ASTM C359 Standard Test Method for Early Stiffening of Hydraulic-Cement (Mortar Method) is used to determine the degree to which a cement mortar develops early stiffening. This is done by comparing the behavior of the first series of penetration and the remix procedure to differentiate a relatively less serious and less persistent tendency to early stiffening (false set) from one that is more persistent and more serious (flash set).

Early stiffening resulting from false set is not likely to cause a cement to fail the applicable time of setting requirement, while that resulting from flash set (depending on severity) can cause a cement to fail.