Material-Level Shrinkage and Tensile-Performance Screening of High-Strength Polyethylene-Fibre Engineered Cementitious Composite (PE-ECC) Mortar: MgO Versus Liquid Shrinkage-Reducing Admixture

High-strength polyethylene-fibre engineered cementitious composites (PE-ECCs) provide tensile strain hardening and distributed cracking, but their high paste content and low water-to-binder ratio produce substantial shrinkage. This study conducted a multi-criteria screening of an MgO expansive agent and a shrinkage-reducing admixture (SRA) in a PE-ECC repair mortar for hydraulic-concrete applications. The reference matrix contained 65% P·II 52.5 cement, 10% silica fume, 25% blast-furnace slag, a water-to-binder ratio of 0.27, and 2.5 vol.% PE fibre. Drying shrinkage, compressive and flexural strengths, tensile response, crack number, and strain-based crack-opening estimate were assessed. The reference mortar reached 90.1 MPa compressive strength and 6.6% tensile strain but developed 1489 με of 28 d shrinkage. 2% MgO reduced the 28 d shrinkage to 1384 με, whereas higher dosages shifted from contraction to net expansion and reduced tensile performance. The liquid SRA reduced 28 d shrinkage to 982, 930, and 874 με at 1%, 2%, and 3%, respectively. The 1% mixture retained 82.3 MPa compressive strength, 5.7 MPa tensile strength, 4.4% tensile strain, and a crack-opening estimate of 66.4 μm. Under the adopted criteria, 1% SRA provided the most balanced combination of dimensional stability, strength, and tensile ductility, making it a candidate for application-specific evaluation in hydraulic-concrete repair.