Accelerator admixtures

There are also examples described in the literature [167,1691 where the admixture accelerates the nucleatlon. This may be encountered in cases where the admixture reacts with the macrocomponent to form less soluble substances. Admixtures that have a common Ion with the macrocomponent can decrease its solubility, this leading to a rise In supersaturation and thus to a decrease of Induction periods of nucleatlon (101,102,244]. Another reason can be given In the case of admixtures with a significant hydration ability they remove water from the hydration spheres of the macrocomponent 182,170,174] and in this way decrease the solution stability [133]. 

Water-reducing and retarding admixtures are required to extend the setting time from 1 to 3.25 hours with respect to the reference concrete. Water-reducing and accelerating admixtures accelerate the setting time between 1 and 3.5 hours. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

Accelerator in Ready-Mix Concrete. Calcium chloride accelerates the set time of concrete giving it a high early strength development. It is not an antifreeze, but by using it duriag cold weather protection can proceed in a timely manner (31—34). In Russia, calcium chloride forms a component of several antifreeze admixtures (33). Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available (21,35). There is no consensus on what the safe levels of calcium chloride in concrete are. 

Retarders and Accelerators. Materials that control hardening of cement may be either organic or inorganic. Retarders are often incorporated in oil well cementing and hot-weather concrete appHcations, whereas accelerators may be useful for cold-weather concrete appHcations in which higher rates of reactivity are desirable. In most cases, these admixtures are used in low concentrations, suggesting that they act by adsorption. 

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. 

Diaikanol aminoalkyl phenols as admixtures enhance the strength [675]. The additives are useful in very small amounts and do not affect the initial properties of the fluid. The strength additive does not cause set acceleration or early set strength enhancement but provides enhanced compressive strength of the cement in later stages. Addition of small amounts of potassium ferricyanide and nitrile-trimethyl phosphonic acid promotes the formation of complex compounds and thus increases the strength of cement rock [1771]. 

Water, methanol, and n-hexane do not influence the photooxidation of PVC (43), but the photodegradation is accelerated by ferric chloride (70,71) and certain other compounds containing iron (70,71,72). Purification of the polymer might be expected to enhance its photostability by removing deleterious impurities such as iron compounds that are derived from metal equipment. This type of result was obtained in one recent study (58) but not in others (30,59). In contrast, the photo-oxidative degradation of PVC should be enhanced by admixture of the polymer with materials that are unusually susceptible to photooxidation themselves. Such behavior has been observed for impact-modified PVC containing polybutadiene-based polyblends (69,73). 

The accelerating water-reducing admixtures are simple blends of either calcium chloride, nitrate, thiocyanate or formate with a lignosulfonate or a hydroxycarboxylic acid salt. In some cases it may not possible to obtain a completely sediment-free solution and agitation of store tanks may be necessary. Typically, a mixture of approximately 33% calcium chloride and 4% calcium lignosulfonate by weight in water would be used. 

As far as the final hydration products of ordinary Portland cement are concerned, there is an indication from isothermal calorimetry [57] that there is very little difference in the presence or absence of a calcium lignosulfonate water-reducing admixture. In this work, the heat evolved per unit of water incorporated into the hydrate has been determined for two cements, with the results shown in Fig. 1.25. It can be seen that the relationship between the amount of heat evolved and the amount of water combined with the cement is maintained whether the admixture is present or not. This work also indicated that the retardation in the early stages is compensated for at later times by an acceleration. 

When a normal, accelerating, or retarding water-reducing admixture is utilized to increase the workability of a concrete mix by direct addition, it would be reasonable to assume that the extent of the effect would be markedly affected by changes in mix design parameters such as cement content, aggregate size, shape and grading, and the water-cement ratio. A study of many hundreds of results, however, indicates that this is not the case and Fig. 

In view of the known deleterious effect of admixtures containing calcium chloride and the possibility of the same effect being found with calcium formate, it is suggested that accelerating water-reducing admixtures should not be used in those areas where sulfate resistance is of importance. 

