Pore Structure Characterization of Hardened Cement Paste by Multiple Methods

Abstract

Pore structure characterization of hardened cement paste is important to concrete mechanical and transport properties. Hence, we apply indirect methods (NS, MIP, and NMR) and direct methods (XCT, FIB/SEM, and HIM) to provide a general view of the pore structure of hardened cement paste. The results show that the 3D pore network of hardened cement paste is isolated in microscale yet is largely connected by large capillary pores, small capillary pores, and microcracks in nanoscale. In indirect methods, MIP and NMR have a wide measurement range and permit observing most of the porous volume with an average porosity of 8.4–9.9% of hardened cement paste. In contrast, direct methods have a relatively narrow measurement range and thus lead to a large porosity scattering of 1.8–16.4%. The pore size distribution (PSD) curves by indirect methods show that the pore structure is mainly concentrated in three sections of 1–10 nm, 10–100 nm, and around 10 μm, which correspond to the imaging range of HIM, FIB/SEM, and XCT. However, FIB/SEM implies that the most porous part of hardened cement paste (10–100 nm) is underestimated by MIP due to the “ink-bottle” effect. Another “underestimation” of NMR is because C-S-H gels swell during the wetting, and the swelling separates the capillary pores into smaller ones. The microcracks induced by sample preparation contribute 0.1-0.2% (XCT) and 4.5–8.1% (FIB/SEM) of porosity to the porous volume of cement paste, but they have limited influence on the pore size distribution and pore connectivity.