Recycling concrete sludge waste as a supplementary binder in one-part alkali-activated composites: A comprehensive performance evaluation

Abstract

Concrete sludge waste (CS), a by-product of concrete production, poses serious environmental challenges due to its disposal. This study aims to evaluate the potential of unground (UCS) and ground (GCS) CS as partial replacements for ground granulated blast-furnace slag (GBFS) in one-part alkali-activated composites (AACs), thereby advancing sustainable binder systems and reducing reliance on virgin raw materials. The experimental program included flowability, compressive and flexural strength, density, porosity, water absorption, sorptivity, and durability assessments under sulfate attack, freeze-thaw cycling, acid exposure, and elevated temperatures. UCS and GCS replaced GBFS at 5-30 % by weight, activated with sodium metasilicate. Microstructural analysis (SEM, XRD) was conducted to link performance outcomes with matrix development and reaction products. The results showed that UCS reduced flowability significantly (up to -38.5 % at 30 %), while GCS caused only minor changes (-4.6 % at 30 %). At 90 days, UCS at 5-10 % improved compressive strength by 15.5 % and 9.0 %, whereas higher UCS levels caused reductions. All GCS mixtures outperformed the reference, achieving up to 26.2 % strength gains and 46.8 % flexural improvement. Optimal GCS incorporation also enhanced resistance to sulfate, acid, freeze-thaw, and high-temperature exposures, while excessive UCS replacement reduced performance. Microstructural observations confirmed denser matrices with reduced porosity in GCS mixtures, while greenness assessment showed that 10-20 % GCS substitution achieves similar to 3-4 % embodied CO2 savings together with up to 26 % strength gains, thereby combining performance enhancement with sustainability benefits.