The effect of lead nitrate on the early hydration of portland cement

Solution analysis, calorimetry and electron microscopy have been used to study the retarding effect of Pb(NO₃)₂ admixtures on the early stages of hydration of Portland cement. Analyses of cement filtrates show rapid precipitation of basic lead compounds incorporating nitrate and sulphate. The precipitation is accompanied by an increase in the early heat liberation followed by a longer term retardation. Microscopy shows that the precipitate is largely in colloidal gelatinous form and coats the surfaces of the cement grains. The protective effect of these coatings is clearly responsible for the inhibition of hydration.