Action of triethanolamine on the hydration of tricalcium aluminate

Hydration characteristics of tricalcium aluminate and tricalcium aluminate + gypsum were studied following addition of 0.5, 1.0, 5.0 or 10.0% triethanolamine (TEA) at a solution/C₃A ratio of 1.0 after hydration periods of 1 to 60 min. TEA accelerated the hydration of C₃A to the hexagonal aluminate hydrate and its conversion to the cubic aluminate hydrate. The rate of hydration increased with increased amounts of TEA, which also accelerated the formation of ettringite in the C₃A-gypsum-H₂O system.     

Influence of pozzolanic additives on hydration mechanisms of tricalcium silicate

Pozzolanic mineral additives, such as silica fume (SF) and metakaolin (MK), are used to partially replace cement in concrete. This study employs extensive experimentation and simulations to elucidate and contrast the influence of SF and MK on the early age hydration rates of tricalcium silicate (triclinic C₃S), the major phase in cement. Results show that at low replacement levels (i.e., ≤10%), both SF and MK accelerate C₃S hydration rates via the filler effect, that is, enhanced heterogeneous nucleation of the main hydration product (C–S–H: calcium‐silicate‐hydrate) on the extra surfaces provided by the additive. The filler effect of SF is inferior to that of MK because of agglomeration of the fine particles of SF, which causes significant reduction (i.e., up to 97%) in its surface area. At higher replacement levels (i.e., ≥20%), while SF continues to serve as a filler, the propensity of MK to allow nucleation of C–S–H on its surface is substantially suppressed. This reversal in the filler effect of MK is attributed to the abundance of aluminate [Al(OH)₄^?] ions in the solution—released from the dissolution of MK—which inhibit topographical sites for C–S–H nucleation and impede its subsequent growth. Results also show that in the first 24 hours of hydration, MK is a superior pozzolan compared to SF. However, the pozzolanic activities of both SF and MK are limited and, thus, do not produce significant alterations in the early age hydration kinetics of C₃S. Overall, the outcomes of this study provide novel insights into the mechanistic origins of the filler and pozzolanic effects of SF and MK, and their impact on cementitious reaction rates.     

Influence of aluminium hydroxide and lime on the hydration of tricalcium silicate

Experiments by isothermal calorimetry, indicate that the hydration of 3CaO·SiO₂ (C₃S) is influenced very little by gibbsite; it is influenced by bayerite to a somewhat larger extent. In the presence of amorphous Al(OH)₃ the reaction of C₃S with water shows a very complicated course and gives four heat peaks. If CaO is added in addition to this Al(OH)₃, the third and the fourth heat peaks are more pronounced. From qualitative d.t.a., infra-red, electron-microscope and X-ray investigations, as well as from quantitative X-ray analysis, a reaction mechanism is proposed. The quantity of C₃S reacted, determined by means of quantitative X-ray analysis, is greater during the reaction of 2·00 g C₃S with 0·40 g amorphous Al(OH)₃, 0·08 g CaO and 2·00 ml water, than during the reaction of 2·00 g C₃S with 2·00 ml water.     

Influence of precuring on high pressure steam hydration of tricalcium silicate

Samples of C₃s, hydrated at room temperature for 0, 2, 4, 8 and 24 hours, were steam cured at 130, 160 and 190°C for 5, 15 and 24 hours, in order to assess the influence of preliminary curing on autoclave hydration. The room temperature curing duration affects the autoclave hydration of C₃S, mainly at low temperatures. The types and relative amounts of the obtained products are also markedly affected by the preliminary treatment.     

Influence of precuring on high pressure steam hydration of tricalcium silicate II. Effect of fineness of tricalcium silicate

C₃S samples of different fineness were precured at room temperature and subsequently autoclaved. It was found that, as the fineness is increased: 1) the effect of precuring on the hydration rate of C₃S in autoclave is less evident; 2) the precuring time corresponding to the highest amount of C₃SH_1.5 becomes longer; 3) particularly for short precuring times, the quantity of C₃SH_1.5 decreases and the formation of α-C₂SH is favored; 4) the amounts of the two crystalline hydrated silicates are reduced while the formation of CSH is favored.     

