Abstract: A flame retardant comprising a mixture of 10 to 90% by weight of powder A and 90 to 10% by weight of powder B of magnesium hydroxide, the powder A containing 40 to 70% by weight of a powdered double salt of magnesium carbonate and calcium carbonate and 60 to 30% by weight of powdered basic magnesium carbonate; and a flame-retardant resin composition comprising 100 parts by weight of a thermoplastic resin and 30 to 300 parts by weight of the flame retardant. Preferably, the double salt is huntite, and the basic magnesium carbonate consists essentially of hydromagnesite. It is preferred to treat the flame retardant, in the form of a powder mixture, with a surface treatment agent. The flame-retardant resin composition is adapted for use as a sheath or insulation for an electric wire or cable, a fire-arresting material, or a wall material.

Abstract: A magnesia-calcia clinker having a high density, said clinker containing MgO, CaO and Fe.sub.2 O.sub.3 as oxides, having the following chemical composition in percent by weightMgO, CaO and Fe.sub.2 O.sub.3 : at least 99% in totalMgO: at least 10%Fe.sub.2 O.sub.3 : 0.2-5%and having a density corresponding to at least 97.5% of the theoretical density. The above magnesia-calcia clinker can be produced by adding a water-soluble iron compound to sea water, bittern or brine, simultaneously with, or after, the addition of the water-soluble iron compound, adding a calcined product of dolomite, lime or a hydrate thereof to form a precipitate composed mainly of magnesium hydroxide, mixing the resulting magnesium hydroxide with a calcium compound, and dead-burning the resulting mixture.