FUNDAMENTAL ACTIONS OF AIR-ENTRAINING AGENTS

THE FORMATION AND RETENTION OF AIR BUBBLES IN CEMENT PASTES CONTAINING SURFACE-ACTIVE AGENTS, DEPEND GENERALLY ON TWO MAJOR FACTORS: (1) AFTER ANY PRECIPITATION OR ADSORPTION, THE RESIDUAL CONCENTRATION OF THE SURFACE-ACTIVE AGENT IN THE MIXING WATER OF A PASTE MUST BE SUFFICIENT TO GENERATE AIR BUBBLES. (2) THE SURFACE-ACTIVE AGENT SHOULD BE ADSORBED ON THE CEMENT PARTICLES MAKING THEM HYDROPHOBIC SO THAT AIR BUBBLES MAY ADHERE TO THEM. THE CONDITIONS GOVERNING THE FORMATION OF STABLE AIR-ENTRAINED MORTARS AND CONCRETES BY MEANS OF SURFACE-ACTIVE AGENTS ARE SIMILAR TO THOSE FOR CEMENT PASTES. HOWEVER, UNDER SOME CONDITIONS, THE AGGREGATE INFLUENCES THE AIR-ENTRAINMENT PROCESSES BY ENTRAPPING AND RETAINING AIR BUBBLES IN THE INTERSTICES BETWEEN AGGREGATE PARTICLES. THE VISCOSITIES AND BLEEDING PROPERTIES OF AIR-ENTRAINED CEMENT PASTES ARE AFFECTED BY BOTH THE PRESENCE OF AIR BUBBLES AND THE TYPE OF ADSORPTION OF THE SURFACE-ACTIVE AGENT ON CEMENT. ALTHOUGH THE IMPROVEMENT IN WORKABILITY OF MORTARS AND CONCRETES PRODUCED BY AIR-ENTRAINMENT, IS DUE LARGELY TO THE PRESENCE OF AIR BUBBLES, THE HYDROPHOBIC ADSORPTION OF SURFACE-ACTIVE AGENTS CAN INFLUENCE SUCH PROPERTIES AS COHESIVENESS AND RESISTANCE TO SEGREGATION. THE SPECIFIC SURFACE AREAS OF ENTRAINED BUBBLES IN CEMENT PASTES DEPEND TO A LARGE EXTENT ON THE TYPE OF SURFACE-ACTIVE AGENT USED AND THE EFFICIENCY OF MIXING. /CRIC/FESR/RRL/A/