This book gathers peer-reviewed contributions presented at the 3rd RILEM Spring Convention and Conference, held at Guimarães and hosted by the University of Minho, Portugal, on March 9-14, 2020. The theme of the Conference was ¿Ambitioning a Sustainable Future for Built Environment: comprehensive strategies for unprecedented challenges¿, which was aimed at discussing current challenges and impacts of the built environment on sustainability. The present volume is dedicated to the topic ¿New materials and structures for ulträdurability¿, which covers current scientific and technological developments aimed at improving knowledge about degradation mechanisms in construction materials, as well as to the development of new materials with extreme durability. Novel special materials for extreme environments or extreme loading conditions are also included, as well as novel approaches to improve the performance and durability of currently common construction materials. The following subtopics are included: general purpose, constructions, infrastructures and facilities, extreme environments and extreme events, transport and deterioration mechanisms, characterization and mitigation, Supplementary Cementitious Materials, admixtures, additions and other emerging material optimization strategies, smart materials for durable structures.