Adsorption Mechanism of Comb Polymer Dispersants at the Cement/Water Interface

Abstract

Comb polymers are commonly used as dispersants to stabilize highly concentrated cement suspensions. The effectiveness of such polymeric additives to stabilize these suspensions is determined to a large extent by the amount adsorbed. In this study we investigated the adsorption characteristics of various comb dispersant containing different graft densities on surfaces of cement particle. The effect of inorganic salts on their adsorption was also examined in order to elucidate their adsorption mechanism. The results show that the adsorption of comb polymer dispersants on cement surface conforms approximately to Langmuir's adsorption isotherm and the characteristic plateau A _s and adsorption free energy ΔG_ads are largely dependent on the anionic group content of the comb polymers. The A _s and ΔG_ads increase with increasing anionic group content. This information suggests that the adsorption of comb polymers on cement surfaces is dominated by electrostatic interaction between COO-groups on the comb polymers and the positive surface of the cement. This conclusion is supported by effects of inorganics such as calcium and sulfate ions, and diffuse reflectance FTIR spectroscopy. The implication of results for tuning polymers for the required performance in cement manufacture should be noted.

Keywords:

• Adsorption
• cement/water interface
• comb polymer
• salt effect

This study is financially supported by Jiangsu Provincial Fund for Natural Sciences (Grant No. BK2007722), National Basic Research Program of China (973 Program) (Grant No. 2009CB623200) and Jiangsu Government Scholarship for Overseas Studies. We are grateful to Dr. Xiaohua Fang at Columbia University for very helpful discussions.

Notes

C₃S, tricalcium silicate, C₂S, dicalcium silicate, C₃A, tricalcium aluminate, C₄AF, tetracalcium aluminate ferrite.

^a M _n  = number average molecular weight.

^b M _w  = weight average molecular weight.

^c DPw, calculated according to M _w .

^d calculated value, refers to the number of COO^‒ per grams of polymer.