Physical, mechanical and durability properties of materials activated with single component alkali containing waste concrete sludge

Concrete sludge waste, a by-product of concrete production, poses significant environmental challenges due to its improper disposal. This study investigates the potential of unground concrete sludge waste (UCS) and ground concrete sludge waste (GCS) as partial replacements for ground granulated blast furnace slag (GBFK) in alkali-activated composites (AACs). An extensive experimental program was conducted including tests on flowability, compressive and flexural strength, apparent porosity, water absorption, sorptivity and durability under sulphate exposure, freeze–thaw cycles, elevated temperatures and acid attack. UCS substitution significantly reduced the flowability (up to -38.53% at 30%), while GCS had minimal impact (-4.59% at 30%), maintaining better workability due to its finer particles. At 90 days; 5% and 10% UCS modification increased compressive strength by 15.5% and 9.0%; while 20% and 30% UCS resulted in 8.2% and 13.7% strength reduction. In contrast, all GCS mixtures outperformed the reference (100% GBFS) with strength gains of 26.2%; 21.5%; 5.6% and 5.2% at 5%; 10%; 20% and 30% modification levels, respectively. At 90 days; 5%; 10% and 20% UCS modification increased flexural strength by 33.2%; 24.2% and 2.5%; while 30% UCS resulted in a 6.9% reduction. All GCS mixtures showed strength gains ranging from 6.5% to 46.8%; depending on the modification level. The compressive strength loss in AAC mixtures ranged from 69.1% to 77.7% at 800°C. This indicates a significant thermal degradation exhibited by all mixtures due to continuous exposure to high temperatures. Of the tested mixtures, the mixture containing 20% GCS replacement of GBFS showed the lowest strength loss, while 30% UCS replacement scored the highest. 5% and 10% GCS mixtures showed lower strength loss than the reference (34.3%). However, at higher GCS levels (20% and 30%), the strength loss exceeded the reference and the 30% GCS mixture showed the highest degradation (53.6%). Similarly, the 5% UCS mixture had the lowest strength loss. The most severe compressive strength loss (35.3%) occurred in GCS30 after magnesium sulfate exposure, while the highest loss for UCS30 was observed after H₂SO₄ exposure. The addition of UCS and GCS increased the strength loss in both H₂SO₄ and MgSO₄ environments.