Ammonium Nitrate vs RDX: Understanding the Explosives Behind the Delhi Red Fort Blast

Nine people lost their lives when a car bomb detonated near Delhi's Red Fort on Monday evening.

The Delhi Red Fort explosion involved an unspecified quantity of ANFO, a explosive compound containing ammonium nitrate and fuel oil, loaded into a Hyundai i20. The bomb was activated by a detonator that appears to have been manually triggered, suggesting it was a suicide attack.

Jammu and Kashmir Police recovered approximately 350 kg of ammonium nitrate during Sunday raids on two residences connected to Dr. Mujammil Shakeel, an employee of Al-Falah Hospital in Haryana's Faridabad. Intelligence sources informed NDTV that Shakeel and his associates – Dr. Adil Ahmed Rather and Dr. Shahina Shaheed – were members of a Jaish-e-Mohammed terrorist cell that had infiltrated India.

The Faridabad operations – which apparently prompted a panicked fourth member, Dr. Umar Mohammed, to detonate the Red Fort car bomb – also uncovered more than 2,000 kg of additional explosive materials.

When authorities confirmed the raids Monday morning, early reports indicated that the recovered material was RDX, triggering serious concerns within government and intelligence circles.

However, it was later determined that the substance recovered was ammonium nitrate.

What distinguishes these two explosives?

RDX is a single, high-explosive chemical compound.

Put simply, it's an extremely powerful, compact military-grade explosive first deployed during World War II; British forces used it against German U-boats with reinforced hulls.

The Germans actually discovered the compound earlier, during World War I, but didn't utilize it due to costly production processes. Eventually, they too adopted it; explosive shells fired from their fighter jet weapons contained RDX (which they called hexogen).

Today, it's recognized as a white, odorless organic compound – with chemical formula CH2N2O2 – and serves as the key ingredient in other powerful explosives, including C-4 and Semtex.

RDX is classified as a high-brisance charge, meaning it can shatter and fragment steel, concrete, and other extremely hard structures. This refers to an explosive's detonation velocity – the speed at which it explodes – making the resulting shock waves more devastating.

RDX has one of the highest brisance figures among conventional explosives – up to 8,700 meters per second and a relative effectiveness, or RE, factor of 1.5 to 1.6.

For comparison, TNT, a baseline explosive, has an RE factor of 1.0.

This means RDX is at least 50 percent more powerful than TNT.

The RE of the ANFO used in the Delhi Red Fort car bomb is approximately 0.75.

Understandably, initial reports about recovering 350kg of RDX caused significant alarm.

RDX – or C-4, Semtex, or substances containing RDX – have been employed in terrorist attacks worldwide, including by the Provisional IRA (Irish Republican Army) in Northern Ireland and Britain during the 1980s and 1990s. In that case, the PIRA obtained Semtex from foreign suppliers.

Ammonium nitrate is a salt consisting of one ion each of ammonium and nitrate, formed by the reaction of ammonia (NH3) and nitric acid (HNO3), giving it the formula NH4NO3.

It's an odorless, white crystalline chemical widely used as fertilizer but also a powerful oxidizer that, under appropriate conditions, can cause powerful explosions resulting in high-temperature fires that burn for extended periods.

Importantly, unlike RDX, ammonium nitrate alone is not considered an explosive.

It must be mixed with a secondary substance – in the Red Fort bomb this was fuel oil, a petroleum-based product – and triggered by an external detonation to explode.

In fact, it can be combined with almost any volatile substance. However, quality matters; pure NH4NO3 is chemically and thermally stable, meaning it requires that external detonation. A stronger initial blast typically results in a larger and more stable explosion.

Nevertheless, it remains a lower-yield explosive due to reduced output per kilogram.

Its destructive capability, therefore, comes from sheer volume rather than brisance, which explains why large ANFO-based bombs – such as the one used in Oklahoma City in 1995 – can weigh hundreds or even thousands of kilograms.

Ammonium nitrate is a controlled substance in India; its sale and possession are supposed to be monitored under regulations established in 2012 (and amended in 2021). Consequently, manufacture, storage, and use require licenses from the Petroleum and Explosives Safety Organisation.

Unauthorized individuals cannot legally purchase or possess ANFO.

How the terrorists managed to acquire such substantial quantities of ammonium nitrate and other explosive materials represents one of the most critical questions investigators must address.