Effect of Pre-Shear on Agglomeration and Rheological Parameters of Cement Paste.

Mareike Thiedeitz,Dirk Lowke,Thomas Kränkel,Inka Dressler,Christoph Gehlen

doi:10.3390/ma13092173

Abstract

Cementitious pastes are multiphase suspensions that are rheologically characterized by viscosity and yield stress. They tend to flocculate during rest due to attractive interparticle forces, and desagglomerate when shear is induced. The shear history, e.g., mixing energy and time, determines the apparent state of flocculation and accordingly the particle size distribution of the cement in the suspension, which itself affects suspension’s plastic viscosity and yield stress. Thus, it is crucial to understand the effect of the mixing procedure of cementitious suspensions before starting rheological measurements. However, the measurement of the in-situ particle agglomeration status is difficult, due to rapidly changing particle network structuration. The focused beam reflectance measurement (FBRM) technique offers an opportunity for the in-situ investigation of the chord length distribution. This enables to detect the state of flocculation of the particles during shear. Cementitious pastes differing in their solid fraction and superplasticizer content were analyzed after various pre-shear histories, i.e., mixing times. Yield stress and viscosity were measured in a parallel-plate-rheometer and related to in-situ measurements of the chord length distribution with the FBRM-probe to characterize the agglomeration status. With increasing mixing time agglomerates were increasingly broken up in dependence of pre-shear: After 300 s of pre-shear the agglomerate sizes decreased by 10 µm to 15 µm compared to a 30 s pre-shear. At the same time dynamic yield stress and viscosity decreased up to 30% until a state of equilibrium was almost reached. The investigations show a correlation between mean chord length and the corresponding rheological parameters affected by the duration of pre-shear.

Highlights

The rheology of cementitious pastes determines the flowability of concrete
In modern concretes like Ultra High Performance Concrete (UHPC) and Self-Compacting Concrete (SCC) with a high amount of fine particles, the knowledge of rheological parameters such as yield stress, viscosity and structural build-up is essential for a proper placement process
The obtained results are summarized. These comprise the measured median chord after shear measured with focused beam reflectance measurement (FBRM), the mini-slump flow values SF at 13.5 min after water addition lengths d50 after shear measured with FBRM, the mini‐slump flow values SF at 13.5 min after as well as the calculated yield stresses τ0,B (Pa) and plastic viscosities μP (Pa.s) from rheometer data for (Pa.s) from water addition as well as the calculated yield stresses τ0,B (Pa) and plastic viscosities all test series

Summary

Introduction

The rheology of cementitious pastes determines the flowability of concrete. Especially in modern concretes like Ultra High Performance Concrete (UHPC) and Self-Compacting Concrete (SCC) with a high amount of fine particles (dmax < 125 μm), the knowledge of rheological parameters such as yield stress, viscosity and structural build-up is essential for a proper placement process. One main influencing factor on rheological behavior is shear, varying in duration and intensity and leading to (micro-) structural changes and modified flow characteristics. This effect is important in the context of rheological measurements on cementitious suspensions, since every variation of the shear history before the start of a measurement causes a change in the rheological parameters. The effect of pre-shear time on agglomeration kinetics and rheological parameters is investigated These findings could be a base for further fundamental research concerning the effect of shear history (e.g., during mixing, pumping, spraying) on the agglomeration status that itself affects the rheological behavior of cementitious suspensions during casting (e.g., workability, form filling ability)

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Conclusion