Geopolymers: Promising Materials For Underground Applications

By Carlos Montes, Trenchless Technology Center | November 2013, Vol. 68 No. 11
Geopolymer coated concrete pipes being subjected to a laboratory-scale Microbial Induced Corrosion test.

It is of particular importance to make use of the large and valuable fly ash stockpiles which are generated and landfilled every year, including fly ash sources that cannot find a use as a blend with Portland cement. The idea of a 100 percent fly ash-based concrete is of interest to power generation stations, which view it as a ‘green’ beneficiation application for their waste streams. Specifically, a close relationship was developed between the TTC and Cleco’s Dolet Hills power generation station, located in Mansfield, LA. Research conducted at the TTC over a period of five years proved that fly ash generated by this power station was suitable to produce high quality geopolymer products. This partnership yielded significant advancements in the development of geopolymer technology and a large number of peer reviewed technical publications.

However, this technology does not come without its challenges. A key challenge is the inherent variability of fly ash streams. Geopolymer cannot be a widely accepted commercial product unless the fly ash variability problem can be overcome to allow the manufacture of a consistent product with predictable properties. Unlike Portland cement, fly ash is a by-product and is not subjected to quality control. Aside from coal, which is the main source of this variability, each power plant has different operating temperatures and cooling rates which significantly affect the morphological composition of the fly ash. Traditionally this has not been a concern for fly ash use as a Portland cement replacement because the cooling process has little effect on pozzolanic properties. However, it could have a significant impact on fly ash for geopolymer use. Thus, it became necessary to establish a knowledge base that would allow researchers to determine if a particular source of fly ash is suitable to produce geopolymer concrete with certain characteristics.

Research at the TTC demonstrated that the current ASTM C-618 classification of fly ash into Class F and Class C is insufficient to predict its potential as a geopolymer precursor. To develop more accurate guidelines regarding the applicability of a given fly ash stockpile for geopolymerization, the TTC collected over 70 fly ash samples from all across the United States and around the world, and conducted exhaustive statistical studies to understand the effect of the fly ash related variables (like its chemical composition or impurity content) on the resulting mechanical and rheological behavior of the geopolymer mix. This resulted in the world’s largest fly ash database for geopolymer purposes.