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

• Superplasticizers (SP) provide fluidity to mortar and concrete.
• Some cements exhibit differences in fluidity at early or delay addition of SP.
• Effect depends on the contents of alkali sulfates and C3A in cement.
• Phenomenon occurs when the alkali sulfate/C3A molar ratio is < 2.
• Addition of extra sulfate to cement can mitigate the effect in some cases.

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 C3A 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 C3A present in the cement samples play the key role. For cements exhibiting a low alkalisulfate/C3A 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 C3A hydrates. While at high alkalisulfate/C3A molar ratios (~ 2), AFt (ettringite) is formed predominantly, and SP intercalation is impossible. Consequently, similar flow values are attained at early and delayed addition.