Hydration of cement — role of triethanolamine

Following addition of 0.1, 0.25, 0.35, 0.5 and 1.0 per cent triethanolamine, studies have been made of the hydration and hardening characteristics of (a) tricalcium aluminate, (b) tricalcium aluminate + gypsum, (c) tricalcium silicate, (d) dicalcium silicate, and (e) portland cement. Triethanolamine (TEA) accelerated the hydration of 3CaO.Al₂O₃ and 3CaO.Al₂O₃-CaSO₄.2H₂O systems and extended the induction period of the hydration of 3CaO.SiO₂. In portland cement paste TEA decreased the strength at all ages and setting characteristics were drastically altered, especially at higher TEA contents. Evidence was obtained also of the formation of a complex of TEA with the hydrating silicate phase.     

Hydration of cubic tricalcium aluminate in the presence of calcined clays

Calcined clay blended cements play a major role in cement industry's strategy to reduce CO₂ emissions. During their hydration, an accelerated aluminate reaction is often observed to affect the sulfate balance. The objective of this study was to provide insights into the influence of different calcined clays on the hydration of cubic tricalcium aluminate (C₃A). A cementitious model system consisting of cubic C₃A, quartz powder, calcium sulfate and a model pore solution was investigated. The influence of three different calcined clays and one nanolimestone was examined by a successive replacement of the quartz powder and variation of the calcium sulfate. Heat flow and hydrate phase development were followed by isothermal calorimetry and quantitative in-situ X-ray diffraction. Accelerated ettringite formation and sulfate depletion were observed for all calcined clays, while the nanolimestone exhibited the opposite effect. It was found that adsorption of SO₄ ions and/or Ca-SO₄-complexes at the surface of calcined clay particles is the main factor inhibiting retardation of the C₃A hydration in absence of a silicate reaction. In the Al-rich systems a retardation through sulfate adjustment seems to be impeded by additional Al ions, which react with Ca adsorbed onto and leached from the C₃A surface.     

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater-mixed C3A–gypsum system

This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C₃A) in C₃A–gypsum and C₃A–gypsum–Ca(OH)₂ systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C₃A and solubility of gypsum, which accelerated the early hydration of C₃A and the formation of ettringite (AFt), leading to a higher hydration degree of C₃A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)₂, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO₄–AFm) and the formation of hydrogarnet (C₃AH₆), which would further reduce the hydration degree of the C₃A at the later ages. However, in the presence of Ca(OH)₂, the hydration degree of C₃A–gypsum–Ca(OH)₂ at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW-mixed cement concrete can affect the hydration degree of C₃A in cement.     

Initial hydration of tricalcium silicate as studied by secondary neutrals mass spectrometry: II. Results and discussion

The hydration of tricalcium silicate (C₃S) was studied by secondary neutrals mass spectrometry (SNMS), a method that enables determination of the Ca/Si ratio of the formed calcium silicate hydrate (C-S-H) phase with an extremely low information depth. It was found that the magnitude of this parameter within the hydrate layer formed at the surface of the nonhydrated C₃S is not constant and increases with increasing distance from the liquid-solid interface. It was also found that, at a constant distance from the surface, the Ca/Si ratio declines with hydration time. The kinetics of the hydration process is characterized by a very fast initial reaction, followed by a dormant period and a subsequent period of renewed hydration. The rate of hydration becomes distinctly accelerated by elevated temperature and retarded by the presence of sucrose, while NaCl affects the initial hydration kinetics only to a small degree.     

Influence of Fe2O3 on the hydration kinetics of tricalcium aluminate

The influence of Fe₂O₃ on the hydration kinetics of tricalcium aluminate (C₃A) was studied in order to clarify the mechanism of improving hydration resistance of CaO by in-situ synthesized tricalcium aluminate. The Krstulovic-Dabic model was used to investigate the hydration processes of Fe₂O₃-C₃A solid solution and calculate the corresponding kinetic parameters. The hydration products were analyzed by the X-ray diffraction and scanning electron microscope. The results indicated that the Krstulovic-Dabic model simulated the hydration processes of Fe₂O₃-C₃A solid solution at different stages effectively. The hydraulic activity of Fe₂O₃-C₃A solid solution decreased with the addition of Fe₂O₃. Reasonable amount of Fe₂O₃ addition reduced the hydration rate in the initial stage of Fe₂O₃-C₃A solid solution hydration, while the hydration rate of Fe₂O₃-C₃A solid solution was increased with excessive amount Fe₂O₃. The hydration process was controlled by multiple mechanisms due to an incomplete layer of hydration products was formed on the surface.     

