Application of calcium aluminate cement in construction faces the challenges of high manufacturing cost and volumetric instability associated with hydrates conversion. This work introduces a newly developed high-performance Ca20Al26Mg3Si3O68(Q phase)-metakaolin (MK) composite binder. The influence of MK on the conversion and strength development of Q phase paste cured at 40 degrees C was investigated. The mechanism of MK on the stability of synthetic hydrate was studied by solution chemistry, XRD, and NMR. The pure Q phase paste experiences a significant strength reduction due to hydrates conversion, whereas the Q phase paste containing 15% MK exhibits a continuous increase in strength. MK promotes the formation of CAH(10), contributing to the refinement of pore structure and enhanced mechanical property. The Al(V)and Al(IV)dissolved from MK increase theAl(OH)4-concentration in the pore solution, and then the solubility of CAH(10)decreases due to the common-ion effect, thus inhibiting the subsequent precipitation of C(3)AH(6). In addition, the release of dissolved alumina from MK considerably impedes silica dissolution, and consequently, the formation of C(2)ASH(8)is hindered at a higher content of MK.