With today’s concrete mixtures becoming more complex, the possibility for cement-admixture incompatibility problems is increasing. Cement-admixture incompatibility is simply when the concrete experiences the opposite performance that was expected when a particular cementitious-admixture system is used. The occurrence of such problems is ever more evident based on the recent creation of ASTM Sub-committee C09/C01.48, which deals with establishing test methods designed to predict and diagnose material incompatibility issues.
The heart of most incompatibility issues concerns how chemical admixtures can alter the balance of sulfate availability (from the inter-ground gypsum) and aluminate reactivity from the C3A and C4AF phases in the cement.
Remarkably, problems such as poor slump increase and rapid slump loss that can occurr with certain cementitious material-admixture combinations, can be essentially eliminated by delaying the addition of admixture sometimes as little as 30 to 60 seconds after the cement and admixture have mixed together. This delay allows the normal sulfate-aluminate reactivity to occur, thus minimizing unexpected performances.