Experimental Investigation on the Mechanical Properties of Non-dispersible Underwater Mortar under Static and Low-flow Conditions

Repairing underwater damaged structures with ordinary cement mortar often causes slurry loss and uneven repair quality. This study examined a non-dispersible underwater mortar (NUM) modified with UWB-II flocculant (0%-2.0%) under static water and low-flow conditions (2.5 and 5 cm/s) using flowability and compressive strength tests, together with SEM and XRD analyses. The flocculant improved anti-dispersion performance in water, but it also reduced consistency from 120 mm to 66 mm. At 1.0%-1.5%, the mortar gave the best overall performance: 3-day compressive strength reached 15.64-16.91 MPa, 7-day strength was close to 20 MPa, and the 7-day underwater-to-air strength ratio exceeded 82%. At 2.5 cm/s, the 7-day strength decreased only slightly; at 5 cm/s, the strength exhibited a substantial decline and showed limited recovery potential within the tested ages. Within the scope of the present experimental conditions, the results indicate that UWB-II modified mortar can be applied to underwater repair projects with low local flow disturbance.