Making silica fume concrete involves adding silica fume, a highly reactive pozzolan, to a standard concrete mix using a specific batching process to ensure proper dispersion and performance.
What is Silica Fume Concrete?
Silica fume concrete is a type of concrete that incorporates silica fume as a supplementary cementitious material (SCM). Silica fume, also known as microsilica, is an ultra-fine powder recovered as a byproduct from silicon and ferrosilicon alloy production. Its incredibly small particle size and high silica content make it highly reactive, improving concrete properties such as strength, durability, and resistance to chemical attack and abrasion.
The Batching Process for Silica Fume Concrete
The method of adding silica fume to the concrete mix is crucial for achieving its benefits. Unlike traditional cementitious materials, silica fume requires careful handling due to its fineness. Following a specific sequence ensures the silica fume is properly wetted and dispersed, preventing clumps and ensuring it can react effectively within the mix.
Specific Batching Steps
According to best practices, including specific guidance on incorporating silica fume:
- Initial Addition: Silica fume must always be added with the coarse aggregate and some of the water. It is critical never to batch the silica fume alone or first into the mixer.
- Initial Mixing: Mix these materials (coarse aggregate, some water, and silica fume) for 1 1⁄2 minutes to begin wetting and dispersing the silica fume particles.
- Final Additions: After the initial mixing period, add the portland cement and any other cementitious material such as fly ash or slag cement to the mixer.
- Complete Mixing: Continue mixing until all materials are uniformly distributed and the concrete reaches the desired consistency.
Why Batch This Way?
Adding silica fume with coarse aggregate and water first helps to:
- Prevent Clumping: The larger aggregate particles help break up agglomerations of the ultra-fine silica fume as they enter the mixer.
- Aid Dispersion: The initial water addition starts the wetting process, and mixing with aggregates helps disperse the fine particles throughout the partial mix before the primary cement is added.
- Reduce Dust: Adding it with wet aggregates minimizes airborne silica fume dust during batching.
Following this specific batching sequence, rather than adding silica fume alone or with the dry cement, is key to successfully integrating this powerful SCM into the concrete matrix.
