Research has shown that alkali-activated concrete (AAC) is comparable to ordinary Portland cement concrete (OPCC) in terms of mechanical properties and may offer ecological benefits compared to OPCC. This study evaluates the energy and emission of AAC and OPCC across different classifications of concrete compressive strength (40, 60, and 100 MPa). Analysis indicates that the selection of constituent materials can substantially affect the energy and emission of AAC and OPCC. Ordinary Portland cement (OPC) is the principal contributor to the energy and emission of OPCC, accounting for 80% of energy and 91% of emissions of OPCC. The activating solution, meanwhile, is the main contributor to the energy and CO 2 emission of AAC. Normal strength AAC (40 MPa) shows 46% less energy and 73% less CO 2 emission than OPCC. However, high-strength AAC (60 MPa), using metakaolin as a base material, experiences higher energy (8%) than OPCC yet the emission is 40% less than OPCC. A substitution of fly ash for metakaolin results in superior efficiency of AAC compared to OPCC. Two mixtures of ultra-high-strength AAC (100 MPa) result in contradictory findings. One mixture with a sodium hydroxide and silica fume activating solution shows 5% and 30% less energy and CO 2 emission, while the other mixture with a sodium hydroxide and sodium silicate activating mixture is less efficient than OPCC. • AAC of grade 40 MPa shows 46% and 73% less energy and CO 2 emission than OPCC counterpart.- • AAC of grade 60 MPa experiences 8% higher energy than OPCC yet the emission is 40% less. • Attention should be paid to activating solution ingredients to control energy and CO 2 emission. • Sodium hydroxide and sodium silicate concentrations have a fatal effect on embodied energy and CO 2 emissions of AAC.