Hydration induction period

The rate of heat evolution keeps at the low value to about 1 hours for the control cement, and to about 4 hours for the cement with SBR dispersion or powder, indicating the SBR dispersion or power prolongate the cement hydration induction period. After the induction period, the second exothermic peak appears, shown in Fig. 1 (b), which is corresponding to the acceleratory and decelera-tory periods of the cement hydration. Here, the influence of the SBR dispersion and powder is different. The influence of the SBR dispersion on the exothermic peak is much more significant. The appearing time of the maximum rate of heat evolution is delayed from 10 hours to 16 hours with the SBR dispersion addition, and the maximum rate decreases markedly from 3.1 mW/g to 1.8 mW/g. Whereas with the SBR powder modification, the appearing time of the maximum rate of heat evolution is delayed from 10 hours to 12.5 hours, and the maximum rate decreases from 3.1 mW/g to 2.9... 

The delay, t0, preceding the onset of the main reaction may include contributions from (i) the time required for the sample to attain reaction temperature, h, (ii) additions to fh resulting from changes within the reaction sample, e.g. water removal (endothermic) from a hydrate, td, phase transitions, etc. and (iii) slow processes preceeding establishment of the main reaction, which are to be regarded as the true induction period, The effective values of th, td and may show different temperature coefficients so that the magnitude of t0(=th + ta + i) may vary with temperature in a complex manner, perhaps differently from that of the subsequent rate process. 

Treatment of liquid air (containing condensed atmospheric moisture) with fluorine give a potentially explosive precipitate, thought to be fluorine hydrate [1], Contact of liquid fluorine with a bulk of water causes violent explosions. Ice tends to react explosively with fluorine gas after an indeterminate induction period [2],... 

MRH Ammonia, 5.86/25, aniline 6.44/17, dimethylhydrazine 6.69/19 Ammonia dissolved in 99.6% peroxide gave an unstable solution which exploded violently [1]. In the absence of catalysts, cone, peroxide does not react immediately with hydrazine hydrate. This induction period has caused a number of explosions and accidents owing to sudden reaction of accumulated materials [2], 1,1-Dimethylhydrazine is hypergolic with high-test peroxide [3],... 

In the latter part of stage 1, the cement particles in the paste become fiilly coated with a layer of hydrate products. This protective layer hinders the diffusion of the reacting species in and out of the reaction interphase, thus sharply reducing the rate of the various reactions. The system enters into a period of latency referred to as the induction or dormant period processes initiated during stage I, however, will continue throughout the induction period. 

As reaction is preceded by a certain delay, this induction period was reduced by the addition of copper salts, e.g. potassium cuprocyanide K3Cu (CN)4. This substance was supplied to the system dissolved in the hydrazine hydrate. It was found that potassium cuprocyanide reacts with hydrazine even at room temperature to form metallic copper which, if deposited in the pipelines, may cut off the flow of hydrazine into the combustion chamber. To prevent this the system was modified so that hydrazine hydrate flowed from the tank into the combustion chamber through a cartridge containing cupric nitrate, which dissolved in hydrazine hydrate in a sufficient quantity to accelerate the reaction (hydrazine and its reaction with H202 will be discussed in more detail further on). 

Concentrated hydrogen peroxide does not react instantly with hydrazine hydrate, only after a certain induction period. This has been the cause of a number of explosions and accidents, produced by the accumulation of unchanged components and their sudden reaction after the induction period has elapsed. 

The induction time is marked as 1 and includes the time taken for crystal nuclei to form which are not visible to macroscopic probes. The induction time is defined in practice as the time elapsed until the appearance of a detectable volume of hydrate phase or, equivalently, until the consumption of a detectable number of moles of hydrate former gas. The induction time is often also termed the hydrate nucleation or lag time (Section 3.1). (The induction or lag time is the time taken for hydrates to be detected macroscopically, after nucleation and onset of growth have occurred, whereas nucleation occurs on too small a size scale to be detected. Therefore, the term nucleation time will not be used in this context. Instead, the term induction time or induction period will be used. The induction time is most likely to be dominated by the nucleation period, but also includes growth up to the point at which hydrates are first detected.)... 

The metastability of the system prevents hydrate forming immediately at Point D (at the hydrate equilibrium temperature and pressure Figure 3.1b). Instead the system pressure continues to decrease linearly with temperature for a number of hours, without hydrate formation occurring (A to B is the induction period, cf. 1 in Figure 3.1a). At Point B, hydrates begin to form. The pressure drops rapidly to Point C (about 1.01 MPa or 10 atm in 0.5 h). B to C is the catastrophic growth period (cf. 2 in Figure 3.1a). 

Hydrate nucleation (the initiation of growth, occuring during the induction period) is a stochastic process (with significant scatter in the data at low driving force under isothermal conditions). 

The induction period is shortened by adding prehydrated CjS (013), but additions of lime or CH, including that formed from CjS, are variously reported to be ineffective (013,B67) or to lengthen it, though shortening it with cement (Ull). In cement mixes, additions of pfa or some other finely divided materials accelerate hydration after the first day, apparently by acting as nucleation sites for C-S-H (Section 9.3.3). Additions of reactive silica markedly accelerate hydration (S53). Most of this evidence supports hypothesis 3 and tells against hypothesis 4. Hypothesis 3 does not exclude hypothesis 1, as the breakdown of a protective layer could be associated with formation of a new product. 

The fact tliat both conventional water reducers and superplasticizers are more efTective if added some time after mixing provides a strong indication that adsorption probably occurs at least in part on the hydrated phases, as the anhydrous surfaces have by that time become covered with hydration products. Chiocehio cl al. (C57) found that the optimum time for addition was at the start of the induction period. More of the admixture seems to be taken up by the early hydration products, especially of the aluminate phase, if it is added before the early reaction has subsided. 

Since the setting of tricalcium silicate involves the formation of hydrates, crosspolarisation studies between Si and H have been used to provide information about the association of protons with the silicate units in the early stages of the reaction of Si-enriched tricalcium silicate (Rodger et al. 1988). The results indicate the formation of a small amount of hydrated material during the induction period, the end of which is therefore not marked by the sudden onset of hydration, but by a change in the product silicate species, from monomers to dimers. 

Isothermal dehydrations of single crystals (about 1 mm ) of the hexahydrate [111] in vacuum between 213 and 243 K gave or-time curves for conversion to the trihydrate with no induction period and a constant rate of water loss during a large fraction of reaction. The rate of dehydration decreased linearly with the prevailing pressure of water (contrasting with the behaviour of many other hydrates) attributed to the occurrence of the reverse reaction. The activation energy for conversion of... 

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