An examination of the ASTM Lime Pozzolanic Activity Test for class C fly ashes

Abstract

The role of the type of calcium hydroxide (“lime”) used in the ASTM C 311-81 Lime Pozzolanic Activity Test (LPAT) has been studied. Three types of lime (reagent grade Ca(OH) 2, a commercial lime and a mixture of the reagent grade lime and MgO) were used with four fly ashes (three Class C and one Class F) in the LPAT. The same four fly ashes were used in the ASTM Cement Pozzolanic Activity Test. The formation of crystalline reaction products in the lime test was monitored by x-ray powder diffraction. All three Class C fly ashes failed to meet the LPAT seven-day 800 psi compressive strength criterion when the reagent grade lime was used but all three passed the test when the other two types of lime were used. X-ray diffractograms of pastes cured for three days at 55°C showed different reaction products formed from the pastes containing reagent grade lime compared to the other two lime sources. It is proposed that impurities in the commercial lime play a key role in determining the pozzolanic activity of Class C fly ashes.