Which admixture is used to slow the setting of concrete?

Which admixture is used to slow the setting of concrete?

• A. retarding admixture
• B. accelerating admixtures
• C. air entrainment
• D. water reducing admixtures

Answer: (A)

Slowing the setting of concrete is achieved by using a retarding admixture. These admixtures work by interfering with the cement hydration process—typically by adsorbing onto cement grains and binding calcium ions—so the chemical reactions that cause stiffening are delayed. This extends the usable work time of fresh concrete, which is especially helpful in hot weather, for long transport, or when placing large pours to prevent early stiffening or cold joints. Common retarders include lignosulfonates and similar organic compounds. Accelerating admixtures do the opposite, speeding up setting and early strength. Air entrainment primarily improves durability (freeze-thaw resistance) by introducing tiny air voids and doesn’t aim to delay set. Water-reducing admixtures improve workability and reduce water content; their main purpose isn’t to slow setting, though they can influence set timing to some degree depending on dosage and cement.

Slowing the setting of concrete is achieved by using a retarding admixture. These admixtures work by interfering with the cement hydration process—typically by adsorbing onto cement grains and binding calcium ions—so the chemical reactions that cause stiffening are delayed. This extends the usable work time of fresh concrete, which is especially helpful in hot weather, for long transport, or when placing large pours to prevent early stiffening or cold joints. Common retarders include lignosulfonates and similar organic compounds.

Accelerating admixtures do the opposite, speeding up setting and early strength. Air entrainment primarily improves durability (freeze-thaw resistance) by introducing tiny air voids and doesn’t aim to delay set. Water-reducing admixtures improve workability and reduce water content; their main purpose isn’t to slow setting, though they can influence set timing to some degree depending on dosage and cement.