Shrinkage and mechanical behavior of steel slag-based alkali activated mortars: Effect of oil and natural hemp fibers

The excessive drying shrinkage of alkali-activated binders produced with high-calcium precursors remains a critical limitation. This study addresses this challenge using an emerging high calcium precursor, basic oxygen furnace slag, as a main binder component, while exploring two shrinkage mitigation strategies: usage of vegetable oil and natural hemp fibers. The individual and combined effect of both additives on drying shrinkage, crack development, porosity and mechanical behavior was assessed to determine whether combined use yields additional performance benefits over individual application. Oil was found to induce a series of physical and chemical mechanisms, significantly modifying the pore structure and resulting in reduced water sorptivity, open porosity, and shrinkage by 76, 50, and 23%, respectively. Hemp fibers showed better performance when incorporated without oil, with a marginal decrease in shrinkage (6–10%) but substantially enhanced flexural strength (10–23%) and crack control. When combined together, oil effect dominated, partially limiting fibers efficiency and resulting in a slightly higher shrinkage than when oil only was used. Comparison with previous literature indicated that shrinkage of mortars with basic oxygen furnace slag in this study was notably lower than those with blast furnace slag while being comparable to Portland cement mortars. In general, the use of basic oxygen furnace slag and vegetable oil enabled producing high-performance binders with a simultaneously low carbon footprint.