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Setting, setting time, admixture vs additive

Short answer

Setting is the change from plastic to rigid state in hours; hardening is subsequent strength gain. Initial setting is when paste loses plasticity (Vicat needle penetrates 5±0.5 mm from bottom, min 30 min); final setting is when needle leaves imprint but no full penetration, max 600 min. Admixtures are added at mixer in small doses (<5% cement); additives are ground with clinker.

What do you think is the basic difference between i) setting and hardening, ii) initial and final setting time, and iii) admixture and additives?

i) Setting and hardening

  • Setting: the change of the fresh paste from a plastic to a rigid (but weak) state; it takes hours and is tested with the Vicat apparatus.
  • Hardening: the gain of strength after setting, taking days to years; it is tested by compressive strength tests.

ii) Initial and final setting time

  • Initial setting time: the time from adding water to the point where the paste begins to lose its plasticity (the Vicat needle of 1 mm square section penetrates only to 5 ± 0.5 mm from the bottom of the mould). IS 269: not less than 30 minutes (OPC). Concrete must be placed and compacted before this time.
  • Final setting time: the time at which the paste has become hard enough that the needle makes an impression on its surface but the annular attachment fails to do so. Not more than 600 minutes (10 h).

iii) Admixture and additive

  • Admixture: added to the concrete (or mortar) at the mixer, in small amounts (usually below 5% of the cement mass), to change its fresh or hardened properties; e.g. superplasticiser, retarder.
  • Additive: added to the cement at the factory (during grinding), or in large quantities by weight of cement, e.g. gypsum, fly ash, slag; it becomes part of the cementitious material.

✓ Answer checked by Claude Opus 5.5 (high effort)

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