This is a particular type of cat litter sold under the brand names Sophisticat and Tesco's Low Dust Lightweight Cat litter (small granules).
Not all cat litters are safe for bonsai as they may be impregnated with harmful chemicals. However, these brands above are widely used in bonsai - see cat litter in the "Growing Mediums" section.
The attached weblink provides info on sources and also some bonsai articles about its use.
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