Q.: We noticed the smell of ammonia in fresh concrete mixes that were made with cement from several cement hauling trucks. In checking back, it appears that when these cement trucks were not being used for hauling cement they had been used for hauling urea fertilizer. We suspect that the urea was not cleaned out of the trucks before cement hauling. We have made trial mixes and as far as we can tell they behave like mixes that have not been contaminated at all. Is it safe to use this cement in concrete? If not, are there some precautions we can take so that we may be able to use it?
A.: We do not know of any published information on the effect of urea mixed into concrete but the chemistry of urea is such that in small quantities it could not do any harm to the concrete. Inasmuch as there has not been any noticeable change in the setting time of your trial mixes (which could happen if the urea had formed significant amounts of carbonates) the urea does not seem to have had any chemical effect. The small amount of contamination is not likely to do any harm. This does not, however, justify the use of cement trucks without thorough cleaning if they are used for hauling some foreign material before hauling cement.