The EU Circular Economy (CE) Action plan entails that cross-sectoral waste streams be harnessed as valuable resources. However, real-world evidence of multi-sector circular open innovation (OI) ecosystems is sparse. Instead of technological barriers, this study analyzes the organizational preconditions for circular OI ecosystems in building and construction. Given a prevalence of firm-level studies, it analyzes if multi-sector interaction supports the CE through circular OI from a systems perspective. The paper contributes to the nascent literature at the intersection of OI, innovation ecosystems and the CE from a whole-system angle on circular innovations (614,663 cement patents) and a within-system angle on 100 R&D multi-sector collaborations of 633 partners across all life cycle stages of building and construction. This two-pronged approach allowed for a triangulation of the findings based on two independent and encompassing data sets while offering detail on focus and maturity stage of circular OI. Neither construction firms nor concrete and cement producers are driving the circular transition but energy, materials, and R&D institutes, underlining the instrumentality of multi-sector interaction in establishing a CE. Building and construction firms should enter multi-sector project collaborations as test beds for industrial symbiosis between core producers and these symbiotic carrier sectors. Peripheral but vital materials reuse providers serve as intermediaries supporting building and construction in entering a low-carbon pathway in cement production. The focus of policymakers should lie on further supporting the adoption of short to medium term no-regrets measures which reduce costs through increased energy efficiency, renewable fuels and innovative low-carbon materials.