AGGREGATE CEMENT REACTIONS IN MWI-BOTTOM ASH-BASED CONCRETE -A PETROGRAPHICAL ASSESSMENT

摘要

Municipal waste incineration in Flandres produces important volumes of bottom ashes per year. The residues are considered as potential aggregate substitutes. The only legislative requirement for their reuse is a low environmental impact risk. Secondary OPC-based dry concrete bricks containing MWI-bottom ashes have been investigated for their environmental impact potential and durability. Petrographical analyses revealed that the MWI bottom ashes contain refractory products, glass, soot, vesicular slags, metallic particles and composite slags. The vesicular texture of many slag-types is responsible for a high intraparticle porosity, an increased external roughness and a hygroscopic behaviour during mixing. This hampers homogeneous hydration of the cement. Some of the metallic grains tend to be very reactive: microcracking related to swelling has been observed around aluminium particles. SEM and EPMA analyses corroborated the petrographical observations by identifying successive layers of oxidised Al, Ca-Al-gel and hydrated salts, responsible for the swelling. In concrete mixtures using aged bottom ashes, the gel is partly or totally carbonated and the swelling reaction is suppressed. Apparently, the reaction starts with the formation of a Ca-Al-oxide/hydroxide-rich gel when the ashes are exposed to the atmosphere. When the gel gets into contact with sulphates or chlorides in the cement paste, the swelling is triggered. Ageing of the bottom ashes may suppress the reaction as CaO/Ca(OH)_2 in the gel will be carbonated during exposure to the atmosphere, decreasing the availability of Ca.