The phrase "hard clumps" refers to a group of things or substances that stick together tightly and are firm or solid instead of being loose or spread out. Full definition
The perfect cat litter forms hard clumps to make cleanup a breeze, it's dust - free and low - tracking, and it controls odors for days at a time. (businessinsider.com)
I had rock hard clumps of sugar in the pan that wouldn't incorporate with the butter and cream — but the clumps tasted good! (browneyedbaker.com)
Made from traditional clay materials, this litter creates hard clumps from your cat's urine and feces that are easy to scoop up. (businessinsider.com)