Abstract: A modified hydraulic cement product comprises a hydraulic calcium silicate-based cement having a Blain surface area of at least 3,000 cm.sup.2 /g, and/or an iron content of less than 2.5% by weight expressed in terms of the ferric oxide content of the dry cement, in which the cement has been treated with an additive comprising an aldehyde or an amide or substance convertible to an aldehyde or amide under the alkaline conditions prevailing in the cement before or during the setting reaction. Treated cement may be hydrated subsequent to or in parallel with the additive treatment to give hydrated products which characteristically exhibit modified cement morphologies, in particular fibrous cement morphologies, and generally also modified physical strength properties, e.g. tensile strengths of at least about 10 MN/m.sup.2 and compressive strengths of at least about 60 MN/m.sup.2, as compared with untreated hydrated cement products.