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The structural build-up of cementitious paste with nano-Fe 3 O 4 under time-varying magnetic fields was experimentally investigated using small amplitude oscillatory shear (SAOS) technique. Several modes of magnetic fields, such as constant, sudden-changed and linearly-changed, were applied to the cementitious paste. Results showed that the structural build-up of the cementitious paste depended on the magnetizing time and magnetic field strength. Applying constant magnetic fields improved the liquid-like behavior during first minutes and afterwards the solid-like property was enhanced. Both the sudden-increased and sudden-decreased magnetic fields resulted in a sharp decrease in storage modulus. The linearly increasing magnetic field resulted in a slight increase in storage modulus but higher liquid-like behavior. When the magnetic field was linearly decreased from 0.5 T to approx. 0.25 T, the structural build-up was enhanced significantly, and with the continuously decreasing magnetic field from approx. 0.25 T to 0 T, a decrease in storage modulus was observed.