Abstract: The present weed reducing material reduces the necessary weed control work by making joint filling sand a hostile substrate for plant and fungus growth by using an environmentally acceptable slow release mineral additive. The additive, having similar grain size distribution, mechanical and rheological properties, forms an integral part of the joint filling sand. The specific gravity of the additive is lower than quartz sand imparting a tendency for the additive to migrate towards the top part of the joint. The additive leads to pore waters rich in sodium and at high pH, both qualities being maintained over long periods. The mixture of additive and sand is handled and applied using conventional laying techniques and equipment, observing usual precautions for mortar or cement mixes with sand. In the environment, reaction with CO2 in the air or in the soil porosity inactivates the causticity, yielding harmless carbonate salts.