Limestone calcined clay cement (LC3) is a promising material for producing cement with low CO2 emissions and properties similar to, or superior to, those of Portland cement (PC), but at 50 % clinker content (LC3-50). In many applications, lower strengths than PC are acceptable, opening new opportunities to develop suitable LC3 for-mulations with clinker contents below 50 %. In this study, we investigate the microstructural development of low-clinker LC3 and the effect of portlandite (CH) addition on the reaction of metakaolin (MK). In low-clinker systems, the MK reaction is limited by the availability of CH. The addition of CH to the blends effectively enhanced the reaction of metakaolin and promoted the formation of carboaluminates, which significantly improved the compressive strength from 7 to 90 d. Environmental performance indicators showed that all investigated low-clinker LC3 systems had lower embodied CO2 compared to PC while achieving adequate me-chanical properties.