Ceramics-Silikáty 52 (1) 8-15 (2008)

INFLUENCE OF DELAYING ADDITION TIME OF SUPERPLASTICIZERS ON CHEMICAL PROCESS AND PROPERTIES OF CEMENT PASTES

M. Heikal, I. Aiad

This paper presents the influence of delaying addition time of water-soluble polymer based on poly condensate products namely; sulphonate melamine (SMFF) and melamine-urea formaldehyde-free (SMUFF) on physico-chemical properties of cement pastes. The superplasticizers were added by two different methods, a) either together with mixing water (simulta-neous addition time) or b) later addition, after specified time (delayed addition time). The minislump was determined at different time intervals namely 30, 60, 90, and 120 minutes. The electrical conductivity of cement pastes admixed with and without superplasticizers was also studied. The results show that the initial electrical conductivity decreases with the delayed time of admixture addition. Delaying addition time from 10 to 15 minutes resulted in an increase in the height of electrical conductivity maximum. Delaying addition time enhances the fluidity of cement pastes than simultaneous addition time. From the results it has been shown that the enthalpy values corresponding to the decomposition of the calcium silicate, calcium aluminate and sulphoaulminate hydrated phases as well as Ca(OH)₂ decrease in superplasticized specimens when compared to neat cement paste. This is attributed to the retardation effect of the superplasticizer. The maximum retardation was shown at 20 minutes delaying addition time.

Keywords: Superplasticizers, Delayed addition time, Minislump and electrical conductivity