What is false set?
False set is the premature stiffening of cement paste, mortar or concrete a few minutes after mixing, before the cement has actually begun to set. The mix appears to “go off”, but vigorous remixing restores its workability. It is also known as early stiffening, and less formally as rubber set or grab set. The Dutch term is valse binding.
Causes
Portland cement is ground with a small amount of gypsum, which regulates setting. Heat in the cement mill drives water out of part of the gypsum, forming hemihydrate. If too much of it is present, it recrystallises as gypsum soon after water is added and stiffens the mix. Storage at high temperatures can have the same effect. Incompatibility with certain admixtures, such as some lignosulfonate-based plasticisers, is another known cause.
False set or flash set
- False set. Stiffening without significant heat. Remixing, without adding water, breaks it down, and strength is normally unaffected.
- Flash set. Occurs when there is too little soluble sulfate to control the reaction of the aluminate phase in the cement. The mix stiffens permanently, with noticeable heat, and remixing will not restore it.
Rule of thumb: if the mix becomes plastic again after a minute of extra mixing, it was false set. In the laboratory, ASTM C451 (paste) and ASTM C359 (mortar) test for early stiffening, using a remix step to tell the milder false set from the more serious flash set.
Tips and points of attention
- Do not add water. Retempering with water raises the water/cement ratio and lowers strength and durability. Mix longer first.
- Store cement cool and dry. Keep bags out of direct sun and hot rooms.
- Check admixtures. If the problem only appears with a particular admixture, ask the supplier about an alternative product or cement type.
- If it stays stiff after remixing, do not use the batch.
Related terms
- Setting
- Cement
- Portland cement
- Gypsum
- Fresh concrete
- Concrete admixture
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