Cement production causes 7.5% of global anthropogenic CO₂ emissions, arising from limestone decarbonation and fossil-fuel combustion^1,2,3. Current decarbonation strategies include substituting Portland clinker with supplementary materials, but these mainly arise in emitting processes, developing alternative binders but none yet promises scale, or adopting carbon capture and storage that still releases some emissions^4,5,6,7,8. However, used cement is potentially an abundant, decarbonated feedstock. Here we show that recovered cement paste can be reclinkered if used as a partial substitute for the lime–dolomite flux used in steel recycling nowadays. The resulting slag can meet existing specifications for Portland clinker and can be blended effectively with calcined clay and limestone. The process is sensitive to the silica content of the recovered cement paste, and silica and alumina that may come from the scrap, but this can be adjusted easily. We show that the proposed process may be economically competitive, and if powered by emissions-free electricity, can lead to zero emissions cement while also reducing the emissions of steel recycling by reducing lime flux requirements. The global supply of scrap steel for recycling may treble by 2050, and it is likely that more slag can be made per unit of steel recycled. With material efficiency in construction^9,10, future global cement requirements could be met by this route.

水泥生产造成的全球人为 CO ₂排放量占 7.5%，这些排放量源自石灰石脱碳和化石燃料燃烧^1,2,3 。目前的脱碳策略包括用补充材料替代波特兰熟料，但这些主要出现在排放过程中，开发替代粘合剂但尚未实现规模化，或采用仍然释放一些排放的碳捕获和储存^4,5,6,7,8 。然而，废水泥可能是一种丰富的脱碳原料。在这里，我们表明，如果将回收的水泥浆用作当今钢铁回收中使用的石灰-白云石熔剂的部分替代品，则可以重新链接。所得炉渣可满足波特兰熟料的现有规格，并可与煅烧粘土和石灰石有效混合。该过程对回收的水泥浆中的二氧化硅含量以及可能来自废料的二氧化硅和氧化铝很敏感，但这可以很容易地调整。我们表明，所提出的工艺可能具有经济竞争力，并且如果由无排放电力驱动，可以实现零排放水泥，同时还可以通过减少石灰通量需求来减少钢铁回收的排放。到 2050 年，全球用于回收的废钢供应量可能会增加两倍，并且每单位回收的钢可能会产生更多的炉渣。由于建筑中的材料效率^9,10 ，未来的全球水泥需求可以通过这条路线满足。