Concrete setting time is a variable. Many are the factors which contribute to modify the setting time, seasonality is maybe one of the most important condition which have a huge impact on concrete setting time. In this short post we will just forcus on two ways to modify it, having an extension of setting time or shorten it. These can be achieved using common sugar or salt.
It is well known especially to the ones with more service years behind, that in the past, some kilos of sugar were always available on building site or inside the cabin of the concrete truck, during summer time. It was quite common in fact to suffer some delays while pouring concrete and the easiest way to continue use the delivered concrete batch without adding water was just adding sugar in order to prolong its setting time and so complete the pouring.
Is that still a viable solution? Well, you can read a lot on the net about this subject and shortly said the answer can still be: yes. However I’d like to add a ”but” to this answer.
The process of delay triggered by sugar is (of course) chemical, I am not going in depth here because in this blog we have just ”practical chats”; in plane words the process can be described as a sort of shielding effect around the cement particles so as to hinder the growth of the hydration products. This will contribute to lower the hydration temperature (minimizing the resulting cracks) and decrease concrete viscosity and so keep the concrete workable for a longer time. But not “forever”…this means that after a while not only the hardening process will start again, but also segregation phenomenon will appear and pumping operations can be compromised.
On top of that must be remembered that the quantity of sugar added to the mix must be controlled, normally 0,1-0,3% on the cement weight is a reasonable ratio, but still....avoid DIY! You can reap segregation, almost ”eternal” setting time or no setting at all and similar issues.
Ask the technology department of your supplier, or ask expert technologists to help you out if you want to avoid problems on site. It’s not by a chance that the use of raw sugar is almost abandoned in real life and substituted by chemical sugars in the form of proper admixtures which often comes with clear technical sheets and technical supervision.
Today is always better to foresee in which conditions the concrete pour will happen and agree together with the supplier on the best mix possible for the given situation. This will add a quality warranty that could not be granted if you ”do it yourself”.
And what about salt? Well chlorides accelerate the setting process leading to an early strenght ratio that is commonly higher. Does that mean that one can add sodium chloride (the regular salt we us on food) to concrete during winter time in order to fasten the setting process? In theory yes but I would warmly discourage you from doing it. First of all this kind of chloride is very aggressive toward rebars so it would be like a suicide to add it, second there are specific rules about the quantity of chlorides that can be present in the concrete mix and since some chlorides can be already present in the aggregates and/or in some admixture or in the water (in very low percentages) it would be a non sense to add more.
Some say that calcium chloride could be a viable alternative, but even in this case aggressione to the reinforcement is most likely to happen, the concrete itself could be chemically damaged and in order to comply with certain regulations, only limited, very limited amout of it could be used. So is that impossible to accelerate the setting time of concrete? Not really. Salts could be efficiently used to this purpose, but we are not talking about the regular salt or other chlorides. We are talking about other chemical salts that in the proper dosage are harmless to the concrete and the rebars. Which ones? Well if you really want to go in depth in this topic that means you have a real passion for chemistry...but, sorry this is ”Practically concrete” so just ”practical” stuff! Besides, my warm advice is to find help directly from your concrete supplier involving its technological department in order to taylor-make the best solution for your real need on site.