- Created: Friday, 11 August 2006 10:27
- Published: Friday, 11 August 2006 10:27
There is a significant trend among both cement manufacturers and concrete producers to maximize recycling and use of recycled materials. Major motivators include the realization that using recycled materials can achieve cost savings, qualify for tax savings, improve product performance and meet recycling goals. So, if manufacturers and users can meet the regulations, save money and produce a better product all at the same time, that's the cementitious equivalent of a grand slam.
Two recycled cementitious materials come to mind immediately ó fly ash and slag cement. For more than 50 years, unprocessed fly ash from coal-fired power plants has been used as a substitute for a portion of the cement in concrete mixes. However, new environmental regulations have reduced the availability of consistent quality fly ash suitable for use in concrete.
Recently enacted federal standards for NOx emissions from coal-fired power plants have led to the installation of pollution control equipment that causes significant and fluctuating levels of unburned carbon in the fly ash. In some cases, the fly ash also is contaminated with ammonia.
High carbon levels render fly ash unsuitable for use in concrete because the excess carbon negates the effectiveness of air-entraining chemical admixtures. The ammonia gas, which tends to be released from a concrete mix when water is added, has no appreciable effect on the performance of the concrete, but presents worker safety concerns.
While several companies market fly ash for use in concrete, Separation Technologies, Inc. has developed a proprietary, high-volume, electrostatic separation process that produces fly ash with consistent carbon levels from raw fly ash that originally had high and widely varying LOI (loss on ignition) levels. If ammonia is present in the raw fly ash, it can also be removed at the same high volume in the processing line.
This offers two benefits for concrete producers. The consistent LOI level in the fly ash allows standardized batching and dependable performance results. The high-volume processing ensures availability when the bulk tanker arrives for a pick up.
Increasingly, over the past decade, slag has been used with cement for its cost savings and performance benefits. New research by STI indicates that a tri-blend mix of cement, fly ash and slag cement may offer increased cost savings as well as better concrete performance.
STI, assisted by several of its ready mixed producer customers, found that cement, fly ash and slag cement used together produce synergies that make the three-part combination work better than either material by itself with cement.
The spherical shape of the fly ash particles act like ball bearings, allowing the elements of the mix to combine more easily. The improved workability from the fly ash means that water content can be reduced, enhancing the strength of the concrete. The tri-blend concrete also has better early strength as well as better overall strength than concrete produced with either slag or fly ash alone in combination with cement (Figure 1).
Based on the concrete producer's location and size, the average cost of cement, slag and fly ash will vary, but Figure 2 (which includes only the cementitious materials) shows the range of savings that can be expected using slag, fly ash, and the tri-blend mix in place of cement alone.
The savings are based on fly ash to cement replacement of 1:1 (by weight); depending on other factors, the actual replacement ratio could be as high as 1.2:1. These considerations do not apply to the tri-blend analysis ó additional chemical admixtures are not called for, and the ratios by which both fly ash and slag substitute for cement in the tri-blend is 1:1. In a 20 percent fly ash, 30 percent slag cement, 50 percent portland cement blended mix, the cost savings over using cement alone can be as much as 15 percent.
In the case of one specific customer, STI was able to show how the tri-blend offered dramatic per yard savings (Figure 3). The cost per yard figures are for the cementitious materials only, not including aggregates, sand, water and admixtures. Actual net savings from replacing cement with fly ash may be less than the indicated number ($2.03), because of the need for additional chemical admixtures (minimal in cost), and because of the likely need to refine the concrete mix by adding additional fly ash.
The tri-blend is a case where the benefits of the total mix are greater than the sum of the parts. Combining cost savings, superior performance and environmental/recycling objectives, the use of cement, slag and fly ash together is one more step in a continually evolving story.
PRODUCER TRIES MIXES
Rock Hill Materials Co. in northeastern Pennsylvania has been using a ternary blend (tri-blends) in residential mixes for the past year at its three plants. ìThe combination of cement, slag, and fly ash gives us more flowable, less permeable, and more durable concrete in a more cost-efficient mix,î says Jay Robinson, Rock Hill's sales and technical services manager. As long as the ambient temperatures stays above 40∞F, Rock Hill runs ternary mixes from April until early December. The company found that a 50-30-20 combination works well.
ìIt results in concrete with less water and a slower curing time giving a higher psi which doesn't affect next-day strength or set performance,î Robinson affirms. In addition, Rock Hill has not had any call-backs for scaling from the jobs using the tri-blend.
Based on their success with residential mixes, Rock Hill wanted to prove to structural concrete specifiers that the ternary mixes would offer a lower water to cement ratio, resulting in a lower coloumb number. ìWe submitted a ternary mix, along with our normal mixes, on four structural bids last year,î Robinson notes. ìAnd although we didn't actually ship it, the ternary mix specs were also accepted for all four.î
So far this year, Rock Hill has shipped one structural job of more than 600 yd. of lightweight pumped concrete using 60-30-10 mix. Although the high temperature was only 26∞F, the concrete set time was well within specs.