The Use of Recycled Materials in the Production of Portland Cement Concrete

We know from history, that recycling of construction materials has been going on as long as man has been building. Whether from natural disaster, war or pillaging it has been very common to re-use parts of structures to make new ones. For example, over the centuries, thousands of tons of limestone have been removed from the exterior of the great pyramids in Egypt and reused to build buildings as far as one hundred miles away.

In modern society, recycling is the norm in the construction industry. In fact, the recycling of construction materials is an everyday business practice. Products such as mulch, rubber bumpers, particleboard, composite lumber, rebar and steel wire products are made primarily of recycled materials. There are many other products such as expanded polystyrene foam that use a portion of recycled materials as part of the production process for new material.

The practice of using recycled materials is also commonplace in the production of Portland Cement Concrete (PCC). Some recycled materials use is mandated by rules and regulations, some use is determined by the lack of raw products in a geographic region, some use is just good business practice of cost reduction and some use can enhance the quality of the finished product.

Following is a list of the recycled materials used to make PCC and the benefits or reasons for their use.

Coal fly ash is a byproduct of coal burning at electric utility plants. Fly Ash when used in PCC commonly constitutes up to 20% of the cementing material.

Fly Ash lowers water demand, reduces the heat of hydration, increases mix pumpability and reduces segregation and bleeding. Hardened PCC made with Fly Ash has a reduced amount of alkali-silica reactivity.

The benefits of Fly Ash use are less water use, higher strength PCC and more durable PCC.

Slag is a byproduct of iron blast furnaces. The slag is ground into granules finer than Portland cement. When used in PCC, slag commonly constitutes up to 40% of the cementing material.

Slag in plastic cement reduces the heat of hydration, increases mix workability and reduces segregation. Hardened PCC made with slag has a reduced amount of alkali-silica reactivity.

The benefits to slag use are higher strength PCC and more durable PCC.

Silica fume is a waste material recovered from alloyed metal production. When used in PCC as a partial cement replacement or cement addition, silica fume commonly constitutes 5% to 30% of the cementing material.

Silica Fume is a great admixture for harsh conditions (like water treatment plants) because it can make highly cohesive and impermeable PCC that reduces steel corrosion. Also, very high strength PCC is made with silica fume (20,000 PSI).

The benefits to using Silica Fume include higher strength, less permeable and longer lasting PCC.

In order to comply with the Clean Water Act of 1987 (and other strict environmental laws), many producers must use reclaimed water. Reclaimed water is primarily a mixture of water, cementicious material and aggregate fines. Solid content in reclaimed water varies from 2.5% to 10%. There are 3 sources for reclaimed water that can be used in making PCC.

Reclaimed storm water: rainwater that remains on the property it landed on because discharging it into a city’s storm drain is not allowed.

Reclaimed wash water: water that was used to wash down the batch plant, the mixer trucks and, often, the batch plant site (dust control).

Reclaimed water from plastic PCC: water that is separated from the other ingredients while the concrete is still in a workable, plastic state (PCC has not achieved initial set).

PCC producers must comply with ASTM C 94, ASTM C 1062-04 and ASTM C 1063-04, which prescribe the use of reclaimed water, the specifications for the water and the method for testing the water. The specification guidelines are performance based so that the quality of the finished PCC is not diminished in any way.

When reclaimed water is used, the benefits include saving approximately 30 gallons of fresh water per cubic yard of PCC.

Each year in the United States millions of tons of PCC are recycled and reused. In our geographical region, PCC rubble is typically crushed into aggregate base. That base material is used for under slab or under road construction.

In other regions of the US, where quality aggregates are harder to find, PCC rubble is crushed and screened into concrete aggregate. This material is then blended with virgin rock, cement and water to make new PCC.

Concrete rubble can make high quality PCC when proportioned right and used for the correct application such as sidewalk PCC. It is also a great product for making Aggregate Base.

The benefits include reduction of the solid waste stream, cost savings from importing virgin aggregate and reducing the amount of current resource consumption.

Ground glass has been used a both a coarse and fine aggregate in the production of PCC. When used in PCC glass replaces the aggregates in the mix.

There is no real enhancement of the PCC by the use of glass. The benefits include reduction of the solid waste stream, cost savings from importing virgin aggregate and reducing the amount of current resource consumption.