<b> ABSTRACT</b> </br>

The modulus of elasticity of a concrete depends on the elastic stiffness of both the cementitious matrix and the aggregate–ITZ system, which includes any slippage mechanisms under loading between the aggregate and the cementitious matrix within the interfacial transition zone (ITZ). A procedure is presented in this paper to estimate the elastic stiffness of an aggregate–ITZ system within a cementitious matrix, by considering the relative volumes and the porosities of the concrete components. The method was validated by determining the elastic stiffness of both the limestone–ITZ and the electric arc furnace slag (EAFS)–ITZ systems when embedded in a slag-based cementitious matrix. The greater stiffness of the EAFS–ITZ system in comparison with the natural aggregate system explained the higher strength and modulus of elasticity of the concrete following additions of EAFS. Moreover, having determined those parameters, the elastic moduli of concretes with a similar cementitious matrix could then be accurately estimated.

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<b> ACKNOWLEDGEMENTS</b> </br>

Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. The authors wish to express their gratitude for funding this research work to the Spanish Ministry of Universities, MICINN, AEI, EU and ERDF [PID2020-113837RB-I00; PID2021-124203OB-I00; RTI2018-097079-B-C31; 10.13039/501100011033; FPU17/03374]; the Junta de Castilla y León (Regional Government) and ERDF [UIC-231, BU119P17]; the Basque Regional Government through the consolidated research group <b>SAREN</b> [IT1619-22]; Youth Employment Initiative (JCyL) and ESF [UBU05B_1274]; and, finally, the University of Burgos [Y135.GI].