Influence of Silica Fume/Fly Ash/Superplasticizer Combinations in Powder-like Complex Modifiers on Cement Paste Porosity and Concrete Properties

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Title: Influence of Silica Fume/Fly Ash/Superplasticizer Combinations in Powder-like Complex Modifiers on Cement Paste Porosity and Concrete Properties

Author(s): S. Kaprielov and A. Sheinfeld

Publication: Special Publication

Volume: 195

Issue:

Appears on pages(s): 383-400

Keywords: concrete; fly ash; paste (cement); silica fume; strength (compressive); superplasticizer

Date: 7/1/2000

Abstract:
A new generation of complex modifiers for concrete on organo-mineral basis have recently appeared on the Russian construction market. These powder type modifiers with bulk weight around 800 kg/ m3 consist of a mineral part - silica fume (SF) or its combination with fly ash (FA), and an organic part - superplasticizer (SP) or its combination with setting regulator (SR) . Effects of the modifiers on the structural parameters of cement paste and concrete - porosity, phase composition, hydration degree, compressive strength and permeability - were studied. Four types of modifiers with mineral to organic parts in the ratio of 10: 1 were used. The modifiers had different mineral parts: the first consists of pure SF (100%); the second and third of a combination of SF and FA (70:30) and (50:50) respectively; and the fourth of pure FA (100%). The organic part of the modifiers comprises SP on the basis of naphtalen-sulfoacid and formaldehyde polycondensate. Cement paste structure was studied by several methods. A combination of different methods enabled porosity on overmolecular, submicroscopic, microscopic and macroscopic levels to be defined. Peculiarities of cement paste structure provided by the four different types of modifiers were revealed. The influence of silica fume and fly ash proportion on cement paste porosity and phase composition, as well as on compressive strength and permeability of concrete was investigated. It was concluded that the key factor that controls a modifier’s efficiency is the proportion of silica fume in its mineral part. However, at replacements of up to 50% silica fume by fly ash, the efficiency of a modifier remains rather high.