Sentence:  It was about this time that the first experiments were made (in Germany) with basic slag, a material which had hitherto been regarded as a worthless <WSD>by-product</WSD> of steel manufacture. A year or two later field trials were begun in England, with the final result that basic slag has become recognized as a valuable source of phosphorus for growing crops, and is now in constant demand for application to the soil as a fertilizer. 
Meaning of the word marked with <WSD> tags:  A secondary product; something made incidentally during the production of something else.