A calcia clinker having an excellent resistance to slaking and an enhanced modulus of rupture at elevated temperatures is produced by a process comprising the steps of: (1) calcining a lime material at a temperature of 800.degree. to 1300.degree. C. to prepare a calcia; (2) providing an admixture, in the form of fine particles having a specific surface area of 5000 cm.sup.2 /g or more, of the calcia with an additive comprising (A) 0.5 to 3%, in terms of oxide and based on the weight of CaO in the calcia, of at least one inorganic compound of an element selected from the group consisting of titanium, aluminum and silicon and (B) 0.5 to 2.0%, based on the weight of CaO in the calcia, of calcium fluoride; and sintering the above-mentioned compound (A), and; (3) sintering the admixture at a temperature of 1200.degree. to 1650.degree. C.