The action of an admixture in relation to attack on reinforcement can be considered either in direct chemical reaction with the steel or, alternatively, a breakdown of the passive layer imparted by concrete which normally prevents corrosion at the cement/steel interface. In this respect, any accelerating water-reducing admixtures containing calcium chloride can be considered hazardous as far as raising susceptibility of steel reinforcement to corrosion is concerned. It is particularly so at calcium chloride contents in the concrete at or above 1.5% by weight of cement as discussed in the section on accelerators. The use of such materials has been controlled by relevant codes of practice where embedded metal is present in the concrete. 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

Dampproofing admixtures are formulated to affect the properties of the hardened concrete, and not those of concrete in its plastic state. In the case of materials based solely on calcium and aluminum stearates, stearic acid in solid or emulsion form, bitumens and hydrocarbon resins, there will be no effect on the properties of the plastic concrete with regard to air content, workability, mix design parameters, etc. When water-reducing admixtures or accelerators are included in the formulation, the effect on the concrete will be a function of the particular type of material used (see relevant section). The wax emulsions do appear to have an effect on the properties of the plastic concrete because of the lubrication effect of the very small... 

It is obtained either as a liquor or as flake material of approximately 20% moisture content, and for use as an accelerating admixture is normally supplied as a 33-35% solution. 

It is obtained as a fine powder and is supplied normally in this form as an accelerating admixture because of its limited solubility in water (about 15% at normal room temperature). 

Most admixtures of this type do not significantly alter the rheology of cement pastes at early ages. The quicker stiffening of accelerated pastes will, of course, result in higher viscosities at a later age. More complex formulations occasionally include water-reducing admixtures to reduce the water-cement ratio, and their effect will be a function of the water-reducing admixture type and content (see Section 1.3.1). 

Triethanolamine is not an effective accelerator when used alone because of its adverse effect on the resultant strength of the hardened paste and because it can act as an accelerator or a retarder depending on the dosage. It is used as an ingredient in some admixture formulations, however, and investigations have shown that chemical interactions occur [8, 9]. 

It has already been shown that the presence of accelerating admixtures produces hydration products of a different type to those from a plain cement paste because of chemical involvement, predominantly with the C3A phase... 

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. 

Accelerating admixture type Stiffening time (h) for penetration resistance of 0.5 N mm- 3.5 N mm-2... 

Accelerating admixtures reduce both the initial and final setting time of mortar sieved from concrete mixes, determined by ASTM C403 68 or BS 5075 (1975). Typical results are given in Table 5.1 for a 300 kg m cement content mix with a compacting factor of 0.85 0.02 [24] at normal ambient temperature. 

There are only limited data available on the effect of other accelerating admixtures, although one comparative study [46] suggests that calcium formate... 

When concrete is to be placed in cold weather, it is preferable that accelerators or antifreezers be used in combination with air-entraining agents and water-reducing admixtures. These combinations not only reduce the amount of freezable water in the mix but also generally reduce the quantity of antifreezers and accelerators needed to obtain desired effects compared to the amounts that have to be used when these are used separately. In addition these combinations may be useful in increasing the resistance of concrete to frost action and to corrosive agents. 

Admixture formulations containing two or more compounds (multicomponent) in which each component plays a specific role or compliments the corrosion resistance capacities of the other are also used. For example, a mixture of calcium nitrite and calcium formate is used to both accelerate strength of the concrete and inhibit corrosion of the steel during steam curing. 

Two types of calcium nitrite-based corrosion inhibitors are currently marketed, viz. a set- and strength-accelerating type and a normal-setting type. The former increases the early strength development in concrete. This effect increases with the dosage. Both admixtures are compatible with all types of Portland cements and... 

It is important to ensure that when using corrosion inhibitors with other conventional admixtures, they are added separately, at different times of the mix cycle. Corrosion-inhibiting admixtures which also accelerate the set of concrete may require the combination of a retarding admixture when ambient and mix temperatures exceed 35°C. In like manner, set-retarding corrosion inhibitors may require the addition of an accelerator to offset the retardation of early strength development (e.g. use of sodium nitrite in conjunction with sodium benzoate). 

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