO₅ particles, and increased the nucleation time. Dendritic morphology was most pronounced in the samples for which w/s = 1. Growth rates and the ultimate size of the Ca(OH)₂ crystals varied within a given sample. The effects of temperature and the w/s ratio on the heat evolved during the hydration were studied by isothermal calorimetry. The times of nucleation of crystalline Ca(OH)₂ estimated from calorimetry were similar to those derived from growth curves determined by optical microscopy.     

Structure Change in Strontium Oxide-Doped Dicalcium Silicates

A series of strontium-bearing dicalcium silicate (Ca₂Si0₄ solid solutions (C₂S( ss )), (Sr_χCa_1-χ)₂SiO₄ with 0.02 ≤χ≤ 0.10, was prepared and examined by powder X-ray diffrac-tometry. These crystals, heated in the stable-temperature region of the α phase and then quenched in water, were composed of the β phase, with χ≤ 0.08, and the α'_L and β phases, with χ= 0.10. With increasing x, the unit-cell axes of the β phase expanded and the β angle became small with eventual increase in the unit-cell volume. The Rietveld analysis of the β-C₂S( ss ) with χ= 0.08 showed that the Sr²⁺ ions preferentially occupied the seven-coordinated site rather than the eight-coordinated site. This site preference, which was originally established in the parent α-phase structure, seemed to cause the systematic change in the cell dimensions.     

Vibrational Spectra of Hydrothermally Prepared Hydroxyapatites

Apatites can be used as bioceramic materials for tooth and bone implants. Infrared (4000 to 400 cm⁻¹) and laser-Raman spectra were obtained for hydrothermally prepared hydroxyapatites with the general formula X₁₀(PO₄)₆(OH)₂ where X= Ca²⁺ Sr²⁺, Ba²⁺, or Pb²⁺ Fundamental vibrational modes were identified and empirical band assignments made. Analysis of the vibrational spectra led to the reassignment of the v₂ mode to a higher wave-number, lower-intensity band (e.g. ∼470 cm⁻¹ for Ca hydroxyapatite). Effects of solid-state interaction were noted in the vibrational spectra and interpreted by factor-group analysis. An "effective" factor group C_6h, which neglects the locations of the OH⁻ ions, best describes the vibrational spectra of the hydroxyapatites.     

Hydration Kinetics and Phase Stability of Dicalcium Silicate Synthesized by the Pechini Process

Reactive dicalcium silicate (Ca₂SiO₄) has been synthesized by the Pechini process, and hydration kinetics studied. With increasing calcination temperature, the amorphous product first crystallizes to α'_L-phase and subsequently to the ß- and γ-phases. The specific surface area, ranging from 40 to 1 m²/g, strongly depends on the calcination temperature of 700°-1200°C for 1 h. Samples with a high surface area have a high water demand; a water/cement ratio >2.0 is required to produce formable pastes in some instances. Hydration kinetics are determined by XRD, ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR), and differential scanning calorimetry/thermogravimetry (DSG/TG). The hydration rate depends only on the surface area, not on the polymorph. Complete hydration occurs in as early as 7 d. Very little calcium hydroxide (Ca(OH)₂) is formed in the most reactive specimens (calcined at 700° and 800°C), which indicates the Ca/Si ratio in C-S-H gels is ∼2.0, but more Ca(OH)₂ forms from samples calcined at higher temperature. The silicate structure of the hydrated Ca₂SiO₄ pastes is investigated using ²⁹Si MAS NMR spectroscopy and trimethylsilylation analysis.