Hydration Characteristics of Portland Cement after Heat Curing: II, Evolution of Crystalline Aluminate-Bearing Hydrates

The development of crystalline aluminate-bearing hydrates in portland cement mortars during water storage at room temperature for periods of up to 1 year after an initial heat cure for 12 h has been observed by quantitative X-ray diffraction analysis and backscattered electron imaging. Ettringite was present in the mortars immediately after a short-term cure at 20° and 60°C, calcium carboaluminate (C₄ĀH₁₁) at 60°C, monosulfate at 85°C, and hydrogarnet at 85°C and above. Ettringite started to form after an induction period ranging from several days to several months after the initial heat cure at 85/100°C, and developed substantially during the period of expansion of the mortar associated with delayed ettringite formation (DEF). Ettringite growth was also observed in the nonexpansive cement mortars. Development of the ettringite bands occurred exclusively in the expansive mortars. Although monosulfate observed in the mortars that had been heat cured at 85°C sometimes increased in amount on initial storage at room temperature, it appeared to vary little in amount for up to 1 year. The amount of hydrogarnet in the heat-cured cement product did not change significantly during storage at room temperature for more than 1 year. DEF expansion of the heat-cured mortars was attributed to ettringite band formation, which started to form at the surface of the cement product and gradually developed inwards.     

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.     

Studies of Early Stages of Paste Hydration of Different Types of Portland Cements

The early stages of hydration of four different types of portland cements were studied by electron-optical and X-ray diffraction techniques. It was observed that, except for low-heat cement, very little ettringite formed up to 3 hours of hydration and that the alite present in the cements was more reactive than the laboratory form. Ettringite formed earlier in the low-heat cement than in other cements. Ettringite was found to be the stable sulfate-bearing phase in sulfateresistant cement, at least up to 30 months, although in other cements ettringite began to change to monosulfate by 14 days. Direct evidence was found for the formation of gypsum from either CaSO₄±0.5H₂O or soluble anhydrite in some cements.     

Retardation of Tricalcium Aluminate Hydration by Calcium Sulfate

The influence of calcium sulfate on the hydration of 3CaO· Al₂O₃ in the presence of Ca(OH)₂ was studied using conduction calorimetry, differential thermogravimetry, and X-ray diffraction. Sodium sulfate was also used instead of calcium sulfate. A substantial retardation of tricalcium aluminate hydration in the presence of sulfate occurs only when calcium sulfate is used and enough ettringite is formed. When ettringite disappears due to the consumption of gypsum, tricalcium aluminate hydration is renewed. Sodium sulfate does not significantly retard this hydration. The results confirm the hypothesis that ettringite formation is essential for coating 3CaO·Al₂O₃ grains and then retarding their hydration.     

Influence of Tertiary Alkanolamines on Portland Cement Hydration

The physical and chemical effects of small additions of two different tertiary alkanolamines to portland cement were investigated. The strengths of standard test mortars moist cured for more than 1 day were found to be enhanced in some cases by addition of triisopropanolamine, but not by similar amounts of triethanolamine. Thermogravimetric and X-ray diffractometric data indicate that the increased mortar strengths resulted from an increased degree of hydration of the cement. Calorimetry and aqueous-phase analysis show that the higher alkanolamine, triisopropauo-lamine, remains in solution for a sufficient time to catalyze hydration of C₄ AF after all of the free gypsum has been consumed to form calcium sulfoaluminate hydrates, In contrast, the lower alkanolamine, triethanolamine, is mostly adsorbed by the cement within the first hours of hydration. It is hypothesized that the catalytic mechanism involves facilitated transport of ferric ions through the aqueous phase in the form of ferric-alkanolamine complexes.     

NMR Relaxation Study of Adsorbed Water in Cement and C3S Pastes

The deuteron and proton spin-lattice and spin-spin relaxation times T ₁ and T ₂ of adsorbed water in commercial portland cement and tricalcium silicate pastes were studied as functions of the hardening time at room temperature. The time dependence of the water self-diffusion coefficient of tricalcium silicate pastes was also followed. The proton and the deuteron T ₁ and T ₂ decrease markedly as hydration increases and the pastes harden due to the increase in the active surface and the number of adsorptive sites, thus providing convenient tools for studying the nature of the hydration process.     

