[freebeard]

The mechanism is interesting:

The coffee-grounds biochar has the perfect combination of pore structure and carbon content to absorb water, Roychand says. “When the coffee biochar is mixed with concrete, its particles act like microscale water reservoirs distributed throughout the concrete,” he says. “As the concrete sets and starts to harden, the biochar slowly releases the water it stored, rehydrating it when needed, thereby reducing shrinkage micro-cracking. This results in the improvement in concrete strength.”