Adsorption of Admixtures on Portland Cement

Abstract

In an attempt to verify theories on the effect of admixtures on portland cement, the adsorption isotherms for calcium lignosulfonate and salicylic acid on cement have been determined. The adsorption isotherms for these compounds on 3CaO·SiO2, 2CaO·SiO2, 3CaO·Al2O3, and 4CaO·Al2O3·Fe2O3 and on a mixture of these components in the ratio 60/25/5/10 have also been determined. Adsorption isotherms were determined at 24°C by a spectrophotometric method, using a Beckman DU spectrophotometer. As a check on the method, previous work of Ernsberger and France on the adsorption of lignosulfonates on portland cement was repeated and their results were confirmed. Results indicate that in aqueous solution the constituents of portland cement mainly responsible for the adsorption of the two compounds studied are the hydration products of 3CaO·Al2O3 and 4CaO·Al2O3·Fe2O3. Salicylic acid adsorption on cement compounds does not fit the Langmuir form. However, adsorption of salicylic acid on type I portland cement, for equilibrium concentrations above 0.005%, fits the form. Calcium lignosulfonate adsorption on type I portland cement, 3CaO·SiO2, 2CaO·SiO2, and partly on 4CaO·Al2O3·Fe2O3 fits the Langmuir form. Adsorption isotherms of salicylic acid on the major cement compounds were determined from ethyl alcohol solution to eliminate the hydration of the compounds occurring in aqueous solution. The adsorption of salicylic acid on the materials studied was found to be in the order: The effect of water on the adsorption of salicylic acid from ethyl alcohol solution was studied. It was found that the smaller the amount of water present, above a minimum value, the greater was the adsorption on 3CaO·Al2O3, 2CaO·SiO2, and 3CaO·SiO2. In the case of 4CaO·Al2O3·Fe2O3 the adsorption of salicylic acid was proportional to the amount of water present at all concentrations studied. It is observed in certain cases that it is probably not surface adsorption but a compound formation with salicylic acid.