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.     

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     

Substituted Hydrated Calcium Silicates Obtained in Autoclave Hydration

Hydrated calcium silicates containing Al^3+] or Fe^3+] were prepared by autoclaving C₃S and β-C₂S in the presence of C₃A or C₂F at 190°C. Al^3+] and Fe^3+] diffuse into the crystal lattice of α-C₂SH and C₃SH_1.5. Solid solutions containing Al^3+] and Fe^3+] were placed in contact at 25°C with sources of sulfates, either in aqueous stirred suspensions or as pastes. Al^3+] and Fe^3+] remain stable in the solid solution, inhibiting the formation of ettringite. This absence of ettringite can explain the resistance of autoclaved cement pastes and concretes to sulfate attack.     

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.     

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.     

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.     

Saturated Solutions of Anhydrous Phases in the System Lime-Silica-Water: Example of β-C2S

Saturated solutions rerely form when the anhydrous constituents of aluminous and portland cement are stirred in water or in lime solutions of increasing concentrations. Apart from monocalcium aluminate, concentration of ions in solution cannot exceed maximum supersaturation with respect to the hydrate most likely to precipitate. The present work shows such a behavior for β-C₂S suspended in water and in lime solutions at low concentration. In more concentrated lime solutions, a short lifetime saturation state with respect to β-C₂S seems to be reached.     

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.     

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.     

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.     

29Si MAS NMR Study of Dicalcium Silicate: The Structural Influence of Sulfate and Alumina Stabilizers

The effect on β-C₂S of two stabilizing agents, calcium sulfate and alumina, has been investigated using high-resolution ²⁹Si solid state NMR spectroscopy. Syntheses were achieved via the gel route, wet or dry processes. Room-temperature NMR spectra characteristics were analyzed as a function of the sintering temperature. The incorporation of Al³⁺ and S⁶⁺ ions, which finds expression in a noticeable line broadening, is shown to be effective above 1200°C. The ²⁹Si chemical shift is unchanged upon doping, suggesting a mean SiO₄ tetrahedra geometry identical to that in pure β-C₂S. General trends on the structure adopted by C₂S upon Al³⁺ and S⁶⁺ doping are also discussed.     

Specific Surface of Hardened Portland Cement Paste as Determined by Small-Angle X-Ray Scattering

Small-angle X-ray scattering was used to determine the specific surfaces of hardened portland cement pastes subjected to various treatments. Reasonably mature pastes yielded values of the order of 700 m²/g of ignited paste when examined in the original saturated condition. Drying to an intermediate relative humidity, P-drying, and D-drying reduce the surface areas to values of the order of 200 to 300 m²/g, but vacuum resaturation results in recovery of all the lost surface. Oven drying produces an additional loss in surface area, and in this case recovery on resaturation is not quite complete. The effect of w/c ratio appears to be minor, and pastes of hydrated C₃S yield essentially the same surface areas as corresponding portland cement pastes.     

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.     

Preparation of Portland Cement Components by Poly(vinyl alcohol) Solution Polymerization

The four components portland cement-dicalcium silicate, C₂S (Ca₂SiO₄); tricalcium silicate, C₃S (Ca₃SiO₅); tricalcium aluminate, C₃A (Ca₃Al₂O₆); and tetracalcium aluminate iron oxide, C₄AF (Ca₄Al₂Fe₃O₁₀)-were formed using a solution-polymerization route based on poly(vinyl alcohol) (PVA) as the polymer carrier. The powders were characterized using X-ray diffraction techniques, BET specific surface area measurements, and scanning electron microscopy. This method produced relatively pure, synthetic cement components of submicrometer or nanometer crystallite dimensions, high specific surface areas, as well as extremely high reactivity at relatively low calcining temperatures. The PVA content and its degree of polymerization had a significant influence on the homogeneity of the final powders. Two types of degree of polymerization (DP) PVA were used. Lower crystallization temperatures and smaller particle size powders were obtained from the low-DP-type PVA at optimum content.     

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.     

A New Approach to Modeling the Nucleation and Growth Kinetics of Tricalcium Silicate Hydration

The hydration kinetics of tricalcium silicate (C₃S), the main constituent of portland cement, were analyzed with a mathematical "boundary nucleation" model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C₃S particle surfaces. This model more closely approximates the C₃S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C₃S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C₃S pastes in the temperature range of 10°–40°C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the "induction period" is not a separate chemical process as has often been hypothesized.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

Effect of Phosphorus Impurity on Tricalcium Silicate T1: From Synthesis to Structural Characterization

Alite is the major compound of anhydrous Portland cement: it is composed of tricalcium silicate Ca₃SiO₅ (C₃S) modified in composition and crystal structure by ionic substitutions. Alite is also the main hydraulic phase of cement and the most important for subsequent strength development. Using raw meals (rich in Ca₃P₂O₈) as alternative fuels in cement plants raises the question about the effect of phosphorus on C₃S and its consequences on reactivity with water. This paper deals with a systematic study of C₃S triclinic T₁ polymorph doped with P₂O₅ in the range 0–0.9 wt%. All the samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electron-microprobe analysis. The appearance of a phase rich in phosphorus is shown. It displays a structure derivative of the α'_H–Ca₂SiO₄ polymorph, noted α'_H–C₂S(P). As phosphorus content increases, C₃S is more and more decomposed into free lime and α'_H–C₂S(P). The α'_H phase was detected from 0.1 wt% P₂O₅ and located at the interfaces of C₃S grains. Two identification keys are proposed in order to highlight the α'_H–C₂S(P) phase: the XRD angular window at 2θ_Cu=32.8°–33.2° and a smooth aspect on SEM micrographs.     

Effects of Saccharide Set Retarders on the Hydration of Ordinary Portland Cement and Pure Tricalcium Silicate

The effects of aliphatic sugar alcohols (e.g., threitol, xylitol, sorbitol) on the hydration of tricalcium silicate (C₃S) and ordinary portland cement (OPC) were investigated and compared with those of sucrose, a well-established cement set retarder. Only sugar alcohols which contain threo diol functionality retarded the setting of C₃S and OPC, their efficacy increasing with the number of threo hydroxy pairs and, to a smaller extent, with the overall population of hydroxy groups. None, however, were as effective as sucrose. The initial and final setting times increased exponentially with the concentration of saccharide, although the hydration of OPC was less inhibited than that of C₃S. Saccharides function as "delayed accelerators," that is, cement hydration is first inhibited and then proceeds faster than in saccharide-free cement. This behavior is consistent with the theory that the induction period is controlled by slow formation and/or poisoning of the stable calcium silicate hydrate (CSH) nuclei. The early inhibiting influence of saccharides on CSH precipitation is apparently stronger than on the growth of crystalline calcium hydroxide. Saccharides did not negatively affect the degree of hydration and compressive strength of fully set OPC paste; on the contrary, sorbitol yielded modest increases.     

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.