Cement hydration, effect calcium chloride

Setting of Biodentine is partly caused by condensation polymerization of the silicate phase to form a chain structure based on SiO tetrahedra joined at two comers (Q2 structures). This is similar to the setting of Portland cement [61], though it occurs faster due to the accelerating effect of calcium chloride. Setting is accompanied first by a contraction in volume, as the initial solidification occurs. This is followed by expansion as secondary hydration reactions take place to form the various calcium silicate hydrates [83]. 

M.C.G. Juenger, P.J.M. Monteiro, E.M. Gartner, G.P. Denbeaux, A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration. Cement Concrete Res. 35 (2005) 19-25. 

There are numerous studies of the diffusion rate of ions, principally the chlorides, in water saturated paste [138,145-149]. Migration of ions in hardened cement paste is not a simple diffusion. Ions are adsoihed on the surface of hydrated phases, in pores, or even react with hydrates. Therefore the diffusion coefficients cannot be considered in a classic manner. Their values derived from the Pick s equation should be considered as apparent or effective diffusion coefficients. For example it is known that CaCl2 reacts with aluminates and C3(A, Fl CaClj lOH O phase is formed. The basic calcium chloride Ca(OH)2 CaCl2-H20 can be also formed in some conditions. 

Several mechanisms have been suggested for the action of calcium chloride on cement. None of the theories can explain all the effects of CaCl2 in concrete. It is possible that any one mechanism will be able to explain only one or some of the observations. It is likely that an overall meehanism should take into account the amount of the admixture used and the time of hydration and the experimental conditions. 

Calcium chloride also has an effect on the hydration of various other cementitious systems such as pozzolanic cements, slag cements, expansive cements, high alumina cement, g q)sum, rapid hardening cement, etc. (See, for example. Refs. 1 and 40.)... 

The setting and hardening of concrete are accelerated in the presence of calcium chloride. These are related to the effect of CaCl2 on the rate of hydration of cement in concrete. The hydration of cement is exothermic, resulting in the production of heat, and if the heat is produced at a faster rate, larger amounts of the hydrates are formed at earlier times in the presence of accelerators. This is particularly significant in the first 10-12 hours. The influence of different amounts of calcium chloride on the rate of heat development is depicted by conduction calorimetric curves (Fig. The position of the peak corresponding to the... 

One of the limitations to the wider use of calcium chloride in reinforced concrete is that, if present in larger amounts, it promotes corrosion of the reinforcement unless suitable precautions are taken. The use of calcium chloride is banned in many countries. There is, hence, a continuing attempt to find an alternative to calcium chloride, one equally effective and economical, but without its limitations. A number of organic and inorganic compoimds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, halides, nitrites, calcium salts of acetic acid, propionic acid, butyric acid, oxalic acid and lactic acid, urea, glyoxal, triethanolamine, and formaldehyde have been suggested. However, practical experience and research on these admixtures are limited. The effect of these compounds on the hydration of individual cement compoimds and cement has been widely studied by thermal analysis techniques. 

One of the best known accelerators of the hydration of Portland cement is calcium chloride. Generally, up to 2 percent solid calcium chloride dihydrate based on the weight of cement is added. Chlorides may be contained in other admixtures such as some water-reducing admixtures where small amounts of chloride are sometimes added to offset the set-retarding effect of the water reducer. 

Carbonation does not cause any damage to the concrete itself, although it may cause the concrete to shrink. Indeed, in the case of concrete obtained with Portland cement, it may even reduce the porosity and lead to an increased strength. However, carbonation has important effects on corrosion of embedded steel. The first consequence is that the pH of the pore solution drops from its normal values of pH 13 to 14, to values approaching neutrahty. If chlorides are not present in concrete initially, the pore solution following carbonation is composed of almost pure water (Section 2.1.1). This means that the steel in humid carbonated concrete corrodes as if it was in contact with water [2, 3]. A second consequence of carbonation is that chlorides bound in the form of calcium chloroaluminate hydrates and otherwise bound to hydrated phases may be Hberated, making the pore solution even more aggressive [2-4]. 

The granulated blastfurnace slag and siliceous fly ash addition will have a similar effect on the ability of chloride ions binding in cement pastes, because they will also decrease the C/S ratio in C-S-H gel. Uchikawa and Okamura [210] report the following C/S ratio 1.7 for Portland cement, 1.6 in case of 40% slag addition and 1.2 at 40% of sihceous fly ash addition. Simultaneously, these additions will cause the aluminates content decrease in favor of calcium silicate hydrates. 

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