Engineering tool

Slag Basicity Calculator (B2 / B4)

Calculate binary and quaternary slag basicity from a slag analysis, and see whether the slag is acidic, neutral or basic — the first thing that decides which refractory lining will survive it.

CaO (wt %)

SiO₂ (wt %)

MgO (optional) (wt %)

Al₂O₃ (optional) (wt %)

Binary basicity B2 —

Quaternary basicity B4 —

Slag character —

B2 needs CaO and SiO₂. B4 also uses MgO and Al₂O₃ where you have them.

Formulas used

B2 = CaO ÷ SiO₂
B4 = (CaO + MgO) ÷ (SiO₂ + Al₂O₃)

Practical notes

A basic slag attacks a siliceous lining and is tolerated by a magnesia lining; an acidic slag does the reverse. Matching the two is the whole point of the calculation.
B4 is the more honest number where alumina and magnesia are present in quantity, which is usual in ladle slags.

Frequently asked questions

Which basicity ratio should I use?

B2 is the common shorthand and is enough when the slag is mostly lime and silica. Where alumina and magnesia are significant — most ladle slags — B4 describes the chemistry better.

What basicity counts as basic?

As a working rule, below 0.9 is acidic, 0.9 to 1.1 is roughly neutral, and above 1.1 is basic. Treat the boundaries as a guide, not a specification.