Preparation, performance, and stability of alkali-activated-concrete waste-lead- bearing sludge composites

Recycling/valorization of solid-wastes in the cleaner production of building materials not only strongly contributes to the environmental protection from pollutants, but it is regarded as an ideal solution in the conservation of naturally-occurring resources that are used in the construction sector. Accordingly, this work aims at eco-sustainable recycling concrete waste (CoW) and hazardous lead-bearing sludge (LBS) in the cleaner production of alkali-activated cementitious materials. The activation of powdered-CoW with different sodium hydroxide (NaOH) contents yields hardened materials, with modest compressive strengths, in which pirssonite is the dominant binding phase as confirmed by X-ray diffractogram, thermal analyses, infrared spectroscopy, and scanning electron microscopy. These materials demonstrate low stability in humidity-rich-environment as a significant regression in their mechanical properties was recorded after 30-days of curing. The use of sodium silicate (Na₂SiO₃) as alternative to NaOH represents two synergistic positive effects, which are working together, including the enhancement of mechanical properties and stability of the hardened materials at the long term of curing. The positive impact of Na₂SiO₃ is mainly originated from the formation silicate-rich strength-giving-phases with higher binding capacity and stability compared to pirssonite-phase. LBS was beneficially used as an alternative silicate source to avoid the negative impact of Na₂SiO₃ on environment. The incorporation of 10 wt % LBS in NaOH–CoW system has resulted in the creation of hardened material with mechanical properties superior to Na₂SiO₃–CoW one, notably long run stability. The high Pb-immobilization, stability, and acceptable compressive strength are the dominant parameters which directly reflect on the safe and benefit use of NaOH–CoW/LBS10 composite as building brick.