Formwork pressure has become one of the important challenges of modern construction parallel to the increase in use of highly flowable cementitious mixtures. Excessive formwork pressure coupled with the highly flowable nature of these materials could lead to increased formwork leakage, which in turn leads to aesthetic or durability issues. This chapter introduces a magnetic field-based active stiffening control methodology that could be beneficial for reducing formwork leakage during casting. Using the newly developed methodology, an experimental study was conducted on cementitious pastes containing fly ash or Fe3O4 nanoparticles as the magnetizable components. The effects of Fe3O4 nanoparticle concentration, paste rheology, applied pressure, magnetic field strength, flow width and the addition of ultra-fine aggregates on the magnetic field-based formwork leakage control were investigated and are discussed herein. Experimental results show that the active rheology control (ARC) method is effective in reducing the flow rate for formwork pressures in the range of 10–50 kPa, and completely blocking the flow at 10–20 kPa. The ARC method has been found to be more beneficial for more flowable pastes, which generally exhibit more leakage in practice. The efficacy of the active rheology control (ARC) method described is significantly influenced by the concentration of magnetizable particles. The required amount of these particles needed for significant flow reduction is dependent on the rheological properties of the base fluid, the expected pressure level, the flow width and the magnetic strength to be applied.