## 6 Assessment of Carbon Neutrality Performance in Production Stage
### Derivation of Major Construction Materials Based on Contribution to Cumulative CE  
To assist the decision-making process of construction stakeholders regarding building materials, the contribution of major materials to CE was analyzed by identifying products having a cumulative A1-A3 CE of 99% or higher. Major construction materials with a cumulative A1-A3 CE(CO2e) of 99% or higher for both apartment houses and business facilities include ready-mixed concrete, steel rebar, cement, and masonry. In contrast, insulation, aggregates, stones, and gypsum boards were considered major materials based on their cumulative weight (ton) contribution of 99% or higher when accounting for all input quantities in the production stage. However, they are excluded from the list of major materials in terms of A1-A3 CE due to the relatively low quantity used in the building or minimal environmental impact.  
Ready-mixed concrete was identified as the most critical material, contributing over 70% to A1-A3 CE, as its input weight accounted for more than 85% of all major materials. Rebar was identified as the second most significant material, contributing around 10% to A1-A3 CE in apartment houses and approximately 17% in business facilities. Cement was identified as the third most significant material, contributing around 9% in apartment houses and approximately 7% in business facilities. Masonry contributed around 4% in apartment houses and 3% in business facilities.  
The A1-A3 CR and A1-A3 ZCBI were analyzed based on the lowest, average, and highest A1-A3 CE values of EPD-certified ready-mixed concrete, steel, cement, and masonry, which collectively contributed 99% or more to the cumulative A1-A3 CE. For ready-mixed concrete, the A1-A3 CR ranged from 3.00 \(\times\) 10\({}^{1}\) (kgCO2e/m\({}^{2}\)) to 2.97 \(\times\) 10\({}^{2}\) (kgCO2e/m\({}^{2}\)), whereas the A1-A3 ZCBI ranged from 6.21% to 67.16%. This material accounts for 76.95% to 96.94% of A1-A3 CR for major construction materials, highlighting that utilizing low-carbon ready-mixed concrete is crucial for achieving A1-A3 CR and A1-A3 ZCBI.  
For rebar, Hyundai Steel Co., Ltd. products are the sole option available in Korea, and the A1-A3 CE of EPD-certified products was found to be 1 (kgCO2e/m\({}^{2}\)) higher than the value in Korea's national LCI DB. Given that rebar accounts for over 10% of total A1-A3 CE as the second-largest contributor after ready-mixed concrete, the development of low-carbon technologies and diversification of certified rebar products are essential for achieving significant CR across various building types. Consequently, the A1-A3 ZCBI could potentially increase by more than 5%.  
For cement, the A1-A3 CR ranged from 2.13 \(\times\) 10\({}^{0}\) (kgCO2e/m\({}^{2}\)) to 1.08 \(\times\) 10\({}^{1}\) (kgCO2e/m\({}^{2}\)), whereas the A1-A3 ZCBI ranged from 0.45% to 2.25%. Cement is also important as it accounts for at least 7% of A1-A3 CE, but only seven EPD-certified products are available. Therefore, the development of low-carbon cement and the diversification of available products is needed.  
Owing to the absence of concrete bricks, which have a relatively high material input, in the EPD DB, autoclaved lightweight concrete (ALC) blocks were analyzed instead. ALC blocks had an A1-A3 CR ranging from 2.51 \(\times\) 10\({}^{0}\) (kgCO2e/m\({}^{2}\)) to 3.06 \(\times\) 10\({}^{0}\) (kgCO2e/m\({}^{2}\)), and an A1-A3 ZCBI ranging from 0.53% to 0.59%. For masonry, achieving A1-A3 CR will require promoting a variety of EPD-certified products, including both concrete bricks and ALC blocks. Table 11 presents the analysis results of the cumulative A1-A3 CE contribution of major construction materials in terms of carbon neutrality performance in the production stage.