A method for forming a suspension of partly dissolved fly ash in an aqueous mineral acid solution, wherein the method comprises the following process steps: 1) supplying a mass, L, of an aqueous mineral acid solution to a dissolution reactor and then measuring and registering the initial temperature, T 0, and initial, pHo, of the aqueous mineral acid solution, 2) set n = 1, where n is an integer, and select a target pH, pH targ, to be reached in the suspension, 3) for n ≤ N, where N is an integer from 3 or higher, execute steps i), ii) and iii) in successive order: i) adding and mixing an amount S n of solid fly ash into the aqueous mineral acid solution, ii) when the amount S n is dissolved, measuring and registering the temperature and pH of the aqueous mineral acid solution at this n'th step as T n and pH n, respectively, and determining and registering an accumulated amount of fly ash, S n, acc, being added to the aqueous mineral acid solution at this n'th step, and iii) set n = n+1 and go to step i), 4) apply the registered temperatures T 0 to T n and the registered pH-values pHo to pH n, and the registered amounts of added fly ash, S n, acc, and equations: Γ n 2 = β 1 − β 2 Δ T n and Δ T n = γ 1 S n, acc L − γ 2 S n, acc L 2, where Γ n = 10 - pHn, ΔT n = T n -T 0, and β 1, β 2, γ 1, and γ 2 are parameters, to determine the parameters β 1, β 2, γ 1, and γ 2 by regression analysis, 5) determining a total amount, S tot, of solid fly ash to be added to the aqueous mineral acid solution from the relation: S tot = L 2 γ 2 γ 1 + γ 1 2 + 4 γ 2 β 1 − 10 − pH targ 2 β 2, and 6) if S n, acc < S tot, execute steps j) and jjj): j) adding and mixing an amount S n into the aqueous mineral acid solution, jjj) if S n, acc < S tot, set n = n+1 and go to step j), or else: jjjj) stop feeding and mixing solid fly ash into the aqueous mineral acid solution. 1) supplying a mass, L, of an aqueous mineral acid solution to a dissolution reactor and then measuring and registering the initial temperature, T 0, and initial, pHo, of the aqueous mineral acid solution, 2) set n = 1, where n is an integer, and select a target pH, pH targ, to be reached in the suspension, 3) for n ≤ N, where N is an integer from 3 or higher, execute steps i), ii) and iii) in successive order: i) adding and mixing an amount S n of solid fly ash into the aqueous mineral acid solution, ii) when the amount S n is dissolved, measuring and registering the temperature and pH of the aqueous mineral acid solution at this n'th step as T n and pH n, respectively, and determining and registering an accumulated amount of fly ash, S n, acc, being added to the aqueous mineral acid solution at this n'th step, and iii) set n = n+1 and go to step i), i) adding and mixing an amount S n of solid fly ash into the aqueous mineral acid solution, ii) when the amount S n is dissolved, measuring and registering the temperature and pH of the aqueous mineral acid solution at this n'th step as T n and pH n, respectively, and determining and registering an accumulated amount of fly ash, S n, acc, being added to the aqueous mineral acid solution at this n'th step, and iii) set n = n+1 and go to step i), 4) apply the registered temperatures T 0 to T n and the registered pH-values pHo to pH n, and the registered amounts of added fly ash, S n, acc, and equations: Γ n 2 = β 1 − β 2 Δ T n and Δ T n = γ 1 S n, acc L − γ 2 S n, acc L 2, where Γ n = 10 - pHn, ΔT n = T n -T 0, and β 1, β 2, γ 1, and γ 2 are parameters, to determine the parameters β 1, β 2, γ 1, and γ 2 by regression analysis, 5) determining a total amount, S tot, of solid fly ash to be added to the aqueous mineral acid solution from the relation: S tot = L 2 γ 2 γ 1 + γ 1 2 + 4 γ 2 β 1 − 10 − pH targ 2 β 2, and 6) if S n, acc < S tot, execute steps j) and jjj): j) adding and mixing an amount S n into the aqueous mineral acid solution, jjj) if S n, acc < S tot, set n = n+1 and go to step j), or else: jjjj) stop feeding and mixing solid fly ash into the aqueous mineral acid solution. j) adding and mixing an amount S n into the aqueous mineral acid solution, jjj) if S n, acc < S tot, set n = n+1 and go to step j), or else: jjjj) stop feeding and mixing solid fly ash into the aqueous mineral acid solution.