Heat evolution accelerators

When the partial pressures of the radicals become high, their homogeneous recombination reactions become fast, the heat evolution exceeds heat losses, and the temperature rise accelerates the consumption of any remaining fuel to produce more radicals. Around the maximum temperature, recombination reactions exhaust the radical supply and the heat evolution rate may not compensate for radiation losses. Thus the final approach to thermodynamic equiUbrium by recombination of OH, H, and O, at concentrations still many times the equiUbrium value, is often observed to occur over many milliseconds after the maximum temperature is attained, especially in the products of combustion at relatively low (<2000 K) temperatures. 

Accelerating admixtures based on calcium chloride, formate, nitrate, and thiocyanate have no significant effect on the workability, air content, mix stability, or water-cement ratio of concretes into which they are incorporated. The only properties of plastic concrete which are affected are the heat evolution and setting time. 

Tris(pentafluorophenyl)boron [(CgF5)3B], a triarylboron bearing electron-withdrawing perfluorinated phenyl rings, was found to be much more powerful than (05115)36 as a Lewis acid accelerator for the present polymerization. When (0565)36 was added at room temperature to the polymerization system at an equimolar ratio of (0565)36 to the growing species 2, the polymerization took place rapidly with considerable heat evolution, attaining 100% monomer conversion within only about 10 min [6ig. 13 ( )]. This polymerization was estimated to be 150-times faster than that in the absence of (0565)36, and 12.5-times faster than that with (05115)36 as a Lewis acid under similar conditions. 

The addition of CaClj increases heat evolution at early ages. Supported by other evidence, considered shortly, this indicates that the effects on setting and hardening are due to acceleration of the hydration reactions. Early studies, summarized by Murakami and Tanaka (M106), showed that a great many salts have effects qualitatively similar to those of CaClj. With some reservations, these effects occur with C,S pastes as well as with cement pastes, and are thus seen to be due to faster hydration of the alite. 

Fig. 11.5 shows heat evolution curves, obtained by conduction calorimetry, for a CjS paste with and without addition of CaCU. When the latter is present, the main heat evolution peak begins earlier and rises and falls more steeply its maximum is reached earlier. The rate of heat evolution at the maximum is positively correlated with the reciprocal of the time at which the maximum occurs (D18). The linear relation extends to organic retarders. As these probably act by hindering the growth of C-S H (Section 11.2.2), this evidence suggests that the accelerators act by promoting it. 

Many studies have shown that both cations and anions contribute to the accelerating effect and that an approximate rank order can be established for each type of ion (E8,MI06,C60,K57,K58,D18,W31). For the present we shall exclude from consideration such ions as Mg or F" for which precipitation of additional phases complicates the situation. The sequences found by different investigators vary somewhat, and are also influenced by concentration and possibly by the nature of the counter ion. Kantro s (K57) results for the cumulative heat evolution at 12 h for a 3% addition of CaCl2-2H20 and equivalent concentrations of the other chlorides or calcium salts are reasonably typical ... 

The Ceramicrete process is based on the acid-base exothermic reaction. As a result, the exothermic heat evolution and its rate depend on the size of the waste forms produced. Larger forms generate more heat, which does not dissipate rapidly. Thus, the setting slurry heats up and accelerates the acid-base reaction, and the mixture is able to set even in cold surroundings. 

Accelerators increase the rate of hydration thereby providing earlier heat evolution and strength development. 

Fig. 4.21 Effect of accelerators on the rate of heat evolution during CjS hardening (according to [35]) 1 set accelerator (CaClj) or high temperature of paste, 2 accelerator of hardening (NaCl), 3 reference, 4 set retarder (low temperature)...

The rate of hydration heat evolution in Portland— fly ash cements may vary greatly, mainly because of distinct variations in the reactivity of the individual fly ashes employed. As a general rule, however, the amount of heat released within the first days of hydration is lower than that of a control Portland cement. If, on the other hand, only the Portland clinker fraction is considered, the amoimt of released heat is usually higher, indicating an accelerating effect of the fly ash on the hydration of chnker constituents. A... 

Rapidly dissolving in all resins, include, blsphenol a, b, c, g A and vinyl ester variable gel times, strong heat evolution, relatively short mould release times no accelerator required above 70-80°C... 

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