Ceramics-Silikáty 50 (2) 98-105 (2006)

The effect of fly ash on cement hydration in aqueous suspensions

D. Jozić, J. Zelić

Fly ash is waste material formed in coal-using thermal-power plants, and is frequently used as addition to cement and mortars/concrete. Its physical and chemical properties depend exclusively on the quality of coal used and on technological conditions of burning. This study has examined the effect of fly ash from the Plomin 2 Thermal Power Plant, Croatian Power Supply (HEP), Plomin, Croatia, on early stages of cement hydration in aqueous suspensions (w/s = 4, t = 20°C). According to the ASTM C618 Standards, fly ash used, belongs to the F class fly ash, i.e. to low CaO content fly ash. Fly ash was added as a replacement for cement in amounts of 0, 5, 11, 15, 20, and 30 wt.%. Conductometrical measurements in suspensions have indicated that the replacement postpones the start of precipitation of Ca(OH)₂, prolongs the duration of that precipitation, and reduces the overall cement hydration rate. XRD and DTA-TG/DTG measurements indicate that the share of Ca(OH)₂ in the solid part of the suspension decreases when the fly ash addition increases, while the results of the DTA-TG/DTG analysis corrected to the cement weight confirm the role of fly ash as nucleation site where hydration products settle, while its pozzolanic activity has not been observed in this period of hydration up to 72 hours.

Keywords: Cement, Fly ash, Conductometry, TG/DTA, XRD