## 1 Introduction  
Active rheology control (ARC) or active stiffening control (ASC) is a developing concept that offers a solution to the conflicting rheological requirements during the different stages of the casting process [1, 2, 3, 4]. During pumping, for example, the concrete should be fluid enough to facilitate the pumping process; however, once the concrete is in the formwork, high fluidity gives rise to increased formwork pressure and formwork leakage. The ability to adjust the fresh concrete properties to match the desired behaviour could thus be of advantage. The concrete can be fluid while flowing in the pumping pipes, and then actively stiffened while in the formwork to prevent formwork leakage. ARC or ASC can be achieved by applying an external signal to the concrete that triggers a change in the material behaviour [3]. Based on this principle of ARC, an innovative active control methodology has recently been proposed in which a magnetic field is applied to the cementitious material to invoke a change in the rheological behaviour due to the presence of 'active' ingredients in the mixture. The active ingredients used were magnetisable particles in the form of Fe\({}_{3}\)O\({}_{4}\) nanoparticles [5, 6, 7, 8] and fly ash [9, 10]. Although magnetorheological control is a fairly new concept for cementitious mediums, this is a technology already widely used in other industrial applications such as brakes, hydraulic valves, and shock absorbers [11]. The interest in magnetorheological (MR) fluids for industrial applications arises from the rapid and reversible field-dependent rheological changes in the fluid [12]. When a magnetic field is applied, the magnetic particles acquire dipole moments and align with the magnetic field to form chains or clusters, giving the fluid an additional yield stress component. To initiate flow, these chains or clusters must be deformed or broken. Building out from the principles of MR fluids, a magnetic-field-based active stiffening control methodology was developed to study the efficacy of ASC for formwork leakages of flowable pastes under high pressure. This was performed using a small-scale laboratory test setup in which a pressurised column with a narrow opening at the bottom was filled with cementitious material to simulate formwork leakage under pressure [9; 13]. Using the ASC methodology, reductions in both the flow rate and net mass of leaked material were observed for pressures lower than 30 kPa. This was attributed to the formation of magnetic clusters due to the application of a magnetic field across the outflow width. Although the formation of clusters was less effective at higher pressures, the study showed that clusters formed under a pressure of 10 kPa were able to withstand pressures of up to 100 kPa [9]. Following these promising results obtained using cementitious pastes, the current study is aimed at studying the effects of fine aggregates on the ASC methodology, in a first step to upscaling to mortar phase. Mortar can be considered as a concentrated suspension, with rigid particles of different sizes suspended in the bulk fluid. In such a system, there are strong interactions between particles due to their close proximity. The concentration, shape, size distribution, and surface properties of the particles all affect the strength of these interactions. In general, there is a net attraction between particles which leads to flocculation [14]. Convergence of such a fluid towards a bottleneck, such as in the case of formwork leakage through a gap, is likely to cause blockage of flow. In general, clogging of granular material is a complex phenomenon that is dependent on the outlet size and type, the physical properties of the particles, and the particle packing [15]. In the case of mortar, however, the presence of the cementitious pastes in which the grains are embedded also influences the contact network between particles [16].