Sulfonated naphthalene formaldehyde, a common dispersant for portland cement, was found to be incompatible with some cements, the incompatible combinations producing gelation rather than dispersion of cement–water mixtures. The gelation was found to be prevented by the addition of alkali hydroxide or alkali sulfate. The objective of this study was to explore the mechanism behind these incompatibilities. Three hypotheses were tested: ettringite bridging, superplasticizer consumption, and polymer bridging. Results showed that the ettringite formation was responsible for the gelation and that alkali addition inhibited the ettringite formation.