THE blocks used to make the artificial fish reefs are the real giveaway.Inspect them closely and you'll see bits of paper clips, other metals, glass, and ceramics. "They've got just about everything you place in your garbage can that's noncombustible," notes Frank Roethel, a chemical oceanographer at the State University of New York in Stony Brook. The university's Waste Management Institute is experimenting with three kinds of blocks made in part from incinerated garbage ash. The blocks differ in strength and quality but include a similar ratio of about 64 percent ash, serving as a substitute for gravel, and 21 percent sand. Portland cement, which makes up the other 15 percent, acts as the binder. The strongest of the three kinds of blocks being developed at the university is a smooth one of poured concrete that can withstand pressure of 3,500 to 4,200 pounds per square inch. The university is securing permits to use the blocks in a sea wall as protection against shore erosion. However, the real ash-block showpiece on campus is the recently completed boathouse next to the university's Marine Sciences Research Center. Of the three varieties of blocks used at Stony Brook, those in the boathouse look the most like ordinary concrete. They are smoother and stronger than the reef blocks but rougher and only about half as strong as the poured concrete blocks. As an experiment, one wall of the boathouse includes both regular blocks of bottom ash - the heavier material left on the grate after incineration - and several blocks combining bottom and fly ash - the finer, and presumably more toxic, material that moves toward the smokestack during the burning process. Fly ash accounts for only about 15 percent of all incinerated ash. "If you're interested in stabilizing the most ... toxic part of the total ash stream, then you want to stabilize some of the fly ash," explains Larry Swanson, director of the university's Waste Management Institute. "We wanted to show that we could make a construction-grade, quality material out of combined ash as well as the bottom ash." "We wanted to create a worst-case environmental scenario," adds Dr. Roethel. "If we can demonstrate that this material behaves in an environmentally typical manner as a substitute for sand and gravel, we should be using it that way - as a resource."