Should Portland Cement Be Dispersed


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Title: Should Portland Cement Be Dispersed

Author(s): T. C. Powers

Publication: Journal Proceedings

Volume: 42

Issue: 11

Appears on pages(s): 117-140

Keywords: none

Date: 11/1/1945

A development of definitions of wetting and dis-persion is followed by a discussion of dispersion of portland cement. From elementary principles it ap-pears that a wetting agent is unnecessary, for portland cement is highly hydrophilic. The dispersed state of portland cement in water is defined as that state in which interparticle attraction in a fresh paste is absent or so weak that it has no appreciable effect on the physical properties of the fresh paste. Experiments and reasoning from general principles indicate that dispersion would be undesirable because it would increase the rate and amount of sedimentation and promote particle size segregation in cement paste; it would destroy the plasticity of the pastes and give them the properties of a fluid, a probably undesirable change* it would have no beneficial effect on rate of hydration during the early stages through an increase in exposed surface area because the whole surface is normally exposed to water even when the particles are flocculated. A reduction in interparticle attraction short of actual dispersion should reduce the water required for a given slump, but it would not improve work- ability except in unusually rich mixes. It would in-crease bleeding. Air entrainment requires an increase in paste content and reduction of water content to maintain a given slump. It reduces strength but improves frost resistance. It improves workability and reduces bleed- ing. Air entrainment together with some reduction in interparticle attraction affects paste content and water requirement in the same way as air entrain-ment alone, but the increase in paste content is smaller and the reduction in water content is greater than when there is no reduction in interparticle at-traction. Air entrainment offsets the undesirable effects of reducing interparticle attraction on plasticity and reduces bleeding. Some agents do not affect the chemical processes of hardening; their effects on strength can be pre- dicted from the voidscement ratio. Others tend to retard hydration unless they contain an accelerating agent. Such agents have different effects with differ-ent cements.