The contribution of Portland cement (PC) manufacture to carbon dioxide levels in the atmosphere has been well catalogued. The problem is of global relevance because such emissions circulate globally regardless of where they originate.
The pursuit of alternatives to PC is of equal concern (LCCG/CGB, 2022) and within that activity the possible role of alkali-activated cementitious materials (AACMs) is of growing importance. There is active research and product developments, but are these efforts having a significant impact? AACMs or geopolymers have been an option for some 120 years (GCM, 2016; Roy, 1999; Singh et al., 2015; Turner and Collins, 2006) but have never become mainstream within the construction sector. This is because of three significant reservations, despite their good durability, chemical and sulfate resistance:
- 1.
the problem of strength/temperature/time restraints of resulting concretes that limits their application, notwithstanding the search for effective ‘activators’ (Luukkonen et al., 2018; Nadoushan and Ramezanjanpour, 2016)
- 2.
the availability of suitable precursors on an engineering scale that could compete with PC-based construction
- 3.
additional costs associated with precursor chemical acceleration.
In addressing these limitations is the additional problem of transferring new materials and technologies into ongoing best practice. Construction practice and specifications are understandably conservative and new products and techniques are viewed as carrying risk and that in turn can result in insurance issues. Designers and contractors perceive reducing risk by complying with national standards. Unfortunately, at the present time, attempts to develop standards for AACMs have had limited success (BSI, 2016; Provis, 2018).
There appears to be growing emphasis in a partial carbon reduction by developing minimum-PC-content concretes using supplementary cementitious materials in multiple combinations while also having sufficient PC to provide activation and having equivalent mechanical properties. This is not a novel approach and, while resulting in lowering the carbon dioxide footprint, will not eliminate it. A pragmatic half-way house?
The chemistry controlling the dissolution of encased aluminosilicates (Walkley et al., 2017) and their subsequent hydration is key to improving their practical application. Alternative ‘activators’, other than concentrated alkalis, coupled with accelerators holds the key. The patent literature makes mention of sodium sulfate, carbonate and calcium oxide used separately and in combination, but the problem of strength capability limits their application.
In addition, the chosen precursors are also important (Bougara et al., 2010; Kashani et al., 2019) as well as the source and type of ground granulated blast furnace slag (for instance, steel or iron based) are factors, together with the selection of calcined clays (Rakhimov et al., 2018; Sposito et al., 2021) as a global precursor.
A tempting alternative option to current approaches is to use less PC-based concrete in both domestic and industrial buildings and superstructures. Is this a naive suggestion? We can envisage longer lasting, more durable, thermally insulating and high-strength concrete now. However, the transformation of radical change into specifications and the adoption of new materials options is cautious. Perhaps that is how it should be?
One has to consider the ultimate objective behind these developments. Are we seeking to replace PC with an alternative binder that has global capacity? Are we seeking minimum-cement-content concretes having adequate performance set against functional need? Perhaps some designs and the resulting structures are too ambitious. Do they have to be taller, longer, more slender or rapidly built and cheaper? Perhaps we have too many possibilities as a result of wide-ranging research programmes while not taking full advantage of what already exists?
In looking ahead to the future of concrete and related research opportunities, the review by van Damme (2018) may well be a good start.
