Anything that is water soluble is going to be a pretty poor building material, unfortunately. The organics floating about (protozoa, algae, fish) also generally

Anything that is water soluble is going to be a pretty poor building material, unfortunately. The organics floating about (protozoa, algae, fish) also generally decompose, another undesirable property. The remainder- very very fine silt and microplastics- might be useful. With all the chemical processing that would be needed to stabilize the salts, mechanical filtering and such, I think we're better off continuing to use bricks and ground sourced gravel and cement. At least the holes we dig can be repurposed into sanitary landfills.