Effect of SO3 content in circulating fluidized bed fly ash on expansion performance of circulating fluidized bed ash-slag unfired brick

Circulating fluidized bed boiler fly ash and bottom slag (CFB ash-slag) have good pozzolanic activity and can be used to prepare unfired bricks. However, the high content of sulfur-fixing minerals in CFB ash-slag can cause volume expansion and cracking of unfired bricks. Therefore, in this paper, CFB fly ash is used as cementitious material, CFB bottom slag as sand and carbide slag as curing agent to prepare CFB ash-slag unfired brick. The influence of SO₃ content in CFB fly ash on the expansion and mechanical properties of the unfired brick is studied, and the dissolution rule of SO₄^2- of CFB fly ash was tested. The hydration products and microstructure of CFB ash-slag unfired brick were analyzed by microscopic methods such as XRD, TG-DTG, and SEM. The results show that with the increase of SO₃ content in CFB fly ash from 1.960 % to 10.677 %, the expansion rate of CFB ash-slag unfired bricks increased from 0.047 % to 1.074 %, the compressive strength increases first and then decreases at each immersion age. With the increase of immersion age, the compressive strength of unfired bricks increases when the SO₃ content of CFB fly ash is less than 4.878 %, and decreases first and then increases when the SO₃ content is more than 4.878 %. The microscopic test results show that with the increase of SO₃ content in CFB fly ash from 1.960 % to 10.677 %, the dissolution of SO₄^2- of CFB fly ash gradually increases, and the amount of ettringite generated by hydration increases from 16.01 % to 23.86 %, resulting in an increase in the expansion rate of CFB ash-slag unfired brick, and the compressive strength increases first and then decreases. In the late period of water immersion of CFB ash-slag unfired bricks (28d, 90d), the ettringite generated by hydration and other hydration products can further dense the pores caused by the expansion of the bricks in the early period of immersion, which increases the compressive strength of the unfired brick in the late period of immersion. Related studies have revealed the expansion law and mechanism of CFB ash-slag unfired brick, and provides a theoretical basis for its large-scale application.