## 1 Introduction
It has been found that in ternary cements containing PC, FA and LS, LS played a dual role; LS (i) contributed as a filler and (ii) participated in chemical reactions with the aluminate hydrates from the PC hydration [12; 17]. In addition, following a study of eight different combinations of FA and LS content combined with PC, this team of researchers concluded that the consumption of Ca(OH)\({}_{2}\) was more pronounced for the 30%FA 5%LS and 65%PC blend rather than the 35%FA and 65%PC blend and was related to the carboaluminates formation and consequently LS appears to interfere with pozzolanic reactions. Consequently, with respect to compressive and flexural strength they inferred that although a 5% replacement of cement by LS reduced the strength of the formulation, a 5% addition of LS to 30%FA and 65%PC blend, will not cause strength reduction. Furthermore, several research papers have reported a number of benefits of the combined effect of FA and \(\upmu\)S on Portland cement blends such as increased early age compressive strength and slump of concretes [18]. Moreover, the C/S ratio decreased with increasing \(\upmu\)S content, which effectively caused an increase in compressive strength. Lastly, the maximum strength was obtained for the PC50 FA40\(\upmu\)S10 formulation. It is interesting to note that although very few studies have been reported on ternary cement blends, no studies have been found on quaternary cement blends containing all the above-mentioned elements, that is to say PC, FA, LS and \(\upmu\)S. In fact, recently published reviews recognize the need for research on ternary and quaternary cementitious composites and on optimization of cement binders containing various pozzolanic materials at different proportions [19]. This gap is bridged by the current research. In fact a step by step methodology for the development of cementitious matrices is presented, based on strength and sustainability criteria. In greater detail, the following items were pursued in this study:* Low carbon footprint binary and ternary cementitious matrices (Figure 1) suitable for further modifications with the addition of silica fume were identified
* Best performing cementitious matrices were selected in terms of embodied energy and compressive strength
* Divergent results on the effect of the addition of limestone in composite cement binders were clarified
* The pozzolanic reactivity in ternary and quaternary cement paste formulations was investigated
* The combined effect of highly pozzolanic constituents in low PC content formulations was assessed
* The current knowledge on cement binder optimization was broadened. In greater detail, apart from the industrial cement CEM II/A-L42.5 conforming to BS EN 197-1 [20] which was studied on its own, given the name B1 in the binary combinations (Figure 1) three more binary limestone--cement combinations were created with reduced Portland cement (PC) content in a gradual manner (B2 contained 80%PC, B3 contained 70%PC and B10 contained 60%PC. At the same time, ternary blended cements were studied at various fly ash contents starting from 20%. Then a 35% FA content formulation was studied in two different pastes (B5 and B8). The specific amount is the upper limit according to ACI [2] for normal other than mass concretes in binary combinations and with respect to the European Standards used in this research, according to BS EN 197-1 [20], the Portland cement (PC) clinker content should not be less than 65% in pozzolanic blends containing limestone. Therefore, the ternary ones pose a significant challenge due to the further reduction of PC content. Next, the performance-based concept was adopted, using as key indicators the 28 day compressive strength of the ten formulations, the hydration reactivity at 7 and 28 days of curing and the embodied CO\({}_{2}\), as a result of the reduction of PC content and increase in the limestone and fly ash content.