Using a new partial calcination strategy, which partially decomposes hydromagnesite into magnesium oxide at a lower temperature for a short time. It also explores the potential of preparing cementitious materials by mixing them with calcined clay. The research showed that in addition to MgO, partially calcined hydromagnesite also contained undissolved amorphous magnesium carbonate, which could enhance the rate of MgO hydrated into Mg(OH)2 and significantly improved the early mechanical properties of the cementitious material, but the development rate of strength in the later stage decreased. Compared with completely calcined hydromagnesite, while the content of amorphous magnesium carbonate accounts for 3.0% of the total mass, the 1 day strength was increased by 167%. Partial calcination strategies can effectively reduce carbon emissions and improve the early strength of cementitious materials.