NHL 2 - Feebly hydraulic: slow setting, highly breathable (contains 25% marl and 75% limestone)
NHL 3.5 - Moderately hydraulic: general-purpose lime (contains 50% marl and 50% limestone)
NHL 5 - Strongly hydraulic: fast setting, more durable and weather resistant (contains 75% marl and 25% limestone)
The clay content (present in the marl rock) is what gives NHL its hydraulic properties — the higher the clay, the stronger and faster the set in the presence of water.