The role of inorganic polymer technology in the development of ‘green concrete’

Abstract

The potential position of and drivers for inorganic polymers (“geopolymers”) as an element of the push for a sustainable concrete industry are discussed. These materials are alkali-activated aluminosilicates, with a much smaller CO₂ footprint than traditional Portland cements, and display very good strength and chemical resistance properties as well as a variety of other potentially valuable characteristics. It is widely known that the widespread uptake of geopolymer technology is hindered by a number of factors, in particular issues to do with a lack of long-term (20+ years) durability data in this relatively young research field. There are also difficulties in compliance with some regulatory standards in Europe and North America, specifically those defining minimum clinker content levels or chemical compositions in cements. Work on resolving these issues is ongoing, with accelerated durability testing showing highly promising results with regard to salt scaling and freeze–thaw cycling. Geopolymer concrete compliance with performance-based standards is comparable to that of most other high-strength concretes. Issues to do with the distinction between geopolymers synthesised for cement replacement applications and those tailored for niche ceramic applications are also discussed. Particular attention is paid to the role of free alkali and silicate in poorly-formulated systems and its deleterious effects on concrete performance, which necessitates a more complete understanding of the chemistry of geopolymerisation for the technology to be successfully applied. The relationship between CO₂ footprint and composition in comparison with Portland-based cements is quantified.

Introduction

Inorganic polymer concretes, or ‘geopolymers,’ have emerged as novel engineering materials with the potential to form a substantial element of an environmentally sustainable construction and building products industry [1], [2]. These materials are commonly formed by alkali activation of industrial aluminosilicate waste materials such as coal ash and blast furnace slag, and, as will be discussed in detail in this paper, have a very small Greenhouse footprint when compared to traditional concretes. Given correct mix design and formulation development, geopolymeric materials derived from coal ash (Class F and/or Class C) can exhibit superior chemical and mechanical properties to ordinary Portland cement (OPC) [1], and be highly cost effective. A key attribute of geopolymer technology is the robustness and versatility of the manufacturing process; it enables products to be tailor-made from a range of coal ash sources and other aluminosilicate raw materials so that they have specific properties for a given application at a competitive cost. Applications of particular interest at present include low or high strength concretes with good resistance to chloride penetration, fire and/or acid resistant coatings, and waste immobilization solutions for the chemical and nuclear industries. The specific properties of geopolymers which lead to particular suitability in each of these applications will be outlined briefly in this paper.

Despite these key technological attributes, and environmental and cost savings compared to OPC, the main drivers for the uptake of the technology by existing players in the cement and concrete products industry may not form a compelling business case for geopolymer production by these organizations at this time. This is particularly so for large cement companies, where the low profit margins and high financial risk involved with the introduction of a revolutionary (rather than evolutionary) technology in low- and high-strength concrete applications respectively must be taken into account. The successful adoption of geopolymer technology in the future will be governed by a host of factors to be discussed here, including the ability to add significant value to coal ash and/or slag waste streams, wider understanding of the benefits of the technology, as well as the potential to significantly reduce carbon dioxide emissions compared to OPC manufacture. This paper discusses the practical technical, environmental, and commercial drivers in more detail, as well as how geopolymer technology may form part of an overall ‘Green Concrete’ industry. Later in the article, the commercial drivers for adoption of geopolymer technology are explored in a broader context that takes into account not just the desirable application of the material, but also the market reality.