**Ioanna Papanikolaou \*, Chrysoula Litina, Amir Zomorodian and Abir Al-Tabbaa**

Department of Engineering, University of Cambridge, Trumpington St., Cambridge CB2 1PZ, UK; cl519@cam.ac.uk (C.L.); az412@cam.ac.uk (A.Z.); aa22@cam.ac.uk (A.A.-T.)

**\*** Correspondence: ip324@cam.ac.uk

Received: 24 November 2020; Accepted: 14 December 2020; Published: 21 December 2020

**Abstract:** Cementitious composites are the most widely used construction materials; however, their poor durability necessitates frequent monitoring and repairs. The emergence of self-sensing composites could reduce the need for costly and time-consuming structural inspections. Natural graphite, due to its low cost and wide availability, is a promising additive to generate an electrically conductive network which could ultimately lead to a self-sensing mechanism. Despite several studies using natural graphite as a conductive additive, the effect of its fineness on the cementitious composite's performance has not been explored. This study experimentally investigated the effect of three graphite products of varying fineness on the early age, mechanical, and electrical conductivity performance of cement pastes. The fluidity of the graphite-cement paste reduced significantly with increasing graphite fineness, and graphite did not affect the cement hydration. The finer the graphite, the lower the effect on the mechanical performance, as confirmed by compressive strength testing and micro-indentation. Electrical conductivity testing showed that the percolation threshold depended on the graphite fineness and was found at ~20 wt % for the fine and medium graphite, while it increased to 30–40 wt % for the coarse graphite. This is the first study that has investigated holistically the effect of graphite fineness on the performance of cement pastes and will pave the way for using this material as an additive for self-sensing structures.

**Keywords:** cement; graphite; additives; electrical conductivity; self-sensing
