Beginning Of Hydration Research Articles

Influence of silica fume on the rheological properties of the matrices of high-performance concretes

The influence of two types of silica fume (SF) on the rheological properties of cement pastes with water-cement ratios typical of very high performance concrete was investigated. The sensitivity of cement to the sulphonated naphthalene formaldehyde type of superplasticizer (SP) and its optimum content was determined from apparent viscosity as afunction of three parameters. It was found that the values characterizing apparent viscosity of the pastes depend non-linearly on the amount of C3A, and that the relation is linked to the content of free CaO in cement. Values of pH for the cement suspensions are also highly correlated with the content of free CaO, and may be regarded as an indicator for selection of cements that have good workability with SP. The quality of SF had no significant influence on the rheological behaviour of the cement pastes with SP that had the lowest content of C3A, free CaO and alkalis. For all the cements with higher contents of these components, a decrease in viscosity was observed only after the addition of the lower reactivity SF. It has been proved that SF can improve the efficiency of the SP and may reduce the water content provided that its reactivity is limited. The chemical processes at the beginning of hydration are restrained by the same phenomenon.

Study of the strength of very young concrete as a function of the amount of hydrates formed—influence of superplasticizer

The strength of microconcretes has been measured at the end of mixing and for some hours afterwards. At the same time, measurements of the heat of hydration have been made. During the ‘dormant period’ the hydration, slight as it is, produces in fact an increase in strength: the flocculated structure of the fresh cement paste can explain the immediate reinforcement of the bonds as soon as hydration begins. When a superplasticizer is used, besides the well-known hydration delay, the beginning of hydration does not signify now that the strength increases: no doubt this is a consequence of the dispersion of the structure by the admixture. After the setting, the filling of pores is easier with a dispersed structure than with a flocculated one, because of the better distribution of the cement particles: a sample with superplastizcier, to attain the same strength, will need less hydrates than a plain sample.

Germination of Monocolpate Angiosperm Pollen: Evolution of the Actin Cytoskeleton and Wall during Hydration, Activation and Tube Emergence

The monocolpate pollen grain of Narcissus pseudonarcissus L. has two preferred sites for tube emergence, one at each end of the colpus. While the cellulosic microfibnls of the inner layer of the intine are disposed circumferentially in the centre of the grain, the microfibrils in these terminal sites are shorter and randomly oriented Soon after the beginning of hydration, inclusions of the vegetative cell begin movement, firstly in a rotatory manner, and then in a pattern focused on one or both germination sites, where the intine bulges as hydration progresses. These changes are associated with the evolution of the actin cytoskeleton. Actin is present in the unactivated grain in the form of fusiform bodies. During hydration these dissociate to form finer fibrils, initially randomly disposed. Then, correlated with the change of the pattern of movement in the vegetative cell, the actin fibril system becomes polarized towards the germination sites, where shorter fibrils accumulate. Callose, absent from the ungerminated grain, is deposited within the cellulosic wall in these locations, forming a shallow dome which eventually develops into an annulus subtending the inner callose lining of the emerging tube. The transition to cylindrical growth is associated firstly with the development of zonation in the cytoplasm of the vegetative cell, with the tip occupied by a population of wall precursor bodies (P-particles) and a dense aggregate of short actin fibrils; and then with the establishment of the ‘inverse fountain’ pattern of movement characteristic of the apical part of the extending tube.

Conductance of tricalcium silicate in water

In this investigation of the earlier hydration of tricalcium silicate, the molar conductance was measured as a function of concentration at 25°C in the range of complete dissolution of tricalcium silicate in water (up to 0.3 g/L). Also the conductance of saturated and supersaturated solutions of tricalcium silicate at different water-solid ratio was carried out. The results sustained the electrical conductivity theories for dissolution. The ions in crystalline tricalcium silicate, Ca 2+, O 2−, and SiO 4 4− hydrolyze during dissolution process. The molar conductance of the completely dissolved tricalcium silicate in bi-distilled water is plotted as a function of the square root of molarity according to the Onsager's equation, a straight line is obtained indicating that tricalcium silicate is a strong electrolyte in water. The limiting molar conductivity λ o of tricalcium silicate solution was measured (1215 S. cm 2 mol −1 at 25°C). Based on the conductance data, a discussion on the dissolution and hydration mechanism of tricalcium silicate is given. The effect of sucrose (non-ionizable material) on the electrical conductivity of tricalcium silicate solution was studies. It was observed that the sucrose accelerates the hydration process in “through solution” reaction at the beginning of hydration, then the hydration is retarded at different times depending on sucrose-solid ratio, due to the adsorption of cationic and anionic species on the surface of tricalcium silicate.

Evolution microstructurale des composites du systeme ciment alumineux-granulat calcaire. I. Mode de propagation de la fissure

The transitional layer, formed at the interface between a calcareous aggregate and an aluminous cement paste, changes versus the hydration time and leads to a zone rich in hydrated carboaluminate, which is formed by epitaxic growth on calcium carbonate. This phenomenon is responsible for a noticeable change in the crack propagation process in the solid. In the system calcium aluminate - siliceous aggregates, the process is always intergranular. In the case of aluminate - calcareous aggregate it is intergranular at the beginning of hydration and becomes transgranular after some months. Then the crack propagation concerns the carbonate grain itself but neither the surrounding zone nor the interface aggregate - transitional zone.

アルミナの水和現象

The hydration of amorphous and crystalline aluminas (γ and η) has been investigated by the dipping treatment in water at the temperatures of room to 95°C. Amorphous aluminum hydroxide, pseudoboehmlte, bayerite, hydrargillite and nordstrandite are formed by the hydration of amorphous alhmina. By the dipping at low temperatures, although the formation of amorphous aluminum hydroxide or pseudoboehmite is observed in the beginning of hydration, alumina trihydrates such as bayerite, and hydrargillite and/or nordstrandite appear in the dipping for about ten days and their formed amounts increase with increasing the period of dipping. With rising the dipping temperature, however, the rate of hydration is promoted, and pseudoboemite is formed at frst and then bayerite and nordstrandite are formed, afterwards the amount of nordstrandite formed increases when the period of dipping is extended more longer. Accordingly it is inferred that the hydration of amorphous alumina proceeds in the following sequence:amorphous alumina→amorphous aluminum hydroxide pseudoboehmite→alumina trihydrateOn the other hand, the hydration of crystalline aluminas is little caused by the dipping treatment in water at room temperature, but η- and γ-aluminas are hydrated to alumina trihydrates and pseudoboehmite, respectively, when the dipping temperature is rised.

Etude du phytochrome detectable, in vivo dans les graines de Cucurbita pepo L. au cours des differentes phases de la germination

In dry gourd seeds all the phytochrome is in the Pfr form. The increase of phytochrome content from the beginning of hydration involves two phases, A and B, in the embryonic axis as well as in the cotyledons. Cycloheximide does not prevent the appearance of Pr during phase A. We assume that Pr is gradually released from an inactive complex. On the other hand phase B is inhibited by cycloheximide; this could mean that a de novo synthesis of Pr occurs.