In this experimental work, mechanical and microstructural properties of calcined diatomite powder (CDP) modified high strength mortars (HSMs) at ambient and high temperatures were researched. In the HSM mixtures, Portland cement was replaced with CDP at ratios of 0%, 5%, 10%, 15% and 20%. Several experiments regarding the hardened properties were carried out at 14, 28, 56 and 90 days at the ambient temperature of 25 degrees C. HSMs were heated up to high temperatures of 400 degrees C to 1000 degrees C at the age of 56 days, separately. Then, high temperature resistances of HSMs were quantified in terms of the residual hardened properties and the changes in the microstructural properties after heating. The residual performance was found to be higher in the HSMs modified with CDP than in the reference HSM (R-HSM) which was made 0% CDP. Based on the test results, it was seen that the HSM modified with 15% CDP (15CDP-HSM) exhibited the highest mechanical properties at ambient and high temperatures. (C) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.