Synopsis Lead compounds may occur in rocks used as aggregates in concrete and those soluble in lime water will affect the rate of hardening of the concrete. The set will be delayed for 7 days by 0·8 % lead soluble in limewater, for 1 day by 0·15 %. Smaller amounts will delay the set and lower the early strength, but subsequent strengths will be as good as or better than for concretes with an equivalent aggregate without the lime-soluble lead.