Portland cements with varying compositions were prepared to examine for the validity of 3% limitation for MgO in the Japanese Specification for the cement. Limestone, dolomite, shale, quartz-sand and rouge were used as raw materials. The ratio, SiO₂: Al₂O₃: Fe₂O₃, remained same for all clinkers. In Group 1, MgO contents varied from 1.25 to 12.00%, H. M., i. e. CaO/(SiO₂+Al₂O₃+Fe₂O₃), being 2.00 for all 4 clinkers. In Group 2, H. M. was changed from 1.77 to 2.10, MgO content being kept constant at 8.00% for all 3 clinkers. Briquettes made of the raw mixtures were fired in a shaft kiln with retort carbon. The clinkers and the cements were examined for their qualities prescribed in the Specification, strength and expansion on heating in autoclaves, and microstructures. Results are: (1) MgO seems rather to promote the chemical combination of CaO. (2) All cements except one with the lowest H. M. passed the tensile strength test. In Group 1, that containing 8% MgO was as strong as those having less MgO and a commercial cement. In Group 2, the strength varied with H. M., the cement with 2.10 H. M. showing the highest strength among all made for the study. (3) Briquettes cured in water for 4 weeks were heated for 6 or 16 hours at 139-140°C or 3.5 atm. in autoclaves, and then were tested for the strength. Among Group 1, that containing 12% MgO disintegrated. Among Group 2, that with 1.77 H. M. was deteriorated. However, the all others showed marked increases in the strength, the cement with 2.10 H. M. and 8% MgO being still strongest. (4) The expansion test was made with neat cement bars cured 1 day in molds and 1 day in air. They were heated for 20 to 1290 minutes at 139-140° in an autoclave. In Group 1, the expansion increased with the MgO content; That with 12% MgO expanded over 3%, while the linear expansions of the others were under 0.4%. In Group 2, the expansion was rapidly reduced with increase of H. M. (5) No new crystal or periclase was observed in the thin sections. However, cryptocrystalline masses were found in the clinker containing 12% MgO. (6) Conclusions are that the MgO content can safely be allowed up to 8% and also that a part of the MgO reacts probably with Fe₂O₃ at first, the remainder forming a solid solution with 3CaO⋅SiO₂ to a certain limit.

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