Magnesium oxide powder, vulcanizing agent composition for rubber, rubber composition, and production method for magnesium oxide powder

A method for producing a magnesium oxide powder, the method comprising the steps of:

• producing magnesium oxide by firing a magnesium compound at a temperature of 1,200° C. or higher and 2,500° C. or lower;
• and
• obtaining a magnesium oxide powder having Doo is 50 μm or less by carrying out any one or both of pulverization and classification of the magnesium oxide, Doo being a particle size at which a cumulative size based on a volume-based cumulative particle size distribution curve by sieving test is 90%, wherein
• a BET specific surface area of the magnesium oxide powder is 2.0 m2/g or less, and
• a citric acid activity of the magnesium oxide powder is in a range of 500 seconds or more and 2,500 seconds or less, the citric acid activity being an elapsed time measured by a citric acid activity measuring method including steps of:
• adjusting a temperature of a mixed solution to 30° C. ±0.5° C., the mixed solution containing 100 mL of a citric acid aqueous solution of a concentration of 0.13 mol/L, and 2 mL of a phenolphthalein solution of a concentration of 1%;
• adding 2 gm of magnesium oxide powder to the mixed solution after the step of adjusting;
• stirring the mixed solution 10 seconds after the step of adding;
• and obtaining the elapsed time obtaining the elapsed time from the step of adding to a time the mixed solution turns to pink.