Hydration accelerators

The main non-chloride, non-corrosive accelerating admixtures available on the market are of two types (1) accelerating admixtures which accelerate hydration but do not depress the freezing point of water and (2) accelerating admixtures for use in sub-freezing ambient temperatures which depress the freezing point of water. The former contain salts of formates, nitrates and nitrites and are effective for set acceleration and strength development. However, their effectiveness is dependent on the ambient temperature at the time of placement. 

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 fineness of cement may be conveniently characterized by the Blaine method, even though the values obtained by this procedure are systematically lower than those found by the more accurate, but also more complicated, BET method. The specific surface area of ordinary Portland cement typically ranges between 280 and 350 m /kg (Blaine), whereas high early strength cement may be groimd up to a specific surface area of 450-500 m /kg (Blaine). This higher smface area contributes to an accelerated hydration of the cement, but also increases its water requirement. 

In mixes in which calcinm aluminate cement is the main constituent, the reaction mainly responsible for fast setting and strength development is a very fast hydration of the calcium alirminate phases. The accelerated hydration responsible for fast setting and strength development is dne to the increase of the pH value of the mix, brought about by... 

It is used when high early strength is needed. It is has more CjS than Type I cement and has been ground finer to provide a higher surface-to-volume ratio, both of which accelerate hydration. Strength gain is double that of Type I cement in the first 24 hours. 

Because of the accelerated hydration, the silicate phases show rapid setting characteristics in the presence of CaCl2. In a 38 028 mixture containing 0, 1, and 2% CaCl2 the setting times have been found to occur at 790, 525, and 105 minutes, respectively. ]... 

In the system C3S-C3A-CaCl2-H20, more C4AH13 is formed and better strengths attained than that without CaCl2.t ° The main factor may be the accelerated hydration of the C3S phase. 

Schwarz, W., Novel Cement Matrices by Accelerated Hydration of the Ferrite Phase in Portland Cement Via Chemical Activation Kinetics and Cementitious Properties, Adv. Cem. Based Mater., Vol. 2, 1995, pp. 189-200. 

In summary, a wealtli of experimental data as well as a number of sophisticated computer simulations univocally indicate that two important effects underlie the acceleration of Diels-Alder reactions in aqueous media hydrogen bonding and enforced hydrophobic interactionsIn terms of transition state theory hydrophobic hydration raises the initial state more tlian tlie transition state and hydrogen bonding interactions stabilise ftie transition state more than the initial state. The highly polarisable activated complex plays a key role in both of these effects. 

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

Because of hydrate formation, the sodium salts tend to be difficult to dry. Excess water over that of hydration is beheved to accelerate the decomposition of the xanthate salts. The effect of heat on the dryiag of sodium ethyl xanthate at 50°C has been studied (84) ... 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

For example, when cementing a casing string run to shallow depth or when setting a directional drilling kick-off plug, it is necessary to accelerate the cement hydration so that the waiting period will be minimized. The most commonly... 

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