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2014SumShotcreteEMag

Technical Tip Retarders versus Set Stabilizers By Dan Millette When concrete is cast, a retarding admix- ations can arise, such as in mining or tunneling,-counterintuitive to use both a retarder and anaccelerator for the same mixture, but many situture is often used to keep the mixtureworkable until finishing is completed. This seems most common with concrete used in where a mixture is batched, travels over an hour flooring applications where more finishing time to the work site, is then sent underground, and is required. If concrete is being used a consider- must be transported from the point of underground able distance from the ready mix plant, or the delivery to the work area. I have seen this take up ambient temperature at the construction site is to 5 hours in many cases. And what if, for very warm, these are good reasons to use a example, there is a further delay while the sprayer retarder. Even at normal or lower temperatures, is being repaired? And when you are finally ready concrete loses its workability with time. A retarder to use the mixture, you need to accelerate it at the can be dosed at a specific amount depending on nozzle. This is fine if the retarder is wearing off, the extended time of setting that is required to but if the retarder was dosed to hold off the set give some flexibility to the operation. As seen in time for 6 hours and you are ready to use the Fig. 1, temperature has a very significant effect mixture in 2 hours, it would be correct to assume on the rate of cement hydration. you would need a lot more accelerator to over- Then why can we not use retarders for shot- come the effect of the retarder. That is why set crete? But we can and do. Retarders are great for stabilizing admixtures or set stabilizers are used shotcrete for many of the same reasons as for in many shotcrete mixtures. concrete, because shotcrete is concrete: extended First, let’s take a look at how retarders work. travel distances; time required for placement, The purpose of the retarder is to delay the setting especially if you need to move a pump during time of the cement paste in the mixture. When placement; hot weather at the job site, and so on. water is added to cement, there is a rapid initial But what must be considered is if we are using reaction (hydration) that occurs, after which an accelerator at the nozzle for any particular there is very little formation of further hydrates reason—are retarders still suitable? It seems for approximately 2 to 3 hours. This is called the dormant period but can be significantly short- ened due to cement type and/or temperature. The hydration rate increases at the end of the dormant period and a lot of calcium silicate hydrate and calcium hydroxide forms relatively quickly, which is what corresponds to the setting time of the mixture. A retarder added to the mixture will delay the end of the dormant period and therefore the start of setting and hardening. This is done by a mechanism based on absorption. The larger admixture anions and molecules are absorbed by the surface of the cement particles to hinder reac- tions between cement and water. In other words, the retarding admixture coats the cement particles. Most retarding admixtures are made with sugars or sugar derivatives while others contain ligno- sulfonate, which is a waste product from the pulp and paper industry. Many retarders are also water Fig. 1: Effect of temperature on the rate of hydration (Rixom and reducers. ASTM C494 Type B designates that the Mailvaganam 1999) admixture is a retarder only. Sugars retard the 52 Shotcrete • Summer 2014


2014SumShotcreteEMag
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