Title:
Mechanisms of Delayed Ettringite Formation in
Portland Cement Systems
Author(s):
Yan Fu and James J. Beaudoin
Publication:
Materials Journal
Volume:
93
Issue:
4
Appears on pages(s):
327-333
Keywords:
cement; ettringite; expansion; gypsum; precast concrete; temperature.
DOI:
10.14359/9818
Date:
7/1/1996
Abstract:
Sulfate adsorption from internal sources in systems containing gypsum and C-S-H gel derived from either hydrating C3S or portland cement was studied with respect to delayed ettringite formation. The influence of C3A addi-tion on sulfate desorption was also investigated. Research indicates that C-S-H gel will adsorb sulfate faster at elevated temperatures, resulting in quick depletion of the gypsum phase in C-S-H-gypsum mixes. The critical temperature for sulfate adsorption by C-S-H gel is above 65 C. Sulfate adsorbed at high temperatures is desorbed more slowly than that adsorbed at normal temperatures. The slower release of sulfate from an internal sulfate source may be a critical condition for delayed ettringite formation in high-temperature-cured portland cement paste. Two competitive reactions in terms of gypsum consumption are discussed. These involve both C-S-H gel and the hydrated calcium aluminates. Further evidence in support of a hypothesis for preferred nucleation of delayed ettringite in cracks in portland cement paste is presented. Results presented indicate that the reactants for delayed ettringite formation in cracks can come from sources away from the cracks. Diffusion of reactant ions through concrete pore solution appears to be a key element of the process responsible for initiation of delayed ettringite nucleation in cracks.