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2014FallShotcreteEMag

Shotcrete Corner Shotcrete Accelerators for Wet-Mix By Dan Millette and Marc Jolin Umixture during batching, but rather intro- reduced. Sodium silicate can also decreasebeen known to decrease ultimate strength by asmuch as 25%. Durability can also be considerablynlike traditional concrete accelerators,shotcrete accelerators are not added to the duced to the mixture at the nozzle in wet-mix waterproofing characteristics because of the shotcrete application. Dry-mix will sometimes leaching of lime when the concrete is subjected have a powdered accelerator incorporated into the to continuous moisture. One other, potentially mixture that begins to react as soon as the water negative, effect is that these accelerators increase is added at the nozzle. These accelerators can the risk of alkali-silica reaction (ASR) and provide fast setting or stiffening of the mixture, leaching of water-soluble portions. early-age strength development, the capability to Sodium silicates are relatively low in cost, so attain increased layer thickness, improved over- they are popular in instances where shotcrete is head spraying performance, and reduced sagging, needed to attain a quick stiffening and then gain all resulting in a higher productivity. some early strength rather quickly. They are not Shotcrete accelerators work on the C3A recommended for permanent exposed shotcrete. (tricalcium aluminate) fraction of the cement, Because of the high alkalinity of this type of influencing the rate of hydration, resulting in heat accelerator and the high alkalinity of the shotcrete evolution and early-age formation of calcium mixture, the mist that blows back from spraying silicate hydrate (C-S-H) gel (Fig. 1). There are the mixture can burn skin and eyes, so adequate two main types of accelerators used in wet-mix protection must be used. in North America. The first type of accelerator The second type of wet-mix accelerator is an that we will look at is sodium silicate-based aluminum-sulfate- or aluminum-hydroxide-based accelerator (water, glass, and modified silicates). product that is commonly called alkali-free. These These have been around a long time and are still are often combined with various amines to boost being used in many applications. They work very their effectiveness. These accelerators are a little quickly and allow the material to build well. more sensitive than sodium silicates to several A sodium-silicate-type accelerator actually environmental and material conditions and do not gels the water in the mixture to increase cohesion always give as fast a stiffening effect, but they within the mixture as well as decrease the set time also do not have quite as severe a detrimental of the cement. Sodium silicate-based accelerators effect on 28-day strength when used at lower have a high alkali content and can be quite caustic, dosages. But even with these accelerators, a as the pH is above 11. Although these accelerators dosage of 10% by weight of cement can give up will display fast stiffening of the mixture, they to a 25% reduction in 28-day strengths. Most can significantly decrease the final or 28-day modern-day specifications where accelerators are strength of the mixture—up to a 50% reduction. required insist on alkali-free accelerators only, as Even a dosage of 5% by weight of cement has alkali-free accelerators do not have any effect on ASR. Most alkali-free accelerators range from pH 2 to 3, which is fairly acidic. When spraying these, the acidity is tempered by the alkalinity of the concrete mixture so it is not as great a hazard to skin and eyes. But these accelerators are also very corrosive. Storage tanks for alkali- free accelerators must be either plastic or Fig. 1: C-S-H gel at work stainless steel—never store these accelerators in 44 Shotcrete • Fall 2014


2014FallShotcreteEMag
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