Upcycling low-grade coal fly ash for the production of fly ash fibers and their applications in fiber-reinforced cement composites

Fly ash (FA) fibers were produced from low-grade coal FA, which exhibited high SO3 content, in combination with dolomite and basalt. To identify the optimal combination of these raw materials, glass samples were cast with varying ratios of these materials. Subsequently, an alkaline resistance test was conducted by immersing the ground glass samples in a 1 M Ca(OH)2 solution for a duration of 75 days. The results of the alkaline resistance test revealed that the formulation consisting of 70 wt% FA, 20 wt% dolomite, and 10 wt% basalt, exhibited the highest alkaline resistance with a weight loss of less than 2 wt%. Based on this formulation, FA fibers were then produced and utilized as a reinforcing phase in fiber-reinforced cement composite (FRCC) samples, ranging from 0.1 to 0.8 wt%. Mechanical testing showed that, after 28 days of air curing, the FRCC samples containing 0.8 wt% FA fibers exhibited the highest modulus of elasticity (MOE) at 16,890 ± 434 MPa, along with a modulus of rupture (MOR) of 13.86 ± 0.98 MPa and an impact strength of 1792 ± 134 J/m2. These results meet industry requirements, with a MOR and MOE exceeding 5 MPa and 7000 MPa, respectively, as well as an impact strength exceeding 1000 J/m2. Using the Archimedes method, the density of the FRCC samples containing 0.8 wt% FA fibers, is 1.71 ± 0.03 g/cm3, falling within the industry standard range of 1.5 to 2.2 g/cm3. Furthermore, according to the cost analysis, the production cost for FA fibers is $324.40 per ton, compared to $5000 and $1000 per ton for commercially available fibers. Consequently, utilizing FA fibers would result in significant savings for the raw material costs for FRCC production.