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.     

Formation Processes of β-C2S by the Decomposition of Hydrothermally Prepared C-S-H with Ca(OH)2

A mixture of CaO and silicic acid prepared with a Ca/Si ratio of 2.0 was hydrothermally synthesized at 80° to 200°C, and the thermal decomposition behavior of the products (C-S-H with Ca(OH)₂) was analyzed using XRD, ²⁹Si MAS NMR, and the trimethylsililation method (TMS). It was found that the main silicate anion structure of C-S-H was a mixture of a dimer and a single-chain polymer (larger than Si₅O₁₆) and that polymerization advanced with an increase of the synthesizing temperature. On heating, the products decomposed to form β-C₂S. It was found that the decomposition was gradual and that the-higher the temperature of hydrothermal synthesis, the lower was the temperature of the decomposition into β-C₂S. Although the decomposition proceeded to form a monomer (β-C₂S) from the polymer and dimer, this dimer was resistant to heat and did not decompose unless heated to above 400°C.     

α-Dicalcium Silicate Hydrate: Preparation, Decomposed Phase, and Its Hydration

α-C₂SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390–490°C, α-C₂SH dissociates through a two-step process to form an intermediate phase plus some γ-C₂S. This appears to be a new dicalcium silicate different from known dicalcium silicates—α, α'_L, α'_H, β, and γ phase—and is stable until around 900°C. At 920–960°C, all the phases are transformed to the α'_L phase. The intermediate phase has high crystallinity and is stable at room temperature. ²⁹ Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the α'phase at higher temperature yields β-C₂S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day ( w/s = 1.0, 25°C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields β-C₂S at the lowest temperature.     

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.     

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.     

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.     

Reaction of Beta-Dicalcium Silicate and Tricalcium Silicate with Carbon Dioxide and Water Vapor

The carbonation-reaction kinetics of beta-dicalcium silicate (2CaO·SiO₂ or β-C₂S) and tricalcium silicate (3CaO. SiO₂ or C₃S) powders were determined as a function of material parameters and reaction conditions and an equation was developed which predicted the degree of reaction. The effect of relative humidity, partial pressure of CO₂, surface area, reaction temperature, and reaction time on the degree of reaction was determined. Carbonation followed a decreasing-volume, diffusion-controlled kinetic model. The activation energies for carbonation of β-C₂S and C₃S were 16.9 and 9.8 kcal/mol, respectively. Aragonite was the principal carbonate formed during the reaction and the rate of carbonate formation was coincident with depletion of the calcium silicates; C-S-H gel formation was minimal.     

Reaction of Hydraulic Calcium Silicates with Carbon Dioxide and Water

The carbonation of wetted powders of beta-dicalcium silicate (β·2CaO·SiO₂=β-C₂S) and tricalcium silicate (3CaO·SiO₂= C₃S) was studied as a function of reaction conditions. The water/solids ratio is an important parameter and there is an optimum value for each silicate. Relative humidity and the partial pressure of CO₂ also strongly affect the reaction. The rate of carbonation can be conveniently represented by plotting the degree of carbonation against the logarithm of time. C-S-H and calcite are the initial reaction products. Subsequently, carbonation of the C-S-H produces silica gel, whereas aragonite may form if the system is allowed to dry out.     

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.     

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.     

Highly Reactive β-Dicalcium Silicate: II, Hydration Behavior at 25°C Followed by 29Si Nuclear Magnetic Resonance

The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca₂(SiO₃)(OH)₂) was studied over a period of 7 to 224 d using ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q² and Q¹ silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q¹ and Q² formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q²/Q¹ increases as the reaction progresses and as the curing time becomes longer. From the values of Q²/Q¹, it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q²/Q¹ ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD.     

The Pore Structure of Hydrated Cementitious Compounds of Different Chemical Composition

The pore structure ofβ-C₂S, C₃S, and portland cement pastes was investigated using mercury porosimetry and H₂O and N₂ adsorption. The β-C₂S had more total macro- and mesoporosities than C₃S and portland cement pastes of a similar degree of hydration. C₃S and portland cement pastes had similar total porosities but differed in the porosity size distribution. In the mesopore range, the various test methods gave different results. These differences are discussed on the basis of the various models proposed for cement paste. It is shown that shrinkage could be correlated with the volume of pores <0.03 μm, but not with total porosity.     

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.     

Highly Reactive β-Dicalcium Silicate: IV, Ball-Milling and Static Hydration at Room Temperature

The ball-milling hydration of highly reactive β-Ca₂SiO₄ synthesized from hillebrandite, Ca₂(SiO₃)(OH)₂, was investigated and compared with its static hydration under a water/solid ratio of 10. In static hydration, the hydration is completed in 8 d to yield C-S-H with a Ca/Si ratio of 1.81 and Ca(OH)₂. There is no major change after this period. In the case of ball milling, the hydration is completed in 2 d to yield C-S-H with Ca/Si = 1.81 and Ca(OH)₂. After this, the two products react to form a C-S-H(II)-like monophase hydrate having a Ca/Si ratio of 1.98. The morphology and structure of this hydrate are different from those of the earlier hydrate, and afwillite is not formed. ²⁹Si MAS NMR measurements indicated the C-S-H to be a mixture of dimers and single-chain structures. Dimeric species are the main species up to completion of the reaction, but polymerization progresses very rapidly after the completion.     

Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

Structural Properties of Calcium Silicate Pastes: I, Effect of the Hydrating Compound

The chemical and physical properties of C₃S, β-C₂S, a C₃S/C₂S blend, and portland cement pastes cured at 25°C were investigated. The H₂O specific surface areas of the calcium silicate samples follow a common linear relation when plotted against a CIS ratio. The β-C₂S had higher capillary porosity and N2 surface area, resulting from increased mesopore volume at the expense of micropores. All calcium silicate pastes had similar polysilicate content vs time curves, indicating an aging process which is not sensitive to the starting composition of the hydrating calcium silicate. The polysilicate content of portland cement was much lower than that of the corresponding calcium silicate pastes. Strength-capillary porosity relations for the various systems are discussed.     

Adsorption of Admixtures on Portland Cement Hydration Products

The adsorption of calcium lignosulfonate and salicylic acid was studied on the hydration products of the four principal components of portland cement. To investigate the adsorption as a function of development of hydration product, the determinations were made after varying hydration times. The times allowed were from 5 min to 24 hr for tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF) and from 1 hr to 28 days for β-dicalcium silicate (β-C₂S) and tricalcium silicate (C₃S). Samples were characterized with respect to surface area and poresize distribution. The effect of gypsum on the adsorption was also investigated. The results indicate that the amounts of salicylic acid and calcium lignosulfonate adsorbed on the hydration products of C₃A, and of calcium     

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.     

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.     

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.