Studies of Early Stages of Paste Hydration of Cement Compounds, II

The paste hydration of mixtures of alite, C₃A, and C₄AF with and without gypsum and/or NaOH was studied by electron-optical and X-ray diffraction techniques. In the absence of both gypsum and NaOH, a foil-like reaction product and hexagonal calcium aluminate hydrates were formed first. Any CO₂, present formed calcium carboaluminate. With time the foils changed to splines of CSH, and hexagonal aluminate hydrates changed to cubic C₃AH₆. When no gypsum was present, NaOH solution retarded the formation of hexagonal aluminates at the very early stages of hydration; it did not have much effect on the later hydration processes. With gypsum but without alkali, a foil-like product and ettringite formed first. Later the foils changed to splines of CSH and ettringite to monosulfate. Alkali, in the presence of gypsum, hastened the formation of splines of CSH. The results suggest that the hydration of alite, even after 14 days, goes through the solution phase.     

Chemistry of the Aqueous Phase of Ordinary Portland Cement Pastes at Early Reaction Times

The chemistry of the aqueous phase of ordinary portland cement paste at early ages (<2 h) has been analyzed in terms of the concentrations of the elemental components in the pore fluid. The concentrations of calcium, sulfur, aluminum, and silicon are rationalized by plotting the data on "phase diagrams." To simplify the analysis, the portland cement system is described using two subsystems: (i) CaO-Al₂O₃-CaSO₄-H₂O, modified by the presence of sodium and potassium, and (ii) CaO-SiO₂-H₂O. During the first 10 min of hydration, the calcium, sulfur, and aluminum concentrations all decrease, roughly in proportion, which suggests a precipitation process, a conversion of calcium sulfate hemihydrate to gypsum, and the initial formation of ettringite. The CaO-Al₂O₃-CaSO₄-H₂O subsystem seems to move from a phase assemblage of gypsum, Al₂O₃·3H₂O, and ettringite to an assemblage of gypsum, calcium hydroxide, and ettringite during the first 15-30 min after the water and the cement are mixed. The silicate equilibrium is approached more slowly. The intensity of mixing has relatively little effect on the concentrations beyond the first few minutes.     

Cement Composition Effects on the Rheological Property Evolution in Concentrated Cement–Polyelectrolyte Suspensions

We have studied the rheological property evolution and hydration behavior of white and ordinary portland cement (type I) pastes and concentrated cement–polyelectrolyte suspensions. Cement composition had a marked effect on the elastic property evolution ( G '( t )) and hydration behavior of these suspensions in the presence of poly(acrylic acid)/poly(ethylene oxide) copolymer (PAA/PEO), even though their affinity to adsorb such species was nearly identical. Both white and ordinary portland cement pastes exhibited G '₀ values of ∼10⁴ Pa and fully reversible G '( t ) behavior until the onset of the acceleratory period ( t = 2 h), where the pastes stiffened irreversibly. In contrast, cement–PAA/PEO suspensions exhibited G '₀ values of ∼1 Pa and G '( t ) behavior comprised of both reversible and irreversible features. Interestingly, ordinary portland cement–PAA/PEO suspensions experienced a gel-to-fluid transition on high shear mixing at short hydration times (<1 h), and the particle network did not rebuild until ∼24 h of hydration. In sharp contrast, white portland cement–PAA/PEO suspensions remained weakly gelled throughout the initial stage of hydration even after high shear mixing. At longer hydration times (>1 h), both cement–PAA/PEO suspensions exhibited G ' _i ( t ) ∼ exp( t /τ_c) with τ_c values of 5.6 and 1.3 h for ordinary and white portland cement, respectively. Our observations suggest that hydration phenomena impact interparticle forces during early stage hydration and, ultimately, lead to initial setting through the formation of solid bridges at the contact points between particles within the gelled network.     

Formation of Ettringite Immediately After Gaging of a Portland Cement

The formation of ettringite during the initial stages of hydration in commercial portland cement was determined by electron microscopy (diffraction and replica techniques) and by infrared spectrography. The initially formed ettringite is stabilized by a high So_d^2- concentration in the liquid phase of the cement paste. Hemihydrate and potassium carbonate acted as stabilizers while providing a high sulfate concentration in the liquid phase.     

