A study published in Applied Sciences by Almssad et al. highlights a critical breakthrough in sustainable construction: the development of concrete mixes that reduce carbon emissions without sacrificing life safety during a fire. As the global construction industry seeks to meet net-zero targets, reducing the amount of cement clinker—which accounts for nearly 9% of human-caused carbon dioxide—has become a priority. However, lowering cement content often changes how concrete reacts to extreme heat. One of the most dangerous reactions is explosive spalling, where trapped steam pressure causes surface layers to violently detach. This research proves that strategic combinations of industrial by-products and specialized fibers can mitigate these risks effectively.

The research team tested seven distinct concrete formulations, comparing a standard cement reference against mixes containing ground-granulated blast-furnace slag, biochar, and hydrochar. The standard reference mix reached a high initial strength of 53.3 MPa, but its dense, low-permeability structure proved to be its downfall during fire testing. Without pathways for steam to escape, the reference mix experienced severe explosive spalling and a mass loss of 19.2%. This demonstrates that high daily strength does not guarantee safety in emergency conditions.

In contrast, the most balanced performance came from a mix that replaced 50% of the cement with slag and added polypropylene (PP) microfibers. These fibers act as a built-in safety valve; as temperatures reach approximately 165°C, the fibers melt, leaving behind a network of tiny channels. These channels allow internal steam pressure to vent safely to the atmosphere, preventing the buildup that leads to explosions. This specific mix showed a significantly lower mass loss of only 8.84% and, most importantly, remained structurally intact after being heated to a high temperrature.

The study also explored the use of biogenic carbon fillers, revealing a sharp contrast between biochar and hydrochar. Biochar, while excellent for carbon sequestration, actually increased the risk of catastrophic failure. At a 10% replacement level, biochar-modified concrete suffered the most severe damage in the study, with a massive 33% mass loss. The researchers attribute this to biochar’s highly water-absorbent nature, which traps moisture that turns into high-pressure steam. Hydrochar, however, showed promising results at higher doses. Mixes with 10% to 20% hydrochar exhibited no spalling and minimal mass loss, as the material’s unique microporosity provided pre-existing escape routes for vapor.

Ultimately, the findings present a clear roadmap for future building regulations and sustainable design. While there is an initial trade-off in mechanical performance—as biogenic additives tend to lower pre-fire compressive strength—the gains in fire safety and environmental impact are substantial. For load-bearing structures where fire resilience is the highest priority, a combination of slag and fibers or high-dose hydrochar offers a superior alternative to traditional cement. This research provides the evidence needed to specify lower-cement concretes that maintain high safety margins, ensuring that the buildings of the future are both green and fire-secure.

Source: Almssad, A., Al-Gburi, M., Viktor, A., & Mohammadullah, A. (2025). The Use of Slag, Biochar, and Hydrochar as Potential Concrete Additives: Effects on Compressive Strength and Spalling Resistance Before and After Fire Exposure. Applied Sciences, 15(13248).