Early stiffening includes both false set and flash set, and is the early development of rigity in the work-ability or plasticity of cement paste, mortar, or concrete.

 

Flash set or quick set is evidenced by a rapid and early loss of work-ability usually with evolution of considerable heat.  Remixing WILL NOT regain work-ability if flash set is occurring.  Flash set is generally no longer a problem since the introduction of gypsum to control set time.

 

False set is the rapid development of rigidity without the evolution of much heat. Remixing WILL regain work-ability if false set is occurring.  False set is also known as rubber set, hesitation set, grab set, or premature stiffening.

 

When the clinker is ground, gypsum (hydrated calcium sulfate) is introduced.  If the clinker is not sufficiently cooled before mixing it with the gypsum, some of the water in the gypsum may be driven off, dehydrating the gypsum and forming plaster.  The presence of plaster may cause false set.  Because the plaster crystals are weak and the amount is small, remixing can destroy the matrix.

 

To test for false or flash set, a mortar is mixed and a “boat” is filled. (The boat is a small rectangular mold 2” high by 2 wide by 6” long)  A rod is placed on the surface of the mortar and then released, penetrating into the mortar.  This penetration distance is then measured at 3,5,8 and 11 minutes after the water and cement come into contact.  The mortar is then remixed for one minute and the penetration is measured one more time.

 

If the cement is experiencing false set, the penetration will decrease significantly and then increase after the remix.

 

If the cement is experiencing flash set, the penetration will decrease significantly and not increase after the remix.

 

It is important that ready-mix drivers be aware of false set since it is not uncommon and is easy to fix if caught in time.

 

It should be standard practice to put a few revolutions on the drum at mixing speed at the job site before discharge.