Importance of adequate soluble alkali content to ensure cement superplasticizer compatibility

The effect of soluble alkalis on cement/superplasticizer compatibility has not yet been thoroughly understood. This paper establishes the impact that soluble alkalis have on the compatibility of cement and polynaphthalene sulfonate superplasticizer during the first few minutes of hydration. The amount of soluble alkalis that go into solution during the first few minutes is a key parameter in controlling fluidity and fluidity loss of a cement paste made with superplasticizer. The optimum soluble alkali content for increasing initial fluidity and decreasing fluidity loss with time was found to be on the order of 0.4%-0.5% Na2O equivalent in the six cements under study. Furthermore, this optimum al kali content is independent of the superplasticizer dosage and cement type. Ln cement with an optimum amount of soluble alkalis, the C(3)A content has practically no effect on fluidity loss. (C) 1999 Elsevier Science Ltd. All rights reserved.