Effect of the addition of cement on the loess stabilization: a case study of Gorgan loess from northern Iran

In the northern reaches of Iran, beneath the city of Gorgan, lies a vast expanse of a peculiar soil known as loess. To the untrained eye, it appears as ordinary, cream-colored earth, but to a geotechnical engineer, it is a hidden hazard. This soil is formed from wind-blown dust that settles in a loose, honeycomb-like structure. While it can support weight when dry, it possesses a dangerous secret: the moment it becomes saturated with water, its fragile internal bonds dissolve, and the ground suddenly collapses, sinking rapidly under the weight of buildings and roads. This phenomenon, known as collapsibility, has caused significant structural damage in the region, where loess deposits can reach depths of over 70 meters. The challenge for engineers is to find a way to strengthen this soil without making it brittle or expensive to treat, ensuring that the ground remains stable even when the rains come.

Researchers from the University of Tehran and the University of Qom set out to solve this problem by testing a common, inexpensive material: cement. Their goal was to see if mixing cement into the Gorgan loess could transform it from a collapsing hazard into a solid foundation. They took samples of the local soil and mixed them with varying amounts of cement, ranging from nothing at all up to 9 percent by weight. To understand how time affects the process, they did not just test the mixtures immediately. Instead, they let the samples sit in sealed containers for periods of seven, fourteen, and twenty-eight days, simulating the curing process that happens in real construction projects. They then subjected these hardened samples to a battery of tests, measuring how much they could be compressed, how much they swelled, and how much force they could withstand before breaking. They also looked closely at the soil under powerful microscopes and analyzed its chemical makeup to see what was happening on a microscopic level.

The results were striking. The untreated loess was extremely weak, with a compressive strength of just 0.1 megapascals, a measure of how much pressure the soil can take before failing. However, as the researchers added more cement and allowed more time for the mixture to cure, the soil became significantly stronger. The most dramatic improvement occurred in the samples treated with 9 percent cement. After twenty-eight days of curing, these samples were 8.5 times stronger than the untreated soil. The researchers observed that the longer the soil was allowed to cure, the stronger it became, indicating that chemical reactions were continuously bonding the soil particles together over time. This increase in strength was not just a matter of the cement acting as a filler; it was the result of a chemical transformation where the cement reacted with the minerals in the soil to create new, durable bonds.

Beyond just getting stronger, the treated soil also became much more stable and less prone to the dangerous sinking that plagues the region. The researchers measured the soil's tendency to collapse when wet and found that adding cement drastically reduced this risk. In the untreated samples, the potential for collapse was classified as moderately severe. But in the samples with 9 percent cement, this risk dropped to a "slight" level, meaning the soil was far less likely to suffer sudden settlement. Similarly, the soil's tendency to compress under pressure or swell when wet was significantly reduced. The more cement that was added, the less the soil moved, creating a material that behaves more like a solid rock than a loose, water-sensitive dirt.

To understand why this transformation happened, the team peered inside the soil using scanning electron microscopy and chemical analysis. The untreated loess revealed a chaotic structure with large, open pores and weak connections between particles, resembling a loosely stacked pile of sand. In contrast, the cement-treated samples showed a completely different picture. The microscopic images revealed that the cement had created needle-like structures and a gel-like substance that filled the empty spaces between the soil grains. These new formations, known as calcium silicate hydrate, acted like a glue, binding the individual particles into a tight, interconnected network. This process, called flocculation, turned the loose, open structure into a dense, compact mass. The chemical analysis confirmed that the amount of calcium and other binding elements had increased, while the loose, dispersive nature of the original soil had been replaced by a solid, cohesive structure.

The study concludes that adding cement is a highly effective method for stabilizing the problematic loess found in northern Iran. By increasing the cement content and allowing sufficient time for the mixture to cure, engineers can create a soil that is not only much stronger but also resistant to the sudden collapse that occurs when water is introduced. The research demonstrates that a simple addition of 9 percent cement can turn a geotechnical hazard into a reliable foundation, offering a practical and accessible solution for construction in areas prone to this specific type of soil failure. The findings suggest that with the right mix and enough time, the ground beneath our feet can be made to hold firm.