<p>The ecotoxicological evaluation of construction and demolition waste (CDW) is essential for its sustainable reuse, but assessment is challenged by the characteristically high pH of its leachates. This study performed a comparative evaluation of three plant-based bioassays (<i>Lemna minor</i>, <i>Desmodesmus subspicatus</i>, <i>Sinapis alba</i>) to identify the most robust method for assessing the phytotoxicity of seven distinct CDW leachates under environmentally relevant, unadjusted pH conditions. Furthermore, a validation experiment with model compounds (CaCl₂, Ca(OH)₂, NaOH) was conducted to unequivocally isolate the primary drivers of the observed growth inhibition. The <i>L. minor</i> bioassay demonstrated superior performance, showing clear, dose-dependent inhibition where the algal and mustard seed assays showed limited applicability due to pH tolerance or methodological constraints. Chemical analysis and validation experiments confirmed that the severe phytotoxicity of fresh CDW leachates is driven primarily by high alkalinity (pH &gt; 11) and the associated buffering capacity, rather than by leached trace elements or specific ion phytotoxicity. High calcium concentrations at neutral pH induced only negligible, sublethal stress. We conclude that the <i>L. minor</i> bioassay is a sensitive and discriminating tool for this challenging matrix. Our results demonstrate that high, stable pH is the primary factor of phytotoxicity in CDW leachates, and that calcium concentration serves as a reliable indicator for this hazard. Testing unadjusted leachates provides an environmentally relevant approach for guiding the safe and sustainable management of CDW.</p> Graphical Abstract <p></p>