Performance and durability of Fe-rich inorganic polymer composites with basalt fibers

The use of inorganic instead of organic polymers in composites receives increasing interest from research institutions and industry because of their fire resistance and lower energy input upon manufacturing. It remains however unclear whether the fibers can resist the alkalinity of the inorganic polymer in a larger timeframe. This is investigated in the present work for basalt fibers. Inorganic polymers were synthesized from an iron-silicate slag, originating from non-ferrous metallurgy, and potassium silicate solutions (SiO2/K2O = 1.6-2.0, H2O/K2O = 16-24), which were mixed in a solution/slag mass ratio of 0.3. To this mixture, 3 wt.% of basalt fibers (12.7 mm long, 13 µm diameter) was added. Curing occurred at room temperature. The 3-point flexural strength as a function of time increased for all batches. The highest flexural strength was recorded when an activating solution with SiO2/K2O = 2.0 and H2O/K2O = 16 was used; values were 29 ± 1 MPa and 37 ± 5 MPa, after 7 and 90 days, respectively. Without fibers, the strength was 13 ± 1 MPa after 90 days. The results show that the basalt fibers retain their structural performance in time, suggesting that basalt fiber reinforced inorganic polymers can be durable. However, a repetition of a similar mix with a different precursor (requiring more solution to be mixable), lead to the contradicting conclusion of having degraded during the curing period. The amount of solution is therefore considered to be an important factor for the durability of fiber reinforced inorganic polymers.