Collaborative green and low-carbon transformation in the construction and building materials is an important path to realize China’s carbon peaking and carbon neutrality goals. This paper reviews two types of assessment methods on greenhouse gas (GHG) emission reduction from the life cycle perspective, applicable to low-carbon building materials. Taking the partial substitution of limestone with calcium carbide sludge in the production of cement clinker as a typical case, with the production of Portland cement clinker as the baseline scenario, the two assessment methods are applied to quantify the emission reduction of low-carbon cement clinker, compared with the baseline. A comparison was made on the differences between the two emission reduction assessment methods. The results show that the emission reduction amount is 293.26 and 253.77 kgCO2eq/t respectively, using two assessment methods. The key influencing factors on the differences include the accounting boundary, the calculation method for industrial process emissions, and the emission factors.