Glass microsphere-tailored lightweight Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC): Hollow-heterogeneity design for crack saturation and fiber bridging

Advanced cementitious composites for lightweight structural systems exposed to demanding or extreme environments require high strength-to-weight efficiency, stable damage distribution, and reliable crack-width control. However, reducing density in Ultra-High-Strength Engineered Cementitious Composites (UHS-ECC) often disrupts the matrix–flaw–fiber balance that governs strain-hardening and multiple cracking. This study introduces glass microspheres (GMS) as engineered hollow heterogeneities to develop lightweight UHS-ECC with controlled crack saturation and preserved fiber bridging. The developed composites exhibited densities of 1481–1816 kg/m 3 , compressive strengths of 97.8–150.1 MPa, tensile strengths of 10.77–17.55 MPa, and tensile strain capacities of 3.59–7.38%, extending the performance envelope of lightweight ECC. Increasing GMS volume significantly reduced density, whereas tensile strength and tensile strain capacity initially increased and then decreased. Hydration heat and phase characterization results showed that GMS incorporation did not suppress bulk hydration of the reactive matrix. Pore structure and micromechanical analyses further revealed that 30 vol% GMS volume produced the most favorable balance among hollow heterogeneity, crack saturation, and fiber bridging, leading to the highest strain-hardening indices and the slowest crack-width growth. These findings establish a micromechanics-based route for designing lightweight UHS-ECC with high strength-to-weight efficiency.