A mixture of fly ashes as road base construction material

Abstract

By order of NOVEM, a research was carried out by TNO Environmental and Energy Research into the formation, occurrence and disintegration of ettringite in stabilizations of a mixture of fluidized bed fly ash and powder coal fly ash. Two series of specimens were manufactured, one from fluidized bed fly ash only and one from a mixture of fluidized bed fly ash and powder coal fly ash. From both series of specimens the compressive strength and the formation of minerals (especially ettringite) were determined at different times between 2 and 890 days after manufacture. The quantity of ettringite present in the specimens and the compressive strength both show a maximum. During the first couple of weeks calcium hydroxide, calcium sulphate and aluminium hydroxide are transferred into ettringite (until one of these components has disappeared). The formed ettringite fills the pores and thus contributes to the strength development. However, if too much ettringite is formed, the specimens “explode” due to expansion. In the course of time (100–200 days after manufacture) the formed ettringite again disintegrates, as a consequence of carbon dioxide attack. Then the compressive strength decreases, but stabilizes at a lower level. If the content of calcium oxide in the mixture of raw materials is not beyond 12%, and the mixture is allowed to react before manufacture of the specimens, the formed ettringite does not cause any crack formation in the specimens. It is possible to construct good and cheap stabilizations from a mixture of fluidized bed fly ash, powder coal fly ash and sand, without the addition of binding agents.