Binani Cement Limited
Key factors that control the compatibility between Naphthalene-based super plasticizers and ordinary Portland cements
Synopsis: Super plasticizers improve the workability of concrete at low water cement ratio but this workability is sometimes lost rapidly in the first hour after contact between the cement and water. This is especially the case for naphthalene-based and melamine-based superplasticizers when used with the so called incompatible cements. It is possible to reasonably predict the rheological behavior of a given Portland cement when used with a naphthalene-based superplasticizer once the physico-chemical composition and the properties of the clinker, of the cement, and of the superplasticizer are known. Different Portland cements, having a wide range of C3A contents (6.0 to 11.8%) and SO3 contents (0.09 to 2.90%), made with clinkers having a wide range of alkali contents (0.07 to 0.87% of Na2Oeq), have been selected to highlight the key cement factors that control the compatibility between ordinary Portland cements and the naphthalene-based superplasticizers. The soluble alkalis (in fact the soluble SO4 2- ions from alkalis), the fineness, the C3A content, and the type of CaSO4 of the cement are among the key cement factors that control the rheological behavior of a superplasicized cement paste and concrete. An optimum amount of soluble alkalis content exists and ranges between 0.4 and 0.6% of Na2Oeq in the cement. The rate of the naphthalene based superplasticizers’ adsorption on the cement particles and cement hydrates, which is influenced by these parameters, controls the loss of fluidity of the concrete. Moreover, this study has shown that concrete made with cements that have a low soluble alkali content not only lose their slump when under dosed in super plasticizer but also present severe segregation and bleeding when the dosage of the super plasticizer is slightly higher than the saturation point. In...
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