The influence of the addition of very effective superplasticizers, that are commercially available, employed for maximising the solid loading of very high-strength Portland cement pastes, has been investigated. Cement pastes were prepared from deionized water and a commercially manufactured Portland cement (Ultracem 52.5？R). Cement and water were mixed with a vane stirrer according to ASTM Standard C305. The 0.38 to 0.44 water/cement ratio range was investigated. Three commercial superplasticizing agents produced by Ruredil S.p.a. were used. They are based on a melamine resin (Fluiment 33？M), on a modified lignosulphonate (Concretan 200？L), and on a modified polyacrylate (Ergomix 1000). Rheological tests were performed at 25°C by using the rate controlled coaxial cylinder viscometer Rotovisko-Haake 20, system M5-osc., measuring device MV2P with serrated surfaces. The tests were carried out under continuous flow conditions. The results of this study were compared with those obtained in a previous article for an ordinary Portland cement paste. 1. Introduction The rheological properties of fresh cement pastes are very interesting, since they strongly affect the consistency, workability, and setting characteristics of the cement. Knowledge of the rheological properties of fresh cement pastes may contribute to supply a useful tool for controlling cement production, for achieving further information on the chemistry of cement as well as to a better understanding of flow behaviour of mortars and concrete. Moreover, understanding of how to control the rheological properties of fresh cement pastes is very important for the economical proportioning of concrete and proper mixing and placement methods, in particular for special applications. Fresh cement pastes are highly concentrated suspensions; their rheological behaviour is generally very complex and is dependent on several factors of different nature, such as: (i)physical factors (the water/cement ratio, the cement grain shape and size, etc.),(ii)chemical and mineralogical factors (the cement composition and its structural modifications due to hydration processes, etc.),(iii) mixing conditions (stirrer type and rate, the stirring time, etc.),(iv) measurement conditions (the measuring instruments and the experimental procedures, etc.),(v)presence of additives (water reducing agents, superplasticizers, etc.). A lot of experimental works on cement paste rheology is available in the literature; reference is made here to the most recent papers [1–22]. This work is a part of a research study concerning the influence
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