Introduction

It is difficult to imagine a world without concrete, it is ubiquitous in our built environment. Much is clearly visible, for example in our buildings, roads, dams and bridges, whilst some of it is out of site, below ground in piles, foundations, road bases and tunnels. Concrete is essential to modern civilisation and our reliance on the built environment to provide a high quality of life. It is essential because of the availability of its constituent parts, cost, flexibility, engineering properties, and durability (Monteiro et al., 2017). Next to water, concrete is the most consumed substance on the planet (Miller et al., 2018) and is produced in volumes exceeding 25 Gt per year worldwide (Li et al., 2019), which equates to approximately to the consumption of 19 Gt of aggregate, 4 Gt of Portland cement and 2-3 Gt of fresh water (Miller et al., 2018).

Recent research on world economic growth predicts that the planet’s built area will double in the next 40 years and concrete production will have to rise by 25% by 2030 (Miller, 2018). Quarrying the enormous amounts of the raw materials needed for concrete entails the destruction of natural environments, the loss of biodiversity, water, air and noise pollution.

Producing PC, a key component of modern concrete, requires the production of clinker which is formed when limestone and clay are burnt at c. 1450 °C (Miller, 2018). This not only requires considerable infrastructure, it also consume vast amounts of raw materials with over 3 tonnes of material being required to produce a tonne of PC (BGS, 2005) and energy (Madlool et al., 2011) and generates high levels of GWP CO2 emissions, accounting for c. 8% of the worldwide total (Monteiro et al., 2017). Given these facts, the construction industry must commit to adopting measures to tackle climate change and guaranteeing concrete and cement manufacture sustainability for future generations (Rahla et al., 2019).

The concrete and cement industry have for many years been seeking alternative raw materials sourced from industrial by-products, both as fuel for cement kilns, aggregates for concrete manufacture and partial PC substitutes or SCMs, such as GGBS and FA (Cantero et al., 2019). In recent years have also witnessed a quest for new SCMs derived from industrial biomass waste (Lv et al., 2019; Medina et al., 2019; Nakanishi et al., 2014), ornamental quarry sludge (Mármol et al., 2010; Medina et al., 2017; Sáez del Bosque et al., 2018), burnt clay (Scrivener et al., 2018) and similar.

Alkali Activated Cementitious Materials (AACMs)

AACMs are a class of cements produced at room temperature via a chemical reaction between a poorly crystalline aluminosilicate material (the precursor) and a highly alkaline solution (the activator) to form a hardened solid (Provis, 2018). They replace Portland Cement (PC) either partially or totally and use binders based on by-products for which the carbon-based legacy has already been accounted, primarily Fly Ash (FA) and/or Ground Granulated Blast Furnace Slag (GGBS). As these materials are only latently hydraulic they must be chemically stimulated or ‘activated’ to release their hydraulic properties. Most activators are highly alkaline or caustic liquid materials such as sodium/ potassium hydroxides and/ or sodium silicate in some form.

The total replacement of PC in concrete systems with the use of AACMs has also been keenly researched. AACMs based on Ca-rich precursors, such as GGBS are currently used for the production of mortars and concretes for structural and non-structural applications (Juenger et al., 2011; Buchwald et al., 2015) where the AACM has been manufactured in both precast, and cast-in-situ processes.

Various combinations of alkali activating species are used and AACM’s have been available for over 100 years in some form. However, with readily available and reasonably cheap Portland cement there has been little justification for their use other than in special works requiring high chemical resistance.

However, sustainability and environmental concerns have placed emphasis on AACM development and their increased use. That trend will continue and is likely to be encouraged by way of legislation and regulation.

Read Andrew Frost’s Contribution to Concrete Magazine on AACMs
Read Graeme Jones’ Contribution to Concrete Magazine on AACMs

Sustainability

AACMs are considered to be an environmentally sustainable material (Habert et al., 2011), reducing resource depletion, waste to landfill, pollution and GWP emissions. There is a consensus that alkali-activated cements can offer cradle-to-gate Greenhouse emissions savings approaching 40–80% compared to PC for a performance-equivalent material (Bernal, 2016).

Durability

Concrete can deteriorate when exposed to aggressive environments. The durability of AACMs is addressed in detail in the recent RILEM State of the Art Report (Provis and Deventer, 2014) as well as several review papers (Pacheco-Torgal, et al., 2012) and (Bernal and Provis, 2014). In general, chemical resistance of concretes produced with AACMs is high, but there remain questions over carbonation and freeze-thaw resistance which are not yet fully understood (Pacheco-Torgal, et al., 2012) and (Gillott and Gifford, 1996).