## 4 Conclusions  
This study allows a better understanding of the mechanical and cost characteristics of mixes made with high amounts of FA and RCA, both with and without SP. The following conclusions can be drawn from this study:  
* **Mechanical characteristics:** both FA and RCA are detrimental to the mechanical properties of concrete, namely compressive strength, splitting tensile strength, and modulus of elasticity. The compressive strength and splitting tensile strength mostly decrease with the incorporation of FA, and the modulus of elasticity is mostly affected by the use of RCA. After a given age, the strength development for all the mentioned properties increased over time, for mixes containing both FA and RCA, relative to OPC only mixes. The strength development of the mixes containing FA and RCA further accelerated with the use of SP. Thus, the simultaneous use of FA and RCA in concrete also containing SP is highly advisable;
* **Economic assessment of the concrete mixes:** the cost of the concrete mixes slightly decreased with the incorporation of RCA. On the contrary, the use of a small percentage of SP significantly increased the cost. However, by incorporating either 60% of FA in NA concrete or 30% of FA in RCA concrete, the high cost of concrete with SP can be offset. In addition, by incorporating RCA in FA concrete, even when SP is used, the cost is lower than that of reference concrete. However, the conclusions are not to be generalized because the results may be different for other approaches or assumed scenarios in life cycle inventory modelling, as well as other regions than Portugal;
* **Optimization.** The optimum mixes regarding economy (cost-efficient) are not necessarily those with the lowest cost. Currently, the mechanical characteristics of mixes are also an essential factor to aid decision on the optimum concrete. The cost of concrete mixes significantly increases with the use of SP. However, all the mixes (regardless of the type of binder and aggregates) with SP are considered as suitable solutions (located in the cost-efficient zone, i.e., more cost-efficient that the reference mix) except when 100% of fine RCA is used individually or jointly with 100% of coarse RCA. A low volume of FA is highly advisable in terms of mechanical behaviour and  
Figure 10: Compressive strength versus “MPR–cost ratio” of the concrete mixes.  
cost. Although a high volume of FA significantly decreases the mechanical characteristics of concrete, those mixes are still considered suitable (located in the cost-efficient zone) when SP is used, due to their significant decrease in cost. Concrete mixes with coarse RCA can be considered as cost-efficient. However, the suitability of fine RCA concrete regarding the cost and mechanical characteristics depends on its incorporation ratio. In fact, it is not advisable to use high volumes of fine RCA without FA in concrete, because of the high loss in mechanical performance. Finally, the results show that the optimum mixes in terms of cost and mechanical characteristics are mostly the ones made with simultaneous incorporation of RCA, FA, and SP, rather than with their individual incorporation.  
R.K., [PERSON], and [PERSON] conceived and designed the experiments; R.K. analyzed the data; R.K. wrote the paper and [PERSON] and [PERSON] revised it.  
This research received no external funding.  
The authors acknowledge the support of CERIS unit from IST-University of Lisbon and the Foundation for Science and Technology of Portugal.  
The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.