At a Glance
Industrial carbon dioxide (CO2) emissions are difficult to reduce. They require the development of innovative technologies based on fundamental science and engineering. The Sustainable Cements Group is researching ways of reducing emissions attributed to the cement industry, a project that is supported by CMI funding. Researchers are focusing their efforts on the optimization of clay-based sustainable cements and the discovery of the fundamental steps involved in weathering of silicate-rich minerals and related materials. Recent progress includes the successful synthesis and characterization of nano-zeolites and assessment of their performance in sustainable cements. Researchers are also using synchrotron X-ray reflectometry and small-angle X-ray scattering to probe the permeability and transport of ions through nanometer-thin silica-rich surface layers. These layers are thought to control dissolution of silicate-rich minerals under certain conditions.
Electricity generation and usage, and transportation are often discussed as the key sectors responsible for anthropogenic CO2 emissions. However, the industrial sector (cement, steel and plastics) also contributes to global CO2 emissions. In 2017, this sector accounted for 19% of the world’s anthropogenic CO2 emissions (International Energy Agency), with most of these emissions attributed to cement production (8%) and iron and steel production (7%). Chemical reactions occurring in cement kilns during the process of (Portland) cement production, specifically the decomposition of limestone (CaCO3) into lime (CaO) and CO2, are responsible for CO2 emissions. Emissions also result from the burning of fuels to reach the temperature necessary for formation of cement “clinker” phases (~1450 °C). Researchers are pursuing ways to reduce CO2 emissions associated with the cement industry, some of which are more conservative than others in terms of emission reductions.
The goal is to produce concrete for construction projects that is net neutral (or negative) in terms of CO2 emissions. The Sustainable Cements Group is undertaking research focused on creating more “disruptive” (non-Portland cement) concrete solutions. These have the potential of significantly reducing emissions (by up to 80-90%) when compared with concrete that requires Portland cement. These alternatives can play an important role in lowering the cement industry’s emissions when combined with the more conservative approaches. Large construction projects, such as high-rise buildings and long-spanning bridges, will continue to use Portland cement-based concrete. However, there are many situations where concrete is used in low- or non-structural settings, such as sewers, sidewalks, roads, residential buildings and retaining walls. It is these applications where new and innovative alternative concretes can have a significant and lasting impact in reducing the CO2 emissions associated with the Portland cement industry.
The Sustainable Cements Group has been working on the development of clay-based alkali-activated cements. This research aims to lower the alkalinity (i.e., pH) required to make a durable and long-lasting product. The vast availability of kaolin-based clays around the world make this technology a viable approach to producing low-CO2 alternative concrete. Thermally treated kaolin clay (also known as metakaolin) forms a mechanically-hard cement-like substance when mixed with alkaline activators such as sodium silicates. The high alkalinity (pH 13-14+) required to obtain alkali-activated metakaolin cements, however, makes this technology prohibitive from on-the-job safety and a CO2 emissions perspective because it only allows for 55% reduction in CO2.
Current research performed by Christine Pu (graduate student) and White (PI) could potentially lead to a product that has a CO2 savings of at least 70%, while also cutting the alkalinity in half. The result is a material that is much more practical to work with from a safety viewpoint. To do this, the researchers are creating and testing nano-zeolites as seeding agents (for use at < 1 wt. % in the cement). The molecular structure of the nano-zeolites is similar to the disordered gel that forms in the alkali-activated metakaolin pastes.
The researchers hypothesize that by using the nanoparticles, the rate of formation of the paste can be accelerated, offsetting the deceleration caused by lowering the alkalinity. The accelerating effect of nanoparticles has been observed in hydration of Portland cement, where nano-calcium-silicate-hydrate (a nano-form of the gel that gives Portland cement concrete strength, trademarked as X-Seed by BASF) is seen to shorten the set time (time from mixing to forming a hard material) and improve early strength gains. Preliminary findings show successful synthesis of a range of nano-zeolites (and other nano-systems) (Figure 9.1a), proving that these nanoparticles can be easily and readily synthesized. Analysis of the impact of these nanoparticles on the formation of alkali-activated metakaolin has yet to show positive results (i.e., acceleration of reaction). A variety of tests are scheduled to discover why nano-zeolites are, at present, inert, and how the alkali-activated metakaolin system can be re-engineered to exploit the positive effects of nanoparticle seeding.
Complementary to Pu and White’s research, Bastian Wild (postdoc), White (PI) and Ian Bourg (co-PI) are investigating the fundamental chemical/physical mechanisms controlling dissolution of silicate-rich materials in aqueous environments. The dissolution behavior of these materials is integral to a range of natural and industrial processes, including the ability for soils to sequester CO2, the formation of concrete, and the long-term durability of nuclear waste glasses. Yet fundamental mechanisms controlling the dissolution behavior are still little understood and being avidly debated by the scientific community. Wild, a postdoc fellow in the Andlinger Center for Energy and the Environment and supported in part with CMI funding, has shown that the formation of amorphous silica-rich surface layers (ASSLs) on the silicate materials subsequently control dissolution processes under acidic and neutral pH conditions.
Ongoing research is focused on uncovering the permeability properties (i.e., pore structure) of ASSLs and associated transport properties of ions (Figure 9.1c). The aim is to link these microscopic details to macroscopic behavior of silicates in the natural environment and built industry.