Mathematical modeling of tricalcium silicate hydration

Based on conceptual models for the stages in the hydration of tricalcium silicate, a mathematical model was developed. The separate resistances in the mathematical model correspond to the phenomenological stages of the conceptual model. Comparison of model output with available hydration data gave a reasonable fit between the model and the data.     

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.     

Carbonation of the hydration products of tricalcium silicate

C₃S has been hydrated for increasing time and stored for 2.5 years under normal atmosphere, the fresh and aged materials being characterized by X-ray diffraction and infrared spectroscopy. The carbonation occuring during storage gives rise to complete disappearance of CSH gel while portlandite remains in appreciable amount; the siliceous residue is an amorphous silica similar to common silica gels. The carbonates formed are vaterite and aragonite, the latter being relatively more important in samples with a low degree of hydration.     

Induction period of hydration of tricalcium silicate

The length of the induction period of tricalcium silicate hydration has been found to be related to the probability of escape of trapped electrons measured by thermoluminescence on anhydrous sample. Several strong experimental evidences suggest an analogy between the hydration of tricalcium silicate with water vapour and the main reaction of the induction period in paste.On this basis a mechanism is tentatively proposed.     

Low pressure steam hydration of tricalcium silicate

Pastes of C₃S (w/c ratio = 0.5) were steam cured at 25, 40, 60 and 90°C for 1 hour to 30 days. The results obtained have shown that, as the curing temperature rises, the induction period is shortened and the initial rate of hydration of C₃S is increased; at longer curings, on the other hand, such hydration rate is considerably lowered. In order to explain the influence of temperature on the hydration reaction a new hypothesis has been proposed, which takes into account the C/S molar ratio as well as the surface properties of the hydrated silicate.     

Studies on the hydration of tricalcium silicate pastes III. Influence of admixtures on hydration and strength development

A study has been made of the influence of calcium salts, as set-controlling admixtures, on the hydration of tricalcium silicate. It is found that some admixtures modify the morphology of C-S-H gel and for calcium hydroxide. Changes in capillary porosity distribution can be correlated with changes in the morphology of the outermost C-S-H gel. It is concluded that capillary porosity, as controlled by the degree of hydration is still the dominant factor controlling tensile strength.     

Effect of calcium formate on the hydration of tricalcium silicate

The effect of 0, 0.5, 1.0, 2.0, 4.0 and 6.0% of calcium formate on the hydration of C₃S has been studied. Free lime determinations, non-evaporable water content, pH of the liquid phase, zeta potential, thermal analysis and infrared spectral studies have been made for understanding the mechanism of action of calcium formate. Results indicate that calcium formate acts as an accelerator up to 2%. Above this concentration, the excess of it has practically no effect.     

Studies on the hydration of tricalcium silicate pastes II. Strength development and fracture characteristics

The fracture of hardened tricalcium silicate pastes has been studied using optical microscopy and scanning electron microscopy. The path of fracture changes with the age of the pastes. In young pastes, fracture passes primarily through high porosity C-S-H gel avoiding low porosity areas where calcium hydroxide has crystallized in the pores. In more mature pastes, this discrimination is lost as the matrix becomes more homogeneous. The different modes of fracture appear to have no significant influence on tensile strength, the tensile strength depends primarily on porosity and is proportional to the cube of Powers' gel-space ratio.     

