Towards tailored superplasticizers

Superplasticizers (SP) of the “new generation” are essentially polycarboxylate polymers. Polymers of this family can be produced with almost infinite variations in their chemical structure, which allow the fulfilment of specific (tailored) properties. These polymers are more efficient for water reduction and for keeping concrete workability for longer periods. Another class of superplasticizer also available essentially for extreme specifications, is poly(oxyethylen) phosphonates. A few years ago, one objective of superplasticizer development was to produce very robust SPs usable in all types of concrete with limited incompatibilities. It seems now that such a product will probably never exist due to the underlying complexity and variability of cement. More versatile SPs can, however, be obtained by blending different polymers. As the interactions cement/superplasticizer are better understood, tailored SPs for given applications are becoming more readily available. This paper aims at highlighting some key structure-property relationships of these different SPs. The role of the polymer fraction that does not get adsorb onto cement particles is also mentioned

Moisture Diffusivity of Fiber Reinforced Silica Fume Mortars

The moisture diffusivity is of considerable importance for quantitative assessments of creep and shrinkage as well as durability of cementitious material. For this reason, the influence of the composition of repair mortars on their effective moisture diffusivity as a function of the relative humidity of the surrounding air has been investigated. Silica fume, superplasticizer and polypropylene fibers have been added in order to reduce the permeability and to control cracking induced by drying shrinkage. It has been shown that the moisture transport in cementitious materials can be realistically described by a non-linear diffusion process governed by Fick's law. A computer program based on the finite volume method has been used in order to get the best effective moisture diffusivity by comparing experimental results (moisture losses of drying mortar cylinders) with the numerical solution. The applicability of a combined experimental-numerical approach to characterize repair mortars regarding their moisture diffusivity has been demonstrated. The material properties necessary for the characterization and qualification of new materials can be found numerically. Moreover, the diffusivities obtained provide useful input data for further numerical calculations. The positive effect of the addition of silica fume on the moisture diffusivity was clearly shown. The positive combined effect of polypropylene fibers and silica fume with increasing entrained air content was observed. Finally, no significant detrimental effect on the addition of fibers (even at relatively high volumes) has been observed for materials cast under shrinkage free conditions