The use of radiographic testing methods such as X-ray computed tomography (XCT) and digital radiography (DR) to non-destructively evaluate the quality of grouting work suffers a major setback of poor contrast and visibility of the voids which makes detection very difficult. This study explores the feasibility of using high-density modified cement grouts by addition of baryte, bismuth oxide and lead concentrate to alter the attenuation characteristics of the grout. The influence of these additives on the fluidity, hydration kinetics, thermal stability, density, compressive strength and mass attenuation coefficient of grout were evaluated. The results show that the additives increased the fluidity of grout and caused a retardation effect on the hydration of grout. The density of the hardened modified grouts showed enhancements and the compressive strength at all ages were reduced. The synergistic performance of Ba and Bi increased the mass attenuation coefficient difference between the grouts and air from 12.15% (air and unmodified grout) to 116.32% (air and grout with both baryte and bismuth addition) at a photon energy of 160 keV. Subsequently, the visibility of the void in the grout and the contrast between void and grout were enhanced by 203.9% and 158.8%, respectively. This gives a promising outlook to modify the grouting material radiation attenuation characteristics to complement equipment and scanning parameter improvements in the detection and evaluation of voids in grouted segments.