Fire-retardant powder comprising at least 30% by weight of monoammonium dihydrogen phosphate and/ or di-ammonium monohydrogen phosphate, and at least 5% by weight of alkaline bicarbonate, and at least 3% by weight of silica, and at least 5% by weight of a compound selected from the group consisting of: sodium chloride, potassium chloride, potassium bromide, potassium sulphate, magnesium carbonate hydroxide pentahydrate, magnesium chloride hexahydrate, iron(II) sulphate heptahydrate, zinc(II) chloride, and combinations thereof. The invention also relates to building materials preferably comprising natural fibres and comprising at least 5% by weight, and at most 30% of a powder according to the invention.
