Evaluating a new slag for use in concrete

Volume 16, Number 3, September 2011

Millions of cubic metres of concrete are produced in Canada each year, consuming large quantities of coarse aggregate, sand, and ordinary Portland cement (OPC). There is growing interest in finding alternatives for these materials for both economic and environmental reasons. While supplementary cementing materials (SCMs) have been used for many decades as partial replacements for OPC, new sources of replacement materials, such as alternative SCMs and alternative aggregates, will be needed.

With regard to alternative SCMs, one possible source is by-products of industrial processes, such as the residual material from a new plasma-assisted waste conversion technology. This by-product is water cooled to produce a glassy black slag. Since a key goal of the technology is that no remaining material is sent to landfills, the NRC Institute for Research in Construction (NRC-IRC) was asked to evaluate the slag for use in concrete.

Test material is a glassy black slag before grinding for use in concrete

A preliminary analysis indicated that the slag had oxide chemistry very similar to that of blast furnace slag (BFS), a widely used SCM. This result suggested that the test slag could be evaluated for use as an alternative SCM. As large volumes of an alternative SCM need to be produced to make it economically attractive to the construction industry, NRC-IRC proposed that the slag also be evaluated for use as a fine aggregate.

Researchers investigated the chemical behaviour of the test slag produced under five different operating conditions and compared it to a commercial BFS. They then ground the slag to the same size as the commercial product and evaluated it using a variety of tests. The results showed that the slag had similar properties to the BFS. It was consistently amorphous and had oxide content similar to that of the commercial slag. Key elements of chemical performance, including its pozzalanicity (ability to react with the calcium hydroxide in hydrated cement), its hydration behaviour when blended with OPC, and its ability to provide resistance to detrimental alkali-silica reactions, were all comparable to commercial BFS.

These promising results led to a second stage of testing, in which the researchers examined the performance of the material in mortar and concrete samples (see video). They measured the compressive strength of three samples of each in which 10, 20 and 50% of the OPC was replaced with the test material. Early curing time strength results showed similar performance to a commercial BFS.

In parallel tests, the researchers ground the test slag to the size distribution of ASTM standard sand and mixed it into mortar and concrete samples made with OPC alone, so that the slag’s performance as sand could also be evaluated. They found the performance of the ground slag sand to be similar to an ASTM sand.

The researchers are now conducting further tests assessing freeze-thaw action, alkali-silica reactivity, and longer-term strength. Final results will be available in Fall 2011.

For more information

For more information about this research or the testing of materials for use in cement and concrete, contact Dr. Jon Makar at 613-993-3797. Information about the plasma-assisted waste conversion process may be found at: http://www.zerowasteottawa.com/en/.