Climate change is a serious problem threatening mankind today and a recent development from MIT may help reduce carbon dioxide emissions. Roland Pellenq, a senior research scientist in the MIT Department of Civil and Environmental Engineering (CEE) and research director at France’s National Center for Scientific Research (CNRS), along with his colleague Franz-Josef Ulm, a professor of CEE and director of the MIT Concrete Sustainability Hub (CSHub), have led research to develop climate-friendly concrete. Given that the construction industry is responsible for over 10% of the total industrial carbon dioxide emissions, climate-friendly concrete alternatives  would be impactful.

Eco-Friendly Concrete

The term “concrete” originates from the word concretus which means compact in latin. Concrete consists of aggregates (rock, sand or gravel), water and cement; the proportion of these three constituents influences the properties of the concrete obtained. Production of cement involves heating limestone up to 1773 K to eliminate carbon and is responsible for about 6 percent of the total industrial carbon dioxide emissions. Considering the ever-growing demand for new construction projects and building maintenances of existing structures, cement production is expected to increase in the coming years. Since the mixture of cement and water — cement hydrate — is the most crucial constituent, cement hydrate was the focus of the research.  

New Formula for Cement Hydrate Paves the Way

In order to develop eco-friendly concrete, it is important to understand how cement hydrate nanoparticles set at the nanoscale. The packing efficiency is crucial in determining the properties of cement hydrate and hence, the properties of concrete. If not closely packed, voids in the hardened cement could potentially allow aggregates to seep in or the cement hydrate particles could be mobile, thus causing aging or other possible deterioration.

In the absence of any “concrete” research in this direction, Pellenq, Ulm and their collaborators at CSHub first started by defining the three-dimensional molecular structure of cement hydrate. This allowed them to devise a new formula optimizing the composition for cement hydrate particles resulting in a 50 percent increase in particle strength. By increasing the structural strength of the concrete, increased longevity is achieved thus leading to a reduction in the dry cement powder that needs to be manufactured, reducing maintenance costs and carbon emissions.

Ideally, this new formula must be tested over a long duration of time (~decades) while allowing the cement hydrate particles to harden by redistribution of particles and evolve. For such simulations, one defines the complete physics involved in the formation of cement hydrate particles by taking into account all the physical forces existing in the system and then waits for the particles to achieve equilibrium. Katerina Ioannidou at MIT successfully performed these simulations resulting in an algorithm that constantly keeps track of forces on each particle and updates their most-probable positions. It also takes into account the addition of differently-sized particles to fill in voids and ultimately results in system equilibrium while mimicking the particle-wise evolution in the precipitation process. This simulation model holds good when comparing the theoretical values of stiffness and strength of cement hydrate samples with the corresponding values from some feasible experiments.

To summarize, this novel research arms us with a useful means to design the cement hydrate in order to achieve the target properties of concrete based on any specific application. Not only does this innovative tool eliminate the ambiguity associated with cement hydrate so far, but also enables the production of eco-friendly concrete.

Industrial Uses and Benefits of Eco-Friendly Concrete

This novel approach of simulating cement hydrate opens up a plethora of possibilities, enabling one to control the characteristics of concrete by carefully tailoring the hardening of cement hydrate particles at the nanoscale. The researchers at CSHub are currently working towards the development of a denser, stronger and more durable concrete by the addition of polymers to occupy the empty spaces in cement hydrate. This could be particularly useful at natural gas wells where cement sheaths are used around drilling pipes to prevent gases from escaping. According to Dr. Pellenq, filling of voids in cement is essential because a molecule of methane is about 500 times smaller than the pores in today’s cement.

Another interesting idea involves recycling of concrete. Currently, old concrete is partially recycled by using it to replace aggregates in a new concrete mix. Ulm, on the other hand, expects to go beyond what is already done and create a new binder instead of more cement for a new concrete mix. In Dr. Ulm’s own words, “We can make new binder without needing any new cement — a true recycling concept for concrete!”. Though not practical yet, the idea here is to replicate the cohesive forces that exist in cement hydrate. What’s more, the impact of this research is not limited to the construction industry. According to research carried out at CSHub, fuel consumption of vehicles can be reduced by about 3 percent by making concrete road surfaces stiffer. This is demonstrated based on the fact that the interaction between tires and the pavement significantly influences fuel efficiency of vehicles. Moreover, concrete is also a promising candidate for long-term confinement of nuclear waste. A recent study shows that cement can address the problem of containing nuclear waste in decommissioned nuclear plants and can safely store certain radioactive materials (Sr-90 and its daughters), even in accidental conditions.

In short, the current research successfully equips one to design cement hydrate so that application-based concrete properties are achievable. In particular, this innovation has resulted in climate-friendly concrete to target the 3 R’s of sustainability i.e. reduce, reuse and recycle.

Talk about a “concrete” idea to “cement” the reduction of the carbon footprint across multiple industries!

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