4. Mineral Admixtures
In addition to chemical admixtures, concrete can also incorporate mineral admixtures. Unlike chemical admixtures, which work chemically on the fresh phase of concrete, mineral admixtures are typically fine powders that react in a pozzolanic or hydraulic manner to improve the mechanical properties and durability of hardened concrete.
The use of mineral admixtures also plays an important role in supporting sustainable construction, as some of these materials are industrial by-products that can be reused. Here are some common types of mineral admixtures used in concrete:
4.1. Fly Ash
Fly ash is a fine powder produced from the combustion of coal in power plants. It has pozzolanic properties, meaning it can react with calcium hydroxide (a byproduct of cement hydration) to form additional calcium silicate hydrate (C-S-H), which strengthens the concrete.
Fly ash is classified into two types:
- Class F: High in silica, alumina, and iron oxide, suitable for improving durability and reducing alkali-silica reaction.
- Class C: Contains more calcium oxide, which gives it both pozzolanic and hydraulic properties.
The use of fly ash enhances workability, reduces hydration heat, and improves long-term durability. However, it may slow down early strength development.
4.2. Silica Fume
Silica fume is a byproduct of the production of ferrosilicon alloys, appearing as an ultra-fine powder. Its particles are approximately 100 times smaller than cement particles, and it has a very high active silica content (greater than 85%), which gives it excellent pozzolanic reactivity.
Adding silica fume improves:
- Microstructural density (filling capillary pores)
- Compressive strength
- Resistance to chloride penetration and other aggressive agents
Silica fume is commonly used in high-performance concrete, such as for bridges, harbors, and earthquake-resistant structures.
4.3. Slag (Ground Granulated Blast Furnace Slag)
Slag is a byproduct of iron ore smelting in blast furnaces and is ground into a fine powder called Ground Granulated Blast Furnace Slag (GGBFS). Slag has latent hydraulic properties, meaning it reacts when mixed with alkaline activators like Portland cement.
The benefits of slag use include:
- Improved durability against sulfates and aggressive environments
- Reduced hydration heat
- Lighter-colored concrete
Slag is frequently used in projects requiring high chemical resistance, such as foundations in sulfate-rich soils or marine structures.
4.4. Natural Pozzolans (e.g., Trass, Zeolite)
Natural pozzolans are volcanic or mineral materials that contain reactive silica and/or alumina. In Indonesia, examples include trass from Lumajang and zeolite from various volcanic areas.
Benefits of natural pozzolans include:
- Reduced cement consumption (more economical)
- Improved resistance to specific chemical attacks
- Suitable for local projects where natural materials are accessible
Natural pozzolans have lower reactivity than fly ash or silica fume, so their composition must be carefully monitored to avoid compromising concrete strength.
4.5. Metakaolin
Metakaolin is produced by heating pure kaolin at temperatures between 650–850°C, resulting in a reactive pozzolan. The use of metakaolin enhances:
- Early and late strength of concrete
- Resistance to chloride penetration
- Microstructural density of cement paste
Metakaolin is often used in architectural concrete, precast concrete, or structures requiring both high strength and a bright aesthetic appearance.
Now, let’s move on to the next section discussing the Selection of Admixtures.