The weight of hydroton brand horticultural clay pebbles also known as "Leca", Lightweight Expanded Clay Aggregate weighs 23.67 pounds per cubic foot. 47.34Pounds = 21.4731 Kgs. This should be roughly half the weight of typical round rock gravel of similar size.
If there's a landscape supply near you, you could ask them if they have rock bark (or another lightweight large-size gravel). It's like 10% of the cost of clay balls.