The Rheology of Cementitious Materials
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The Rheology of Cementitious Materials
Robert J. Flatt, Nicos Martys, and Lennart Bergström Abstract The introduction of a new generation of dispersants in concrete allow this material to exhibit self-compacting properties in its fresh state and high durability and mechanical strength in its hardened state. These properties translate into many practical advantages for the construction field. Two of the most important are reducing the ecological impact of this sector of industry and reducing the labor-intensive work associated with placing ordinary concrete by vibration. In this article, it will be shown that knowledge of colloidal science has proven essential in the development of this new generation of dispersants for concrete. Indeed, the polymer molecules used in these dispersants are specifically designed to induce steric repulsion between cement particles, reducing their agglomeration and allowing high workability of fresh concrete prior to setting. While the linkage between interparticle forces and the rheological behavior of cement pastes is still only semiquantitative, recent advances in the modeling of concrete rheology show very promising results in terms of handling aggregates with a wide distribution of particle sizes and shapes. However, accurate modeling requires reliable input on the interaction of the dispersant with the hydrating cement at the molecular level, which is identified as a future research challenge. Keywords: cement, colloids, construction materials, rheology, aggregates.
Introduction When explaining what concrete is to students, the analogy to baking a fruitcake turns out to be quite useful. Both “materials” contain solid inclusions (aggregates in concrete, versus fruit or nuts in a fruitcake) and a binder (cement versus dough). In both cases, the consistency of the “batter” may be improved by adding water, and as a result, the porosity of the final product is increased. While in the case of fruitcake this porosity may be an advantage, particularly if the recipe calls for soaking the cake with brandy or kirsch after baking, it can prove catastrophic to the durability of concrete. Indeed, it is through the porous network of the cement paste that chemical agents can enter concrete and cause its degradation. Furthermore, increased porosity of the matrix significantly decreases concrete strength. At first glance, this may suggest that the solution for a more durable concrete is
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Self-Leveling / Self-Compacting Concrete Nowadays, it is possible to optimize the properties of fresh concrete with the use of superplasticizers to combine a high flowability at very low additions of water with negligible segregation of the particles. This type of concrete, which is usually referred to either as self-compacting concrete (SCC) or self-leveling concrete (SLC), can be cast into a frame of reinforced steel without the need for the labor-intensive vibration usually associated with concrete placing. The introduction of SCC in the last few decades has enabled the development of new construction tec
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