The Importance of Bentonite Raw Source for the Performance of Geosynthetic Clay Liners (GCLs)

By GNA Editor

‍Opinion Pieces by Marc Amtsberg (Atarfil) and John Scheirs (ExcelPlas)

Marc has spent 20 years of working in the Geosynthetic sector as both a client and working with major manufacturers and he has observed an ongoing challenge across the industry. Marc has found that remains a general lack of focus on both the Geosynthetic Inputs and Manufacturing Processes that are so critical to product design life.

John has been testing polymers and geosynthetics for 30 years and he has pioneered some of the industry’s latest testing to evaluate long-term durability such as deformulation testing of additives in geosynthetics, thin-film accelerated immersion testing and the use of strain hardening modulus as a surrogate for the NCTL stress cracking resistance test.

An evaluation of the raw material and manufacturing process provide the key insight into expected Geosynthetic performance. Yet this is often overlooked in academic studies, in critical project testing, even for projects that require accountability in EPA regulations.

A reputable manufacturer will have extensive data and must be willing to identify their raw material supplier, their quality processes.  If not willing to reveal their polymer inputs, demonstrated testing and project history then serious doubts should be raised over the suitability of their materials.

The fundamental challenge in using Geosynthetics, is short term testing on very small samples is being used to make performance assumptions for projects that can be millions of square metres. This assumption is only correct, if the product properties are replicated across every square metre. A designer who invests time to understand product inputs and manufacturing process, is better equipped to understand this project risk.

In this new GNA series, Marc and John document the most common technical questions encountered for common landfill and mining geosynthetic lining systems. Many of these properties are not measured by QC Testing, but nevertheless are critical in determining Geosynthetic product life.

Introduction

Geosynthetic Clay Liners (GCLs) are widely used in the geoenvironmental industry for various containment applications, providing impermeable barriers against liquids such as landfill leachates. They consist of a layer of high-swelling sodium bentonite sandwiched between two geotextiles. The hydraulic performance of GCLs relies heavily on the properties of the bentonite used in their construction.

Bentonite, a natural clay mineral, exhibits expansive characteristics and low permeability, making it an ideal material for GCLs. This article explores the significance of the raw bentonite source in determining the performance characteristics of GCLs and highlights the importance of understanding and analyzing the properties of the bentonite raw material.

The Role of Bentonite in Geosynthetic Clay Liners

Bentonite is the key component of GCLs, responsible for their hydraulic performance. When hydrated under confinement, bentonite swells to form a low-permeability clay liner, providing a level of hydraulic protection equivalent to several feet of compacted clay.

The dominant clay mineral in bentonite is montmorillonite, which exhibits significant swelling when in contact with water, expanding approximately 900% by volume or 700% by weight.

Raw Bentonite Variation

Bentonite is a natural material with inherent variations, and its properties can differ significantly depending on its source and mineralogical composition. The dominant exchangeable cation (sodium or calcium) and the types/content of certain impurities play a crucial role in determining the performance of the bentonite.

Sodium cations impact swelling properties, while calcium provides absorbent properties. Consequently, the raw bentonite source can lead to markedly different performances of GCLs, especially considering their relatively thin layer (approximately 7 mm).

Importance of Analyzing Raw Bentonite Source

Despite the significance of raw bentonite properties in GCL performance, analysis of the bentonite source is often overlooked in testing programs and technical papers. However, gaining insight into the raw material's characteristics is essential for understanding the expected performance of GCLs.

While manufacturers may not disclose specific additives or processes used, knowledge of the dominant cation, montmorillonite content, and impurity levels can provide valuable information regarding swelling behaviour, moisture uptake, and retention. These properties are critical for evaluating the long-term hydraulic risks associated with exposure to leachates and challenging liquors.

Insufficiency of Traditional Testing - Are properties - Powdered or Granular/Swell/Fluid Loss sufficient?

Analysis of the raw Bentonite source is rarely documented in testing programs and technical papers, yet the information should be made accessible and provides great insight as to expected performance. The manufacturer may not tell you the specific additions/process they use, but the raw material dominant cation, montmorillonite and impurity content provide a direct correlation to expected swelling behaviour, moisture uptake and retention. These properties drive long term hydraulic risk with exposure to leachates and challenging liquors, we must see greater analysis of GCL raw bentonites and link it to testing results.

Traditional hydraulic testing, swell, and fluid loss testing with deionized water are insufficient for evaluating GCLs' performance under challenging conditions. Bentonite sources vary significantly in their properties, and solely relying on particle size (granular vs. powdered) for analysis is no longer adequate.

Advanced testing techniques, including mineralogy testing, are now essential for understanding the montmorillonite content and percentage of non-swelling impurities. The critical montmorillonite form (natural sodium or soda-activated) also plays a significant role in determining the suitability of bentonite for GCL use. Consequently, testing must go beyond basic properties to establish accurate performance expectations.

Selecting Suitable Bentonite Sources

Only a few bentonite sources are suitable for use in GCLs due to their marked variations in properties. Therefore, it is crucial to understand the raw material's characteristics in conjunction with any beneficiation processes applied. Mineralogy testing, including X-ray diffraction (XRD) analysis, helps reveal the montmorillonite content and the type of activation required for specific sources. Not all natural sodium bentonites are suitable for GCL use, necessitating a deeper understanding of the raw materials and their beneficiation processes to ensure optimal performance.

Conclusions

The performance characteristics of Geosynthetic Clay Liners (GCLs) directly correlate with the properties of the raw bentonite source used in their construction. Analyzing and understanding the characteristics of the bentonite raw material are critical for assessing the long-term hydraulic risks associated with GCLs' exposure to leachates and challenging environments.

Traditional testing methods are insufficient, and advanced techniques such as mineralogy testing are now essential for evaluating the suitability of bentonite sources for GCL applications. By investigating the raw bentonite properties, engineers and regulators can make informed decisions to enhance the overall performance and effectiveness of GCLs as impermeable liners in various geoenvironmental applications.

Take Home Messages

Investigate Raw Site Bentonite properties as they correlate directly to GCL performance characteristics.

Technical Datasheets from GCL manufacturers do not capture risk for leachate.