COMPOSITES Microstructure–toughness relationships in calcium aluminate cement–polymer composites using instrumented scratch testing Kevin Anderson 1 and Ange-Therese Akono 1,2, * 1 Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 N. Mathews Avenue, Urbana, IL 61801, USA 2 Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 205 N. Mathews Avenue, Urbana, IL 61801, USA Received: 23 February 2017 Accepted: 19 July 2017 Ó Springer Science+Business Media, LLC 2017 ABSTRACT We investigate the influence of the microstructure on the fracture properties of calcium aluminate cement/polymer composites. We carry out microscopic scratch tests during which a Rockwell C diamond probe pushes across the surface of a polished specimen under a linearly increasing vertical force. We extend the scratch fracture method to heterogeneous materials. The scratch test induces a ductile-to-brittle transition as the penetration depth increases. Scan- ning electron microscopy imaging shows that the low porosity and the strong cement-binder interphase favor toughening mechanisms such as crack trapping and bridging. Nonlinear fracture mechanics theory yields the fracture toughness in the fracture-driven regime. The fracture toughness of macro-defect-free (MDF) cement is found to decrease as the polymer-to-cement ratio increases. This decrease in the fracture resistance can be explained by the decrease in anhydrous cement content and the increase in the inter-particle distance between cement grains. By evaluating the fracture toughness of the micro- constituents of MDF cement, we show that the high value of the fracture toughness at the composite level stems from tough calcium aluminate phases and a highly packed non-porous granular microstructure. Introduction Calcium aluminate cements (CAC) are non-Portland cements with outstanding properties such as rapid hardening, high thermal stability, high chemical inertia, or high abrasion and wear resistance. They are relevant in many applications including cryo- genic facilities, foundries, industrial floors, fireplaces and orthopedic cements [1–4]. We seek to elucidate the microstructure–toughness relationships in cal- cium aluminate cement/polymer composites using macro-defect-free cement as a surrogate material. Address correspondence to E-mail: aakono@illinois.edu DOI 10.1007/s10853-017-1416-8 J Mater Sci Composites