What Is The Difference Between Basalt Rock Wool and Slag Rock Wool

With the economic development of different countries and regions around the globe, the demand for mineral wool products has increased worldwide. In recent years, more than 200 manufacturers of rock and slag wool have emerged on the market. However, during actual procurement, many buyers find a significant difference in quotations among manufacturers for similar products, which greatly confounds purchasers, leaving them unsure of how to choose.

If you encounter a manufacturer offering unusually low prices, you must be cautious as they might be selling slag wool under the guise of basalt wool. These two products are fundamentally different.

Ordinarily, we differentiate between rock wool and slag wool by their appearance:

Slag wool is loose, with a high content of black slag balls, unevenly arranged, showing obvious signs of burning, with colour tending towards earthy yellow, feeling prickly and causing itchiness.

Basalt rock wool tends to be yellowish-green, with significantly fewer slag balls and almost none visible. Rock wool fibres are fine, neatly aligned, with uniform colour, firm to the touch, and not prickly.

The most important and fundamental method to distinguish between rock wool and slag wool is the acidity coefficient.

What is the acidity coefficient?

The acidity coefficient is an index that measures the chemical durability of materials like basalt wool and slag wool. It refers to the ratio of the sum of silica (SiO2) and alumina (Al2O3) mass in the mineral wool fiber composition to the sum of calcium oxide (CaO) and magnesium oxide (MgO) mass. A higher acidity coefficient indicates better weather resistance and longer service life.

For slag wool, the acidity coefficient is <1.5, usually around 1.2, whereas for basalt wool, it is ≥1.6 and can reach up to 2.0. Therefore, compared to slag wool, basalt wool exhibits better weather resistance and a longer service life.

Below is a comparison of slag wool and basalt wool across four dimensions to help you better distinguish between the two.

1. Comparison of Chemical Composition and Acidity Coefficient

Basalt and diabase contain 77% to 83% of silica (SiO2), alumina (Al2O3), calcium oxide (CaO), and magnesium oxide (MgO), approximately 10% lower than blast furnace slag. The average content of iron oxides (Fe2O3 + FeO) is about 11%, with a maximum of 17%. Thus, the acidity coefficient MK of basalt rock wool boards is typically 1.5 or higher, sometimes exceeding 2.0; in contrast, the acidity coefficient MK of slag rock wool boards is around 1.2.

Here's a specific comparison of the four main chemical components between basalt rock wool and slag rock wool:

2. Difference in Water Resistance

The crystalline region of basalt rock wool boards mainly consists of CS-C2AS-C2S (wollastonite-aluminous anorthite-calcium aluminosilicate), none of which possess hydraulic characteristics, meaning they change very little when exposed to water, endowing basalt rock wool with excellent water resistance. In contrast, the crystalline area of slag rock wool includes CS-C2AS-CAS2 (wollastonite-aluminous anorthite-dicalcium silicate), which can undergo hydration reactions as temperatures rise during production, leading to decreased stability of slag fibres in moist environments.

Moreover, the pH value of basalt rock wool boards is less than 4, classifying them as mineral fibres with exceptionally stable waterproof properties; whereas the pH value of slag rock wool generally exceeds 5 and might even go beyond 6, resulting in only moderately stable or unstable water resistance.

3. Comparison of Thermal Conductivity

Both types of rock wool boards exhibit outstanding thermal insulation performance. However, when operating temperatures exceed 675°C, the cooling process of slag rock wool slows down due to internal structural changes that cause its density to decrease from 3.28 to 2.97, with a volume expansion of about 10%, ultimately leading to the pulverization and disintegration of slag rock wool. By comparison, basalt rock wool boards do not undergo such transformations and can be used at temperatures above 760°C, with a softening temperature ranging between 900 and 1000°C.

4. Distinction in Corrosion Resistance

One of the main functions of blast furnaces is to prevent iron embrittlement during use, so desulfurized sulfur exists in slag in the form of CaS, which then enters slag rock wool. When slag rock wool is used in high-humidity environments, CaS decomposes into calcium hydroxide (Ca(OH)2) and hydrogen sulfide (H2S). Ca(OH)2 makes water alkaline, further reducing the water resistance of slag wool. H2S dissolved in water produces bisulfite acid, which can cause corrosion when it comes into contact with metal. Basalt rock wool boards, made from basalt or diabase, contain no sulfur source, thus avoiding this type of corrosion.

In summary, although both basalt rock wool and slag rock wool provide good thermal insulation, the former outperforms in terms of ageing resistance, water resistance, and corrosion resistance. For engineering projects seeking high quality and long-term stability, choosing basalt rock wool may offer a superior solution.

If you have procurement needs for high-quality basalt wool or wish to learn more details, please feel free to contact us, the ECOFOX® basalt rock wool board factory, industrial rock wool manufacturer.