**Michał A. Glinicki, Daria Józwiak-Nied ´ ´ zwiedzka \*and Mariusz D ˛abrowski**

Institute of Fundamental Technological Research, Polish Academy of Sciences, Pawi ´nskiego 5b, 02-106 Warsaw, Poland

**\*** Correspondence: djozwiak@ippt.pan.pl; Tel.: +48-22-8261281 (ext. 310)

Received: 2 July 2019; Accepted: 2 September 2019; Published: 3 September 2019

**Abstract:** Fly ashes from coal combustion in circulating fluidized bed boilers in three power plants were tested as a potential additive to cement binder in concrete. The phase composition and microstructure of cement pastes containing fluidized bed fly ash was studied. The fractions of cement substitution with fluidized bed fly ash were 20% and 30% by weight. X-ray diffraction (XRD) tests and thermal analyses (derivative thermogravimetry (DTG), differential thermal analysis (DTA) and thermogravimetry (TG)) were performed on ash specimens and on hardened cement paste specimens matured in water for up to 400 days. Quantitative evaluation of the phase composition as a function of fluidized bed fly ash content revealed significant changes in portlandite content and only moderate changes in the content of ettringite.

**Keywords:** clean coal combustion; fluidized bed fly ash; microstructure; phase composition; portlandite; unburned carbon
