Journal Published Online: 01 July 1989
Volume 11, Issue 1

Another Look at the Portland Cement/Chemical Admixture Incompatibility Problem



Reports of incompatibility between portland cement and chemical admixtures have increased over the past ten years. Specifications written by architects and engineers calling for the use of admixtures in concrete often result in strange occurrences, that is, rapid set, accelerated stiffening, increase in time of set, lack of water reduction. This paper addresses the effect of the composition of the cement and that of the chemical admixtures on incompatibility with respect to the problems of rapid set and accelerated slump loss.

The use of natural anhydrite (CaSO4) as a substitute or partial replacement for gypsum (CaSO4 · 2H2O) in the manufacture of portland cement is gaining in popularity. Admittedly, portland cement containing large amounts of natural anhydrite will perform normally, but not necessarily so in concrete containing the common chemical admixtures.

The data presented in this paper attempt to show why large amounts of natural anhydrite in portland cement can cause incompatibility problems in the presence of chemical admixtures. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a “sulfate-starved” system in the concrete, often producing rapid set and an increase in rate of concrete slump loss.

Author Information

Dodson, VH
Construction Products Division, W. R. Grace & Co., Cambridge, MA
Hayden, TD
Construction Products Division, W. R. Grace & Co., Cambridge, MA
Pages: 5
Price: $25.00
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Stock #: CCA10102J
ISSN: 0149-6123
DOI: 10.1520/CCA10102J