## 4 Conclusions  
The present paper is the last part of an extensive research program carried out at the University of Bath, aimed at investigating the effect of nanoparticles in sustainable blended cements. Although a significant number of papers have been published on the effect of various nanoparticles on low carbon footprint composite cementitious formulations [5, 8, 10, 25, 27, 28, 29, 30, 40, 42, 44, 45, 46, 47], the starting pastes and the methodology of selecting formulation on performance based criteria, has never been discussed before. The paper has introduced, analyzed, compared and discussed, a suite of binary (PC + LS) and ternary (PC + FA + LS) cementitious matrices. Three ternary pastes were selected which were further optimized with the addition of silica fume. Competing constituents and areas of improvements to pave the way for the addition of the nanoparticles were identified. Moreover, the pozzolanic reactivity in ternary and quaternary cement paste formulations was investigated, while the lowest possible Portland cement content was employed. In addition to the above, a number of further conclusions were made:  
* Current research can expand on challenging formulations, which allow for significant reduction in CO2 emissions, beyond allowable limits by current codes. * It was demonstrated that optimizing blended or composite Portland cement pastes at high PC replacement levels is an iterative process. This step by step methodology presented in the paper can serve as a basis for other researchers in the field. * In areas where pozzolanic materials are scarce, binary formulations can be developed as the control mix, further enhanced by highly reactive materials, such as nanoparticles of SiO2 or montmorillonite [10, 29, 44, 47]. * In areas where fly ash is abundant and needs to be valorized, ternary blends can be developed and again can be further enhanced with nanomaterials [22, 25, 42]. Similarly, quinary cementitious matrices can be developed with both micro- and nanosized silica [42] for superior strength and denser microstructure. * It is essential that material characterization is thorough and extensive testing is necessary. * The complicated interaction between the constituents was investigated with respect to strength development, changes in the hydration reaction and microstructural enhancement. Lastly, adding to the future research suggestions, apart from the addition of different types of nanoparticles for strength and durability improvements, which has been previously mentioned, matrices containing different types of supplementary cementitious materials can be generated. Some options could include ground granulated blast furnace slag or geopolymers altering water demand, strength and microstructural characteristics. This research has rendered the surpassing of the boundaries defined by the current codes of practice, possible and therefore, further optimizations of blended pastes can be expected in the near future. Conceptualization, S.P. and K.P. ; methodology, S.P. ; validation, S.P. and K.P. ; formal analysis, S.P. investigation, S.P. ; data curation, S.P. ; writing--original draft preparation, S.P. ; writing--review and editing, S.P. and K.P. ; funding acquisition, K.P. Both authors have read and agreed to the published version of the manuscript. All authors have read and agreed to the published version of the manuscript. This research was funded by European Commission funding, FIBCEM project, grant Number 262954. The authors acknowledge the European Commission funding and all partners are thanked for their input and for the supply of materials. The authors would also like to acknowledge the Department of Chemical Engineering at the University of Bath for the use of the TG analyzer. The strength over embodied CO2 calculations of pastes (shown in the respective columns in Tables 6 and 8) were carried out by Dr [PERSON] and her help is greatly appreciated.