Polycrystalline single-phase bulk hydrogrossulars (Ca₃Al₂(SiO₄)_3−x(OH)_4x (0 ≤ x ≤ 3)) of various compositions were synthesized for the first time utilizing hydrothermal treatment at 200 °C of the phyllosilicate strätlingite (Ca₂Al₂SiO₂(OH)₁₀·2.25H₂O) in its mother liquor. The reproducibility of previously reported synthetic methods for both Si-free katoite and hydrogrossulars was evaluated. Analysis of the thermal behavior of hydrogrossulars based on TG-DTA and variable-temperature XRPD showed a significant increase in thermal stability upon silicon incorporation due to the reinforcement of the lattice with SiO₄ tetrahedra. By tracking changes in the lattice parameters upon heating, we showed that the linear thermal expansion coefficients decrease with higher degrees of Si substitution (17.19 × 10⁻⁶ K⁻¹ and 14.20 × 10⁻⁶ K⁻¹ for Ca₃Al₂(OH)₁₂ and Ca₃Al₂(SiO₄)_1.28(2)(OH)_6.88(8), respectively). Temperature-induced partial dehydration of the SiO₄-stabilized lattice results in the formation of tetrahedral Al sites as defects due to the removal of two oxygen atoms from the initial octahedral environment. Near-complete dehydration (450–850 °C) leads to the loss of the long-range structure, which later on crystallizes at 900 °C as a mixture of different minerals, e.g. calcio-olivine (γ-Ca₂SiO₄), gehlenite (Ca₂Al₂SiO₇), krotite (CaAlO₂), or mayenite (Ca₁₂Al₁₄O₃₃).