Aluminum incorporation lowers the intrinsic mechanical properties of single-chain calcium silicate hydrate

In blended cement systems, the principal hydration product is calcium (alumino) silicate hydrate (C-(A-)S-H). The investigation into how aluminum incorporation affects its mechanical properties is essential for optimizing the formulation and properties of cementitious materials. This study employs high-pressure X-ray diffraction to explore the intrinsic mechanical properties of nanocrystalline single-chain C-A-S-H (Ca-to-Si ratio 1.6), resembling chemical compositions of C-A-S-H in blended cement systems. Increasing aluminum incorporation softens all unit cell axes, with basal planar incompressibility governed by the aluminum coordination number, showing an increasing order of Al(IV) < Al(VI) < Al(V). The softening of the basal plane arises from the longer Al O bond length relative to Si O bond, and the different distortion indexes in AlO x polyhedra. The hydrogen bond numbers and angles relative to the basal plane within per unit cell influence the basal planar incompressibility of single-chain C-A-S-H. Aluminum-uptake-induced expansion in basal space contributes to a softened c -axis.