Hydration of tricalcium silicate

Results of following the quantities of free Ca(OH)₂ and of tricalcium silicate (C₃S) during the hydration of C₃S, and also the influence of the presence of free CaO on this reaction are in accordance with the hypothesis of Stein & Stevels with regard to the hydration of C₃S. at the first contact between C₃S and water, a surface hydrate, invisible by electron microscope methods, is considered to be formed and to retard the reaction strongly. This hydrate is thought to change into one which retards the hydration reaction less and changes later into a third hydrate, tobermorite gel.     

Influence of tricalcium aluminate on the microstructure evolution of CaO specimen during hydration

C₃A-containing CaO specimen was prepared and the evolution of its microstructure during hydration process was investigated to clarify the protective mechanism of tricalcium aluminate (C₃A) on hydration resistance of CaO specimen. The slit-shaped micropores were formed on the grain boundary of CaO due to the stacking of lamellar C₄AH₁₃ formed by the hydration of C₃A. The contact area of residual C₃A with the moisture was reduced by the porous C₄AH₁₃ layer at the original site, which resulted in a slower dissolution rate of C₃A grain through the porous layer. In addition, the crack propagation and the formation of macropores were inhibited by the pinning effect of C₄AH₁₃, which was beneficial to the improvement of hydration resistance.     

Influence of amorphous silica on the hydration of tricalcium aluminate

Amorphous silica influences tricalcium aluminate (C₃A) hydration both in pastes and in suspensions. Two heat peaks are found by isothermal calorimetry during the paste hydration of C₃A. The addition of amorphous silica causes the second heat peak to shift towards shorter reaction times and become more pronounced. In suspensions, the change in ion concentration in the water phase is not influenced by the presence of amorphous silica except that the change in concentration occurs more quickly. Quantitative X-ray analysis shows that more 3CaO.Al₂O₃.6H₂O is present in suspensions containing amorphous silica than in silica-free suspensions at equal hydration times.     

Influence of triethanolamine on the hydration characteristics of tricalcium silicate

The hydration characteristics of 3CaO.SiO₂ or β2CaO.SiO₂ are studied by an addition of 0.0, 0.1, 0.5 or 1.0% triethanolamine. The amount of Ca(OH)₂ found at 1, 3, 7 or 28 days was in the order C₃S + 0% TEA > C₃S +0.1% TEA > C₃S + 0.5% TEA > C₃S+1.0% TEA, irrespective of whether lime was estimated by X-ray, DTA, TGA or chemical analysis. The rate of hydration, in terms of the disappearance of 3CaO.SiO₂, showed that hydration proceeded faster in the presence of TEA after 1 day. Additions of TEA increase the induction period, promote the formation of a C-S-H with higher CaO/SiO₂ ratio, increase the formation of non-crystalline Ca(OH)₂ and enhance the surface area of the hydrated silicate product.     

Tricalcium aluminate hydration in the presence of calcium sulfite hemihydrate

This article reports on the study to evaluate the potential possibility of regulating the tricalcium aluminate (C₃A) hydration process by the addition of calcium sulfite hemihydrate. The kind and the form of hydration products were studied in the system: C₃A-CaSO₃·0.5H₂O-H₂O and C₃A-CaSO₃·0.5H₂O-Ca(OH)₂-H₂O by use of XRD, DTA and SEM/EDS methods as well as the kinetics of hydration along with chemical composition development of the liquid phase. The results thus obtained were compared to the hydration process of C₃A with the addition of natural gypsum. The results show that the reaction rate of C₃A with the addition of calcium sulfite hemihydrate differs from the analogous hydration process of C₃A in the presence of calcium sulfate dihydrate. Also, the kind of hydration products obtained in the presence of CaSO₃·0.5H₂O is different.     

Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum

To understand the mechanisms and the parameters controlling the reactivity of tricalcium aluminate in the presence of gypsum at an early age, a study of the hydration of the “C₃A-sulphate” system by isothermal microcalorimetry, conductimetry and a monitoring of the ionic concentrations of diluted system suspensions have been carried out with various gypsum quantities. The role of C₃A source and its fineness were also studied. This work shows the fast initial formation of AFm phase followed by ettringite formation during the period when the sulphate is consumed. It has been highlighted that the time necessary to consume all the gypsum varies with the type of C₃A and it has been attributed to the intrinsic reactivity of each one and mainly to the change of fineness from one C₃A to another. Results are discussed alongside hypothesis from the literature to explain the slowing down of C₃A hydration process in the presence of calcium sulphate.     

Effect of cement type on the corrosion of reinforcing steel bars exposed to acidic media using electrochemical techniques

The effect of different percentages of cement components (tricalcium aluminate C₃A) on the corrosion of embedded reinforcing steel bars was studied in presence of 5% NaCl or 5% MgSO₄ solutions. Different electrochemical techniques namely; half-cell potential measurement, impressed voltage method and impressed current method were used. The C₃A in cement reduced greatly the corrosion of steel bars embedded in concrete subjected to chloride or sulphate solutions. In chloride solution, as the percent of C₃A increased in cement from 2% to 10% the steel corrosion decreased proportionally. The rate of corrosion in 5% MgSO₄ solution was decreased as the percent of C₃A increased from 2% to 6%. From 6% to 10% the steel corrosion rate was rapidly accelerated. In general the presence of chloride and sulphate solutions in surrounding media reduced the destructive effect of sulphate ions on embedded steel bars.     

Hydration characteristics of tricalcium aluminate phase in mixes containing β-hemihydate and phosphogypsum

The tricalcium aluminte phase was prepared from pure chemicals on a laboratory scale. Five mixes were formulated from the prepared C₃A phase, β-hemihydate, phosphogypsum, calcium hydroxide and quartz. Different mixes were hydrated at various time intervals, namely, 6, 24, 72 and 168 h. The kinetics of hydration was measured from chemically combined water and combined lime contents. The phase compositions and microstructures of the hydrated products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA)/TG, scanning electron microscopy (SEM) techniques and FT-IR spectroscopy. This work aimed to study the effect of partial to full substitution of phosphogypsum by β-hemihydate on the hydration characteristics and microstructures of tricalcium aluminte phase. The results showed that the combined lime slightly increases with the increase of amounts of phosphogypsum. The XRD patterns showed the increase in the intensities of monosulphate and different forms of calcium aluminate (C₄AH₁₃ and C₄AH₁₉) with phosphogypsum content. Ettringite is less stable than monosulphoaluminate, so it transformed into monosulpho-aluminate after 24 h, which persisted up to 168 h. The mechanism of the hydration process of C₃A phase in the presence of phosphogypsum proceeds in a similar path as with β-hemihydate. Phosphogypsum reacts with C₃A in the presence of Ca(OH)₂ forming sulphoaluminate hydrates, which are responsible for setting regulation in cementitious system.     

Combined hydration of tricalcium silicate and β-dicalcium silicate

The mutual interaction of tricalcium silicate (C₃S) and β-dicalcium silicate (β-C₂S) in their combined hydration was studied. The rate of β-C₂S hydration was accelerated significantly in the presence of C₃S. The rate of C₃S hydration was retarded, but only in the presence of large amounts of β-C₂S. The stoichiometric composition and the pore structure of the hydrates formed was altered only unsignificantly when both compound hydrated simultaneously.     

