A THERMODYNAMIC PERSPECTIVE ON ADMIXED CHLORIDE LIMITS OF CONCRETE PRODUCED WITH SCMs

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Title: A THERMODYNAMIC PERSPECTIVE ON ADMIXED CHLORIDE LIMITS OF CONCRETE PRODUCED WITH SCMs

Author(s): Vahid Jafari Azad and O. Burkan Isgor

Publication: Special Publication

Volume: 308

Issue:

Appears on pages(s): 1-18

Keywords: Chloride binding; Admixed chlorides; Chloride limits; Cement; Blended cements; Supplementary cementitious materials; Thermodynamic reaction modelling.

Date: 6/1/2016

Abstract:
A thermodynamic modeling investigation was conducted to quantify free and bound chloride concentrations, and pore solution pH, in mixtures produced with different types of blended cements and admixed chlorides. Specifically, the validity of using total cementitious materials content, instead of cement content, as the basis for allowable admixed chloride limits in new construction was evaluated. The effects of replacing OPC with two types of fly ash (class C and F) and one type of slag at different replacement levels were investigated. Assuming chlorides are only chemically bound, the water soluble Cl-/OH- remained rather stable up to 20% class F fly ash, 30% class C fly ash, and 40% slag replacements. Beyond these replacement levels Cl-/OH- were shown to increase, in some cases rather sharply, for the mixtures that were studied in this investigation. The results of this investigation support the ACI committee recommendations for allowable chloride limits to be reported in terms of total cementitious materials content up to 20% class F fly ash, 30% class C fly ash, and 40% slag replacements. Since SCM replacements affect the binding capacity, water soluble chlorides, and the pH of pore solution simultaneously, it is recommended that allowable chloride limits are made in terms of Cl-/OH- rather than chloride contents alone.