## 4 Reduce the Environmental Impacts and Resources Use of Binders  
As mentioned by [PERSON] and [PERSON] [4], reducing the environmental impacts and resource-use of binders are the key drivers for today's technology uptake. The mentioned study made a global review to show the feasibility of using non-conventional materials to reduce cement content and improve the durability of concrete. They show that limestone-calcined clay cement (LC3) is one of the promising binders that can be used in the concrete industry to improve mechanical performance, environmental impacts and cost. The authors then focused on another path to improve the performance of concrete containing LC3. This is due to the fact that concrete with high-performance characteristics leads to long service life, which is the main contributor to sustainability. For that purpose, they made an extensive literature review to see the effect of natural fibers, specifically \"kenaf\", on LC3 concrete, as a step toward green cementitious composite. They concluded that most mechanical (e.g., toughness, tensile strength and impact resistance) and durability (e.g., carbonation, sulphate and chloride resistance) properties can be enhanced compared to ordinary Portland cement concrete, specifically when the natural fiber is about 1% of binder's mass, with a length of about 50 mm. The authors suggested using a combination of LC3 with natural fibers such as kenaf fiber due to the fact that the kenaf plant absorbs a significant amount of CO\({}_{2}\) from the air. [PERSON] et al. [5] showed that cement content can be reduced by decreasing the porosity of the paste matrix. For that purpose, cement was replaced with different components and different grade tailings of waste limestone powder. Then, the specimens were characterized via nuclear magnetic resonance and scanning electron microscopy to understand their microstructure and pore properties. Based on the results, with a given grade tailing of the waste limestone powder, 20% of cement content can be replaced with limestone powder without jeopardizing the mechanical properties. Further, porosity, macropore proportion and the average pore radius decreased by 7%, 46% and 16%, respectively. As mentioned before, the durability of concrete can be an effective key factor to obtain sustainable concrete. Accordingly, [PERSON] et al. [6] studied the influence of the thermo-mechanical activation of bentonite on the mechanical and durability performance of concrete. For that purpose, they studied the mechanical (compressive and tensile strength) and durability (water absorption, sorptivity and acid attack resistance) properties of concrete containing different incorporation ratios (10%, 15%, 20%, 25%, 30% and 35% mass replacements of ordinary Portland cement) of bentonite that was thermally and mechanically activated. Apart from the fact that the environmental impacts of bentonite are way lower than those of ordinary Portland cement, specifically when mechanically activated, both the durability and the mechanical properties of the concrete mixes improved with the incorporation of bentonite in both activation techniques. The results also show that the incorporation ratio of bentonite of 15-25% is the optimum in terms of technical performance of concrete, and specifically for durability. [PERSON] et al. [7] highlighted, through a systematic review of more than 300 peer reviewed articles, that the current methodology for environmental impact assessment of concrete is not conclusive. The reviewed values for the environmental impact of green concrete seemed to spread widely, with a mean quite close to that of ordinary Portland cement concrete mixes.