David G. Brice

Microstructure and durability of alkali-activated materials as key parameters for standardization

Alkali-activated concrete (AAC) has been commercialized as a low-CO2 construction material, but its adoption still faces several challenges, including standardization, lack of a dedicated supply chain, limited service track record, and the question of whether laboratory durability testing can predict service life. This paper outlines how using different precursors leads to the formation of different AAC phase assemblages, and how AAC can be recognized in standards using a performance-based approach independent of binder chemistry. Microstructural assessment of pastes, strength development, water permeability, and chloride migration of two AACs (100% slag and 1:1 slag:fly ash) are presented, and compared to Portland cement concrete. Manipulation of binder chemistry leads to differences in the properties of the AACs; however, both AACs assessed exhibited technical benefits in a performance-based comparison. AACs can meet the requirements of the equivalent performance concept, independent of the binder chemistry, supporting their scale-up, regulatory acceptance, and wider adoption.

Demonstration Projects and Applications in Building and Civil Infrastructure

In the context of a Report such as this, it is of immense value to be able to provide tangible examples of structures and applications in which alkali-activated concretes have been utilised throughout the past decades. A detailed outline of the utilisation of AAM concretes in the former Soviet Union and in China is given in Chap. 12 of the book by Shi, Krivenko and Roy [1], and this chapter will briefly describe some of the applications mentioned in that (more extensive) document, along with applications elsewhere in the world where AAMs have been utilised on a significant scale in the construction of buildings and other civil infrastructure components. An overview of developments and applications in the former USSR has also been presented by Brodko [2] and by Krivenko [3]. Each project reported in this chapter involves at least pilot-scale, and in some cases full commercial-scale, production of alkali-activated concretes utilising largely standard concrete mixing and placement equipment and labour, indicating that these materials are both accessible and useful on this length scale, given sufficient expertise in mix design based on locally available precursors. In the former USSR in particular, slags obtained from local iron production facilities were used in each of the different locations in which the concretes were produced, and activators were sourced in large part from locally available alkaline industrial waste streams.

Conclusions and the Future of Alkali Activation Technology

The key outcome of RILEM TC 224-AAM has been the development of a conceptual framework from which the discussion of standardisation of alkali-activated binders and concretes can proceed. There has been agreement from the members of the TC that a performance-based approach to both materials formulation (as described in Chap. 7 of this report) and testing (as outlined in Chaps. 8,9, and 10) is essential in enabling the scale-up of alkali-activation as a method of concrete production in the global context. However, it is essential to proceed with a degree of conservatism, to avoid becoming ‘the next high-alumina cement problem’ through use of a material in environments and/or systems where it is not fit for purpose. So, it is critical that standards development is conservative to ensure that due care is taken, and to make sure that poor-quality AAM products, and/or products used in unsuitable applications, do not ruin the global reputation of the technology. A key discussion which occupied much of the time of the TC was the issue of how to achieve this – and the conclusion reached was that the use of strict performance criteria (and maybe even criteria which seem excessively strict until a higher degree of certainty regarding performance levels can be reached), and with good scientific foundations, must underpin any testing method applied to these materials.