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. e 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%. e 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. e 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.

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“…The MIP analysis of PC is shown in Figure 6. According to Song, 34,35 there are mainly four kinds of pores in the hardened cement paste, namely gel pores (sizes:<10 nm), small capillary pores (sizes: 10–100 nm), large capillary pores (sizes: 100–1000 nm), and air pores (sizes: >1000 nm). It can be seen from the figure that adding WASM to PC will promote the uniform distribution of concrete pores.…”

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“…Thus, the porosity determined by μ-CT analysis is expected to be remarkably underestimated since the spatial resolution does not allow detection of either gel pores or most of the capillary pores, the latter representing up to 70-90% of the whole pore volume of the cement paste [43,44,46,47].…”

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“…The choice of the porosity values here is not at all trivial, since cement paste possesses an inherent porosity. However, at w/c = 0.5, the cement porosity is very small in size, mainly being the porosity of the gel (submicrometric) or the capillary porosity, residing in the size range of micrometers [36,37]. The porosity linked to the presence of the plastic aggregates is much larger than the gel or capillary porosity of cement, and for this reason, the difference in porosity with respect to the reference sample was chosen.…”