29Si Magic-Angle-Spinning Nuclear Magnetic Resonance Study of Hydrated Cement Paste and Mortar

This paper presents ²⁹Si magic-angle-spinning nuclear magnetic resonance measurements that trace the cement hydration process in cement paste and mortar specimens made from ordinary portland cement, type I. These specimens were moist-cured for 3, 7, 14, and 28/31 d at temperatures ranging from 21° to 80°C. Compressive strength for all tested specimens was also determined. The results show that the degree of hydration ( Q ¹+ Q ²) and the compressive strength increase with curing times and temperatures. However, at 80°C, the compressive strength decreases while the degree of hydration increases.     

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.     

Structure and Properties of Portland Cement Clinker Doped with Zinc Oxide

ZnO added to the raw meal accelerates the rate of portland clinker formation. Due to ZnO doping, the amount of alite and C₂(AF) formed increases at the expense of belite and C₃A. The ZnO is preferentially taken up by the interstitial phase. The initial rate of tricalcium silicate hydration is retarded and the formation of ettringite is moderately accelerated in cements made from ZnO-doped clinkers. The set time of these cements is gradually prolonged and their strength development retarded with increasing degrees of ZnO doping.     

Si MAS-NMR Study of Hydrated Cement Paste and Mortar Made with and without Silica Fume

This paper presents ²⁹Si MAS-NMR measurements that trace the hydration process in both cement paste and mortar specimens made from ordinary portland cement, Type I, when the cement content is replaced by 0, 10, 15, and 20 wt% of silica fume. The specimens were moist-cured for 3, 7, 14, 28, 90, and 180 days at a laboratory temperature of 21°C (69.8°F). Compressive strength for all tested specimens was also determined. The results show that the degree of hydration (Q¹+ Q²)/(Q°+ Q¹+ Q²) increased with increasing content of silica fume, especially at the early ages of 3 to 28 days. In the same manner, compressive strength results were markedly increased up to 14 days and were lowered at later ages, compared to the control mix (0 wt% silica fume).     

Chemical Speciation of Trace Zinc in Ordinary Portland Cement Using X-ray Absorption Fine Structure Analysis

Chemical change of trace zinc in ordinary portland cement (205.1 ppm) was investigated in hydration process using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Intensities of the peaks appearing at the same energy of ZnO in XANES spectra were decreased with cement hydration. The interatomic distances and the coordination numbers of the first and the second shells calculated from EXAFS spectra indicated that ZnO hydrolyzed to zincate ion [Zn(OH)₄]²⁻ with cement hydration keeping their fundamental structure of ZnO₄ tetrahedra.     

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.     

Studies of Early Stages of Paste Hydration of Cement Compounds, I

The paste hydration of C₄AF with and without the addition of lime and/or gypsum was studied by means of electron-optical and X-ray diffraction techniques. It was found that in the paste hydration of C₄AF iron did not separate out but remained in the structure of the hydrated compounds, probably substituting for aluminum. In a neat paste of C₄AF and H₂O, at first di- and tetracalcium aluminate hydrate crystals formed, but later cubic C₃AH₆ crystals appeared. In the presence of Ca(OH)₂, C₂AH_x crystals did not form; otherwise the course of the reaction was the same as without lime. Any CO₂ present formed monocarboaluminate. When gypsum alone was present, ettringite crystals first appeared which later changed to monosulfate hydrate. In the presence of both lime and gypsum, the reactivity of C₄AF was much reduced. Ettringite crystals were somewhat stabilized by lime. The formation of monocarboaluminate was hindered in the presence of gypsum.     

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     

The Effects of Prehydration on the Liquid Hydration of 3CaO·Al2O3 with CaSO 4·2H2O

Liquid hydration and water-vapor hydration of 3CaO·Al₂O₃, were studied. Variable parameters were hydration time, temperature, relative humidity, and amount of gypsum. The hydration products (gel, ettringite, hexagonal hydrates, and 3CaO·Al₂O₃·6H₂O) were studied by electron microscopy, X-ray diffractometry, and thermal analysis. A reaction scheme is proposed. The degree of water-vapor hydration influenced the sequence of the subsequent liquid hydration which, however, was independent of the composition of the water-vapor hydration products. Below a critical degree of water-vapor hydration (≊3% combined water) the reaction with liquid water occurred as if no water-vapor hydration had taken place. Above this value the reaction gave hydration products suggesting a change of the 3CaO·Al²O³ reactivity. A possible correlation with the retardation of strength development of prehydrated cement is suggested.     

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.