1. Fly ash, slag cement, and silica fume are industrial by-products that are used as a partial replacement for portland cement in concrete. These supplementary cementitious materials (SCMs) are pre-consumer materials.
2. Recycled material or recycled concrete can be used as aggregates in concrete.
3. Spent solvents, used oils, tires, and medical waste are used as fuel in many cement plants. Industrial byproducts are used as ingredients for manufacturing portland cement.

Fly Ash, Slag Cement, and Silica Fume (SCMs)

From top: Fly ash, Slag Cement and Silica Fume

These SCMs are used as a partial replacement for the portland cement in concrete. Fly ash is commonly used at replacement levels up to 25%; slag cement up to 60%; and silica fume up to 5% to 7%. When slag cement replaces 50% of the portland cement in a 7500 psi concrete, greenhouse gas emissions per cu yd of concrete are reduced by 45%. Because the cementitious content of concrete is about 7 to 15%, these SCMs typically account for only 2% to 8% of the overall concrete material in buildings.

Testing determines the optimum amounts of SCMs used with portland or blended cement, the relative cost and availability of the materials, and the specified properties of the concrete. When SCMs are used, the proportioned mixture (using the project materials) should be tested to demonstrate that it meets the required concrete properties for the project. SCM frequently enhance the durability of concrete. Some SCMs increase curing times and can affect the construction schedule.

Recycled Aggregates

Recycled concrete can be used as aggregate in new concrete, particularly the coarse portion. The Federal Highway Administration (FHWA) reports that eleven states use recycled concrete aggregate in new concrete. These states report that concrete with recycled aggregate performs equal to concrete with natural aggregates. When using the recycled concrete as aggregate, the following should be taken into consideration:

• Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
• Too many recycled fines can also produce a harsh and unworkable mixture. Many transportation departments have found that using 100% coarse recycled aggregate, but only about 10% to 20% recycled fines, works well.The remaining percentage of fines is natural sand.
• In crushing the concrete, it is difficult to control particle size distribution, meaning that the “aggregate” may fail to meet grading requirements of ASTM C33 – “Standard Specification for Concrete Aggregates”.
• The chloride content of recycled aggregates is of concern if the material will be used in reinforced concrete. This is particularly an issue if the recycled concrete is from pavements in northern climates where road salt is freely spread in the winter. The alkali content and type of aggregate in the system is probably also unknown, and therefore if mixed with unsuitable materials, a risk of alkali-silica reaction is possible.

Recycled Materials in the Cement Plant

The cement industry has steadily increased its use of waste materials to fuel cement kilns, and currently relies on the burning of waste materials to satisfy 10% of these energy needs. Cement plants also burn many industrial wastes, including sludge from the petroleum industry, and agricultural wastes such as almond shells.

Cement producers also include non-hazardous byproducts from other industries in the raw ingredients used to make new cement. Common industrial byproducts used include fly ash resulting from the production of electricity, mill scale resulting from steel making, and foundry sand resulting from metal castings. This practice not only recycles wastes, but also reduces the amount of raw materials taken from quarries. Cement manufacturers even reuse cement kiln dust (CKD), the primary byproduct of cement manufacture, by recycling it back to the kiln as an ingredient for new cement. CKD is also sold to the agriculture industry for use as fertilizer.

Recycled Steel