Active control of thixotropy of magneto-responsive cementitious materials with the intervention of time-varying magnetic fields

Abstract

The thixotropic behaviour of cementitious materials received widespread attention in recent years. However, structural break-down is often ignored in most research. The role of structural break-down in thixotropy of magneto-responsive cementitious materials should be discussed in order to achieve comprehensive active rheology control of cementitious materials, not only from more liquid-like to more solid-like (set-on-demand), but also from more solid-like to more liquid-like. This research investigates the thixotropy of magneto-responsive cementitious materials in combination with the structural break-down. The magnetorheological response of cement pastes subjected to unipolar and bipolar magnetic fields is experimentally evaluated. The effects of magneto-responsive particle size, magnetic field direction, intervention duration, magnetic flux density and magnetic field frequency on thixotropy of cementitious materials containing carbonyl iron particles (CIPs) are studied. The shear stress of magneto-responsive cementitious materials can be changed in time and on demand by the structural build-up and structural break-down of magneto-responsive particles' alignment and/or movement. With the increase of magnetic flux density, the shear stress of magneto-responsive cement paste reaches a minimum first and afterwards increases dramatically. When the magnetic field frequency increases from 0 Hz to 0.2 Hz, the yield stress of magneto-responsive cement paste decreases dramatically, followed by a slight increase. The origins of thixotropy for fresh magneto-responsive cement paste considering the magneto-responsive particle agitation (e.g. vibration and movement, etc.) are presented.