Influence of water activity on hydration of tricalcium aluminate-calcium sulfate systems

The hydration of tricalcium silicate (C₃S)—the major phase in cement—is effectively arrested when the activity of water (a_H) decreases below the critical value of 0.70. While it is implicitly understood that the reduction in a_H suppresses the hydration of tricalcium aluminate (C₃A: the most reactive phase in cement), the dependence of kinetics of C₃A hydration on a_H and the critical a_H at which hydration of C₃A is arrested are not known. This study employs isothermal microcalorimetry and complementary material characterization techniques to elucidate the influence of a_H on the hydration of C₃A in [C₃A + calcium sulfate (C$) + water] pastes. Reductions in water activity are achieved by partially replacing the water in the pastes with isopropanol. The results show that with decreasing a_H, the kinetics of all reactions associated with C₃A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When a_H ≤0.45, the hydration of C₃A and the precipitation of all resultant hydrates are arrested; even in liquid saturated systems. In addition to—and separate from—the experiments, a thermodynamic analysis also indicates that the hydration of C₃A does not commence or advance when a_H ≤0.45. On the basis of this critical a_H, the solubility product of C₃A (K_C3A) was estimated as 10^−20.65. The outcomes of this work articulate the dependency of C₃A hydration and its kinetics on water activity, and establish—for the first time—significant thermodynamic parameters (ie, critical a_H and K_C3A) that are prerequisites for numerical modeling of C₃A hydration.     

The effect of glucose and some glucose oxidation products on the hydration of tricalcium aluminate

The effect of glucose and some glucose oxidation products on tricalcium aluminate hydration has been studied. The results are in agreement with the theory that stabilization of hexagonal C₄AH₁₃ determines the retardation. It is assumed that the glucose oxidation products are more effective retarders than glucose because they are more stable in alkaline conditions.     

Impact of triethanolamine on the sulfate balance of Portland cements with mixed sulfate carriers

The differences between the hydration of Portland cements with single and with mixed sulfate carriers in the presence of triethanolamine (TEA) were investigated, and possible mechanisms were proposed. Without TEA, cements with different types of sulfate carriers (gypsum, hemihydrate, anhydrite, and mixture of these) have a comparable hydration process at the same molar amount of calcium sulfate. At a TEA dosage of 0.5 wt.%, the sample with a mixture of three sulfate carriers shows substantially stronger retardation of the C₃S (This publication uses the cement chemist notation: C₃S = Ca₃SiO₅, C₂S = Ca₂SiO₄, C₃A = Ca₃Al₂O₆, C₄AF = Ca₂(Al, Fe)₂O₅.) hydration than the cements with only one of these sulfate carriers, which is likely caused by the rapid formation of ettringite and the fast depletion of all sulfate carriers. These effects indicate that TEA influences the balance of sulfate carriers with aluminate-containing clinker phases. On the one hand, TEA can disturb the original sulfate balance due to the accelerated dissolution of aluminate-containing clinker phases, especially C₄AF. On the other hand, these effects are closely related to the types and amounts of the sulfate carriers in the cement. A higher amount of sulfate carriers can minimize the TEA-related retardation of the C₃S hydration, and hemihydrate shows the strongest impact at the same calcium sulfate quantity.     

A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl₂) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C₃S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl₂. It appears that CaCl₂ accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.     

The Influence of Water Activity on the Hydration Rate of Tricalcium Silicate

Tricalcium silicate does not undergo hydration at relative humidities (RH's) below 80%. But, the rate at which its hydration rate decreases as a function of the RH has not yet been elucidated. By invoking correspondence between RH and water activity (a_H, unitless), both of which are related to the chemical potential of water, the reaction evolutions of triclinic tricalcium silicate (i.e., T1‐Ca₃SiO₅ or C₃S) are tracked in water + isopropanol (IPA) mixtures, prepared across a wide range of water activities. Emphasis is placed on quantifying the: (a) rate of hydration as a function of a_H, and (b) the critical (initial, a_H0c or the achieved) water activity at which hydration effectively ceases, i.e., does not progress; here identified to be ≈ 0.70. The hydration of tricalcium silicate is arrested even when the system remains near saturated with a liquid phase, such that small, if any, capillary stresses develop. This suggests that changes in chemical potential induced via a vapor‐phase or liquid‐phase route both induce similar suppressions of C₃S hydration. A phase boundary nucleation and growth (pBNG) model is fit to measured hydration rates from the onset of the acceleration period until well beyond the rate maximum when the water activity is altered. The simulations suggest that for a fixed hydrate nucleation density, any water activity reductions consistently suppress the growth of hydration products. Thermodynamic considerations of how water activity changes may influence reactions/hydrate evolutions are discussed. The outcomes improve our understanding of the chemical factors that influence the rate of Ca₃SiO₅ hydration.