Q.: Test cylinders for an upcoming job are supposed to be cured on site. ASTM C 31 (Section 7.3) says that after the cylinders are stripped they should be cured in saturated limewater. How much lime should I add to a 55-gallon drum of water to make a saturated limewater solution?

A.: Hydrated lime (calcium hydroxide) makes a saturated solution when less than 0.2 percent has dissolved. This is only 2 grams per liter or 0.9 pound per 55 gallons.

The definition of a saturated solution is one that contains as much solute (lime) as the solvent (water) will dissolve while in contact with excess solute. The simple answer to the problem is to thoroughly stir 2 to 5 pounds of hydrated lime into 55 gallons of water, allowing the excess to settle. The excess in the bottom will ensure that the solution remains saturated.