The effect of rheological parameters on static segregation of self-compacting concrete mortar

Abstract

The stability of mortar is generally understood as the ability of the suspension to remain homogeneous during and after fresh mortar placement or casting. This is mostly associated with the segregation of the suspension that can be defined at a static and dynamic level. At a static level, this phenomenon is simplified by Stokes’ equation, while at a dynamic level the characterisation becomes more complex due to the horizontal and translation motion of solid particles that must be considered simultaneously. Static segregation consists primarily of the downward migration of solid particles from the liquid medium remaining on top of the suspension (bleeding). Available literature has established a relationship between the rheology of fresh mortar and its stability, stating that viscosity is the determinant rheological parameter of the suspension able to dictate mortar stability. Cement mortar is a suspension with two mediums consisting of sand particles as the solid phase and cement paste as the liquid phase. It is argued that the overall performance of the suspension depends on the individual behaviour of the two phases. In accordance with Stokes’ law, solid particles should overcome the physical characteristics of the intermediate medium to settle effectively. Cement paste has to therefore exhibit microstructural strength to avoid sand particle settling. This is normally attributed to the yield stress of the cement paste. This means that the cement yield stress is the strength of the liquid phase that cannot be overlooked at the expense of overall mortar viscosity, as currently noted in the literature. High performance cement mortars also require the inclusion of superplasticisers whose effectiveness depends primarily on their chemical structure (group function) and the dosage at which they are used. It is thus important to understand the compatibility between the superplasticisers and cements since their interactions affect the cement paste that can in turn alter the stability of the cement mortar. Three different cements and two superplasticisers were used in this study. All cements were CEMI with distinct contents of aluminate and silicate phases manufactured in three different plants. The superplasticisers were poly-carboxylates with a specific molecular structure that defines their impact on the setting time. Mortars with different pastes exhibiting discrete yield stress values were designed. These yield stresses were achieved at optimum dosage of the product resulting from the blending of the two superplasticisers. The products consisted of mixing superplasticisers in different proportions at the set dosage. Rheological measurements were performed both at mortar and paste scale to estimate their yield stress and viscosity values. The Total Organic Carbon (TOC) was done at paste scale to determine the adsorption of superplasticiser on the cement particles within the suspension. This research confirmed that the stability of mortar depends not only on its overall viscosity, but also on the yield stress of its cement paste phase that defines the strength that opposes gravity acting on the sand solid particles to cause them to settle. Moreover, the study highlights the possibility of achieving a high performing superplasticiser by blending two different superplasticisers at an optimum dosage. In particular, mortar with high yield stress cement pastes exhibited more stable suspensions with lower segregation indexes. In contrast, mortar with lower cement yield stress values exhibited higher segregation indexes resulting in a mortar with poor stability. There is no definitive evidence, according to the results, to indicate that yield stress and viscosity have an effect on bleeding. Results from TOC measurements were in agreement with the literature showing that cement pastes with higher adsorption superplasticisers have lower yield stress values and vice versa.