High Strength Admixture

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Apple Chemie has wide range of admixtures used in construction industry which are environment friendly, sustainable, durable and efficient to use . The Apple Chemie products covered in this range are :
>Hyper Plasticizer
>Additive for Self Compacting Concrete
>High range water reducing admixtures
>Shotcreting, Guniting, Accelerators & Admixtures
>Hardening Accelerator Type Admixture
>Anti Freezing Type Admixture

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Effects on Concrete

Water-reducing admixtures are primarily used to reduce the water-cementitious content of concrete, thus increasing strength. In some cases, they can be used to increase the workability or slump of the concrete providing for easier placement. Mid-range water-reducing admixtures were developed to increase the slump beyond the range available with regular water reducers without the excessive retardation that had been known to occur. High-range water reducers, commonly called superplasticizers, were developed for high-strength and high-performance concrete applications.

Superplasticizers, e.g., Multiplast Super can take a 3- inch slump concrete to a 9-inch slump without risk of segregation and without compromising its strength. Many forecasters can benefit from the use of a superplasticizer, especially because of its improved high early strength development.

All water-reducing admixtures increase strength development as a result of better dispersion of the cement. This increases the exposed surface area of the cement particles, allowing for more complete hydration of the cement.

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Set-Controlling Admixtures

Set-controlling admixtures alter the rate of the cement’s hydration and, therefore, the rate of setting (stiffening) of the paste. Coincidentally, they also may affect the hardening or strength gain after the paste has set. Yet controlling admixtures include retarding and accelerating admixtures.

Retarding Admixtures

These admixtures, slow down the hydration process. They may also reduce the setting time of cement. Retarding admixtures fall into two categories: regular and extended-set. Regular, most commonly referred to as just “retarders,”is used to place concrete in hot climates when long travel times are expected or, in case of emergency, when placement is delayed. They are also commonly used for mass concrete pours to prevent cold joints.

Extended-set control admixtures are those used to delay hydration for many hours or even days. These are usually a two-component admixture system. The first component is a retarder (stabilizer) which delays the setting of concrete. The second component is an accelerator (activator) which overcomes the retarder. The concrete typically reaches an initial set in a few hours after the activator is applied.

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How they work Retarders essentially slow early hydration by reducing the rate at which tricalcium silicate (C3S) reacts with water. Furthermore, retarders slow the growth of calcium hydroxide crystals. Both reactions develop the early setting and strength gain characteristics of paste. The effect remains until the admixture is incorporated into the hydrated material, thereby removing it from the solution and allowing for an initial set to occur. The duration of retardation is based on the dose and chemistry of the retarder, cement composition, temperature and the time it was added to the mix.

Accelerators

These admixtures increase the cement’s rate of hydration. They are designed to increase the rate of hydration of C3S, thereby increasing early strength. There are two types of accelerators: rapid and normal.

Rapid accelerators can set concrete in minutes and are used in shotcrete applications, to make repairs against hydrostatic pressure or when the very rapid setting is required. These are typically not used in precast concrete applications.

Standard or normal accelerators are used to speed up construction in cold-weather concreting conditions; however, it is important to note that they are not antifreezing admixtures.

Effect on concrete: Both retarders and accelerators seem to have negligible effects on air entrainment. However, when water-reducing agents are included, such as lignosulfonates, some air may be entrained.

Retarders tend to reduce one-day strengths and usually increase later-age strengths. Retarders may also increase slump loss and cause an early stiffening of the mixture, even though the strength gain has been delayed. Retarders tend to lose their effectiveness as concrete temperature increases. They also tend to increase the plastic shrinkage.

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Accelerators typically increase early strengths. However, later age strengths may be reduced relative to the same concrete without the accelerator. They also tend to increase early-age shrinkage and creep rates, but tests have shown that ultimate values seem to be unaffected.

Combinations

Some admixture chemistries provide for a combination of effects such as water reduction with retardation or acceleration. Advantages of this include reducing the number of admixtures that have to be stored and added to the concrete; less admixture incompatibility; and cost savings. Disadvantages include less flexibility and limited use when an accelerating or retarding effect is not desired. ASTM C 494 lists specifications for these combination admixtures.