Strength enhancement and pore-size refinement in clinker-free CaO-activated GGBFS systems through substitution with gypsum

This study presents an improvement in the compressive strength of the clinker-free CaO-activated GGBFS system with the addition of gypsum. The compressive strength was primarily governed by the amount of ettringite, but more importantly, the morphology of reaction products (e.g., size, slenderness) is proposed as being a more dominant factor. The gypsum content at the best strength produced fine ettringite crystals and removed large pores that were around 100 μm in diameter, resulting in significant pore-size refinement. However, excessive addition reduced the strength and generated a large volume of detrimental pores with sizes of 0.03–0.4 μm, which were probably due to expansive cracking or large voids between the large sizes of re-precipitated gypsum crystals. The SEM images demonstrated that the fine ettringite crystals at the gypsum dosage of the best strength filled the pores effectively, which led to dense matrices; while the coarse crystals of gypsum or ettringite at the excessive addition level produced porous matrices.