Radioactivity and Pb and Ni immobilization in SCM-bearing alkali-activated matrices

Partial or total replacement of Portland cement clinker by SCMs (Supplementary Cementitious Materials) is a priority for the cement industry in its pursuit of global sustainable development and eco-friendly binder manufacture. The most widely used SCMs include industrial by-products such as blast furnace slag, fly ash and red mud. Alkali-activated cements manufactured with SCMs may reduce the need for Portland clinker by up to 90 wt%–100 wt% with no significant decline in material strength. The trade-off, however, is the risk of higher than legally allowable levels of radioactivity and unbound heavy metals (Cd, Hg, Ni, Pb, Cr), which may leach into the soil with the concomitant adverse implications for human health and the environment. This study assessed the mechanical strength, leachability and natural radioactivity of alkali-activated cement paste containing industrial waste-based SCMs (blast furnace slag, fly ash and red mud) and Pb and Ni compounds. Strength was highest in alkali-activated slag and slag/fly ash pastes and lowest in the red mud-containing materials. The addition of Pb or Ni sulphates had no adverse effect on this parameter. Alkaline and OPC pastes showed a high level of immobilization of both lead and nickel ions. According to the radiological findings, the Activity Concentration Index (I) was higher in red mud than in OPC, blast furnace slag or fly ash. With (I) values lower than 1, however, all the hydrated/activated materials studied would be EU directive-compliant. Nonetheless, the use of these new materials will depend not only on the activity concentration index, but also on their physical and chemical properties and the quality tests that must be passed to conform to legal requirements.