## 4 Discussion  
A cradle-to-gate LCA of replacing sand in concrete with CBA was conducted. Five concrete alternatives, i.e., CBA0, CBA25, CBA50, CBA75, and CBA100, with 0, 25, 50, 75, and 100 wt.% of sand replacement, respectively, were considered. Two mixture design methods, i.e., MIX-fixed-SLUMP (concrete mixtures with a fixed slump range of 60-80 mm) and MIX-fixed-W/C (concrete mixtures with a fixed W/C of 0.52), which resulted in different component qualities of concrete mixtures, were used to environmentally evaluate the concrete production using two levels of ReCiPe2016 methods: (i) The ReCiPe2016 midpoint H method, which considers four of the most significant environmental impacts, namely GWP, TE, FRS, and WC, and (ii) the six methodological options of the ReCiPe2016 single-score method.  
The results showed that the trends of the CBA-based concretes designed with the two concrete mix design methods were controversial. First, the environmental impacts/damages were reduced when sand was replaced with CBA for MIX-fixed-SLUMP concretes. Second, the environmental impacts/damages were increased by replacing sand with CBA for the MIX-fixed-W/C concretes. These controversial trends were confirmed via the ReCiPe2016 midpoint method (Figures 3 and 5) and the six methodological options of the ReCiPe2016 single score method (Figures 4 and 6).  
The results of the MIX-fixed-W/C CBA-based concretes confirmed the results presented by [PERSON] and [PERSON] [14] for CS-based concretes, which were also designed with a fixed W/C. In the CS-based study [14], the maximum recommended sand replacement percentage was 40 wt.%. In this study, we observed the same effect for CBA-based concretes that were designed with a fixed W/C. A maximum sand replacement percentage of 25 wt.% is recommended. However, for CBA-based concretes that were designed with a fixed slump range, a sand replacement rate of 50-100 wt.% with CBA is recommended.  
In contrast to this controversial issue of replacing sand with byproducts, replacing cement with byproducts mainly provides environmental benefits. [PERSON] [31] replaced 30% of cement with GBFS and reported a reduction in the greenhouse gas emissions of 47.5% from that of conventional concrete. Additionally, [PERSON] et al. [32] replaced 66% of cement with GBFS and reported a decrease in the environmental impacts, such as abiotic depletion, acidification, and eutrophication, of 40%-70%. [PERSON] et al. [2] replaced 25% of cement with FA and confirmed that the impacts of respiratory inorganics, global warming, nonrenewable energy, and acidification were approximately 20% lower than those of conventional concrete.  
Such discrepancies in the benefits of replacing sand and cement with byproducts can be explained by the different damage contributions of sand and cement in the total LCA of concrete, which are 0.3%-2% and 74%-93%, respectively [33; 34]. Consequently, owing to the relatively small contribution of sand production to the overall environmental damage caused by concrete production, whether the fixed slump or fixed W/C ratio method of designing concrete is better is a sensitive issue when aiming to elucidate the usefulness or harmfulness of byproduct-based concretes.  
The different byproduct modeling approaches (attributional or consequential) and transportation distances (short or long) are also sensitive issues in the LCA of byproduct-based concretes. Therefore, [PERSON] et al. [12] conducted the consequential modeling of the byproducts and reported the environmental sustainability of replacing sand with foundry sand or steel slag, while [PERSON] et al. [13] conducted attributional modeling for byproducts and reported the environmental harmfulness of replacing sand with CS. [PERSON] et al. [12] also investigated the sensitivity of the contribution of the byproduct transportation distance to the total LCA of byproduct-based concretes, and reported that long distances for the delivery of byproducts (100 km or more) mitigate the beneficial effects of replacing sand with byproducts.