Formation of organo-mineral phases at early addition of superplasticizers: The role of alkali sulfates and C3A content

Most superplasticizers (SPs) exhibit significantly higher dispersing performance when added to an already mixed concrete (delayed mode) instead of addition with mixing water (early addition). Here, the specific parameters which determine this effect were investigated. At first, the tendency of four polycarboxylates and a naphthalene-polycondensate to intercalate into C(3)A hydrates was correlated with the flow values of pastes prepared from six different commercial cements at early and delayed addition. It was found that the contents of alkalisulfates and C(3)A present in the cement samples play the key role. For cements exhibiting a low alkalisulfate/C(3)A molar ratio (0.04-1.29), huge differences in paste flow at early and delayed addition were observed, presumably because of chemisorption (intercalation) of SP into C(3)A hydrates. While at high alkalisulfate/C(3)A molar ratios (similar to 2), AF(t) (ettringite) is formed predominantly, and SP intercalation is impossible. Consequently, similar flow values are attained at early and delayed addition. (C) 2014 Elsevier Ltd. All rights reserved.