Effect of cement on the demulsifying behavior of over-stabilized asphalt emulsion during mixing

Abstract

Over-stabilized asphalt emulsion is normally used in cement asphalt emulsion (CA) composites. The mixing stability of asphalt emulsion with cement is vital to the workability and mechanical properties of CA composites. However, the effect of cement on the demulsifying behavior of asphalt emulsion during mixing is not clearly understood. To reveal the demulsifying behavior, effect of pH value and Ca2+ on the rheological properties and mixing stability of over-stabilized asphalt emulsion are studied. Results indicate that cationic emulsions have no obvious change in the tested pH value and Ca2+ concentration range. Ca2+ ion is very harmful to the chemical stability of anionic emulsion. There are three stages in the demulsifying process of pure emulsion: (1) small asphalt droplet coalescence along with trapped water; (2) trapped water diffuses out from coalesced asphalt droplets; (3) complete demulsification. There are two demulsifying behaviors when asphalt emulsion is mixed with fine solid. One is asphalt droplet coalescence, which is dominant in the mixing with limestone powder. The other is the direct droplet adhesion on solid surface, which is dominated in the mixing with cement. This difference is due to that the adsorption ability of cement is much stronger than that of limestone powder. The ions of OH− and Ca2+ released by cement hydration cannot greatly destabilize asphalt emulsion. However, emulsions with better resistance to the invasion of OH− and Ca2+ still have a better mixing stability. Therefore, reducing the adsorption ability of cement and increasing the resistance of emulsions to hydrate ions can improve the mixing stability of asphalt emulsion with cement.