Alkali-activated materials – a sustainable alternative to concrete
Due to its ease of production and use, local availability, relative robustness, versatility and low cost, concrete is by far the most widely used construction material in the world with an annual consumption in volume only surpassed by water. Ordinary Portland cement (OPC) is the essential component in concrete with a global annual production of 4.6 billion tons and is responsible for 8% of the anthropogenic global CO2 emissions. About 60% of the CO2 emissions originate from the calcination of limestone during clinker production while 40% stems from the fuels used for firing cement kilns and electricity for milling. In 2050, the share of CO2 emission attributed to cement production is predicted to increase to 16-24% demonstrating the urgent need to improve the sustainability of concrete.
In this context, clinker-free alternative binders with low associated CO2 footprint including the binder class of alkali-activated cements which has been overlooked for a long time become a promising and needed alternative to ordinary Portland binders. These binders are mostly or entirely made from industrial by-products (e.g. blast furnace slag from steel manufacturing, fly ash from coal combustion). They typically do not require further thermal processing and are inherently rich in reactive calciumsilicates and aluminosilicates that can form an insoluble binder phase with only an activator (typically 10% of the binder) being synthesised at a high temperature. Therefore, alkali-activated binders have been identified as offering the potential for up to 70% CO2 emission savings when compared to OPC.
Basic reaction mechanisms and products
Alkali-activated materials (AAMs) are produced from an alkali metal source (solid or dissolved) and calcium silicate or aluminosilicate-rich solid precursor such as coal combustion fly ash (FA), calcined clays (e.g. metakaolin), metallurgical slags (i.e. granulated blast furnace slag – GBFS, stainless steel slags), mine tailings and even natural pozzolans. The reaction mechanisms and the microstructural properties of AAMs are governed by the chemical composition of the solid reactant, the type and the concentration of the activating solution and the curing temperature. The aluminosilicate-rich materials react by means of dissolution, gelation and polycondensation to form inorganic N-A-S-H gel where sodium is integrated in the gel structure, and networks of zeolites which form via polycondensation. Main reaction products in calcium-rich materials include calcium silicate hydrate gels, typically incorporating aluminum, which are therefore labelled as C-A-S-H gel. At a nano- and microstructure level, these gel structures typically perform similarly or even outperform the C-S-H gel occurring in OPC systems, in terms of mechanical, chemical and thermal resistance.
Material Characteristics – potentials and limitations
As a result of the chemical and microstructural properties of AAMs, they offer several potential advantages as compared to OPC materials, including improved mechanical properties such as a higher rate of compressive strength development or significantly higher tensile strength. The material’s potential is impressively reflected when focusing on durability aspects. The superior durability performance of AAMs manifests in a higher resistance to chemical (acid and sulphate) attacks, to chloride penetration, to fire and to freeze-thaw cycles. Despite these advantages, AAMs possess a crucial shortcoming, which is rapid hardening resulting in very short setting times and large shrinkage deformations. The former leads to substantial and rapid changes in rheology delivering poor workability and compatibility. The latter derive from physico-chemical processes occurring during solidification of AAMs and can be up to four times larger than in OPC [64,16]. This induces a high risk of early-age cracking, which, in turn, endangers mechanical properties and durability performances.