Structural and hydration properties of amorphous tricalcium silicate

Mechanical milling was carried out to synthesize amorphous tricalcium silicate (Ca₃SiO₅) sample, where Ca₃SiO₅ is the most principal component of Portland cement. The partial phase transformation from the crystalline to the amorphous state was observed by X-ray and neutron diffractions. Moreover, it was found that the structural distortion on the Ca-O correlation exists in the milled Ca₃SiO₅. The hydration of the milled Ca₃SiO₅ with D₂O proceeds as follows: the formation of hydration products such as Ca(OD)₂ rapidly occurs in the early hydration stage, and then proceeds slowly after about 15 h. The induction time for the hydration of the milled Ca₃SiO₅ is approximately one half shorter than that for the hydration of the unmilled one. This result means that the mechanical milling brings about the chemical activity of Ca₃SiO₅ for hydration, and may be particularly useful for increasing the reactivity in the early hydration stage.     

Early hydration of large single crystals of tricalcium silicate

A reaction product believed to be an initial hydrate layer was observed to have formed on large pure C₃S single crystals after 5 minutes of hydration. This layer then increased in thickness and became covered with micrometer-sized spheres of poorly crystallized Ca(OH)₂ within 30 minutes. Subsequently, the formation of a new hydrate of acicular morphology was observed to occur on the surface of the first-formed hydrate. This transformation was accompanied by the disappearance of the first hydrate layer and the calcium hydroxide spheres. The observation of a transient initial hydrate is in accord with the view that the rate of early C₃S hydration is controlled by the formation and subsequent disappearance of a barrier layer.     

The tricalcium silicate hydration in the presence of active silica

The influence of active silica upon the tricalcium silicate hydration was studied. On the microcalorimetric curves a considerable acceleration of the process with dormant period elimination was observed. The reason for this is the lowering of calcium ions concentration in the solution as they are consumed, rapidly by the CSH phase formation of a low C/S ratio. Basing on these results the authors presume that the delaying factor of C₃S hydration process is the quasi-stationary layer supersaturated with calcium ions surrounding the anhydrous grains of tricalcium silicate.     

Early hydration of tricalcium silicate II. The induction period

The initial hydration of C₃S was found to be stimulated by adding to the paste prehydrated C₃S and by lowering the Ca⁺⁺ concentration of the liquid phase with oxalic acid. An addition of crystalline calcium hydroxide did not alter the duration of the induction period. Based on these findings the origin of the induction period in C₃S hydration is discussed.     

A study of the microstructure and hydration characteristics of tricalcium silicate in the presence of calcium chloride

The morphological and hydration characteristics of tricalcium silicate treated with 0, 2 and 5 per cent calcium chloride were followed by employing Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA), and X-ray diffraction techniques. The water: solid ratios used were either 0.5 or 0.3. In terms of Ca(OH)₂ estimation CaC?₂ accelerated hydration, except with 5 per cent CaC?₂ at a 0.3 w/s ratio. The maximum amount of Ca(OH)₂ and calcium silicate hydrate was formed at 2 per cent CaC?₂ and a w/s ratio of 0.5. Except at very early times, the rate of reaction was slower at 0.3 w/s with or without CaC?₂ compared with the corresponding samples at 0.5 w/s.     

Early hydration of tricalcium silicate III. Control of the induction period

The duration of the induction period in the hydration of C₃S can be altered by several means. Fast cooling after burning, fine grinding and an excess of water shorten the induction period. The induction period gets prolonged by slow cooling of the sample, by long lasting storage and by doping with A1₂O₃.     

Early hydration of tricalcium silicate I. Kinetics of the hydration process and the stoichiometry of the hydration products

The initial hydration of C₃S in paste form at room temperature was studied. The process is initiated by a short lasting rapid hydration in which about 1 – 2% of C₃S is hydrated and a hydrate with low C/S and high H/S ratio is formed. After a subsequent induction period of 4 hours a renewed rapid hydration is observed in which a hydrate of constant stoichiometric composition, independent on the time of hydration is formed. This hydrate has a higher C/S and lower H/S ratio than the one formed initially. The liquid phase stays supersaturated with respect to calcium hydroxide for several hours after the induction period is terminated.