What can I mix with cement to make it stronger?

To make cement stronger, you can mix it with various materials that enhance its chemical reactions, densify the matrix, reduce porosity, or improve its mechanical properties.

Enhancing Cement Strength: Key Admixtures and Materials

The strength of cement-based materials like concrete is significantly influenced by the addition of specific admixtures and supplementary cementitious materials. These additions can improve the material's density, reduce permeability, and boost its overall strength and durability.

Supplementary Cementitious Materials (SCMs)

These materials react with the calcium hydroxide produced during cement hydration, forming additional cementitious compounds that fill voids, densify the concrete, and enhance long-term strength and durability. They also make mixtures more economical and reduce permeability.

• Fly Ash: A byproduct of coal combustion, fly ash reacts with calcium hydroxide and water to form additional cementitious compounds. This reaction helps to create a denser, less permeable concrete, leading to increased long-term strength and improved workability. It's particularly effective for enhancing later-age strength.
• Slag Cement (Ground Granulated Blast-Furnace Slag): Derived from iron production, slag cement also undergoes a pozzolanic reaction, similar to fly ash. It contributes to higher ultimate strength, improved resistance to chemical attack, and reduced heat of hydration, which is beneficial for mass concrete pours.
• Silica Fume: An ultrafine byproduct of silicon and ferrosilicon alloy production, silica fume is highly reactive. Its spherical particles fill microscopic voids between cement particles, creating a much denser and stronger paste. Silica fume significantly increases compressive strength, reduces permeability, and enhances abrasion resistance.
• Other Natural Pozzolans: These include materials like metakaolin and calcined clay. They exhibit similar pozzolanic properties, reacting with calcium hydroxide to improve strength, reduce permeability, and enhance chemical resistance.

Chemical Admixtures

Chemical admixtures are liquids or powders added to concrete mixtures to modify specific properties, including strength.

• Superplasticizers (High-Range Water Reducers): These admixtures allow for a significant reduction in the water-cement ratio while maintaining the workability (slump) of the concrete. A lower water-cement ratio is directly correlated with higher compressive strength because it results in a denser, less porous hardened cement paste. They enable the production of high-strength concrete with excellent flow characteristics.
• Accelerating Admixtures: While primarily used to speed up the setting and hardening process, some accelerators can contribute to early strength gain, allowing for faster form removal or earlier use of the structure.
• Air-Entraining Admixtures: These create microscopic air bubbles within the concrete, primarily to improve freeze-thaw resistance and durability. While they might slightly reduce compressive strength at very high dosages, they are crucial for the long-term integrity of concrete in cold climates, which indirectly contributes to its lasting strength.

Fibers

Adding fibers to cement mixtures significantly improves the material's tensile strength, flexural strength, and resistance to cracking. Unlike SCMs or chemical admixtures that primarily boost compressive strength or material density, fibers enhance the matrix's ability to withstand pulling and bending forces.

• Steel Fibers: These are often used in industrial floors, pavements, and precast products to provide excellent post-crack strength, ductility, and impact resistance. They help distribute stress and control crack propagation.
• Polypropylene Fibers: Typically used at lower dosages, these fibers help reduce plastic shrinkage cracking and improve impact and abrasion resistance. They are particularly effective in preventing early-age cracking.
• Glass Fibers: Resistant to alkali, these fibers are used in glass fiber reinforced concrete (GFRC) to create thin, lightweight, high-strength panels for architectural applications.
• Basalt Fibers: A relatively new option, basalt fibers offer high tensile strength and chemical resistance, providing an alternative to steel or glass fibers in various applications.

Summary of Strength-Enhancing Additives

The table below summarizes common materials and their primary benefits for improving cement strength:

When selecting materials to mix with cement for increased strength, it's crucial to consider the specific application, desired strength, environmental conditions, and cost-effectiveness. Combining these materials can often lead to superior performance characteristics. For instance, using a superplasticizer with silica fume can produce ultra-high-strength concrete.