Pore Structure Characterization of Sodium Hydroxide Activated Slag Using Mercury Intrusion Porosimetry, Nitrogen Adsorption, and Image Analysis

Abstract

The pore structure of alkali-activated slag has a significant influence on its performance. However, the literature shows insufficient studies regarding the suitability of different techniques for characterizing the pore structure and the influences of Na2O and curing age on pore structure development. In pursuit of a better understanding, the pore structure of sodium hydroxide activated slag paste was characterized by multiple techniques, e.g., mercury intrusion porosimetry (MIP), nitrogen (N2) adsorption, and scanning electron microscopy (SEM) image analysis. The sodium hydroxide activated slag pastes were prepared with three different contents of Na2O (Na2O/slag = 4, 6, and 8%) and cured for different times up to 360 days. The microstructure observation reveals that outer C–(N–)A–S–H and inner C–(N–)A–S–H grow successively around the reacting slag grains, along with crystalline reaction products which are formed in the empty coarse pore space. The increase of Na2O content and curing age lead to a finer pore structure. The MIP measurements show that the total porosity drops about 70% within the first day, and that one peak at most, corresponding to gel pores, was identified in the differential curves of all the investigated samples from 1 to 360 days. On the contrary, only one peak, corresponding to capillary pores, was identified by SEM-image analysis. The differential curves derived from N2 adsorption generally reveal two peaks, and the trend that the pore diameters of those two peaks vary with curing age depends on the content of Na2O. Compared to Portland cement, sodium hydroxide activated slag has a higher pore space filling capacity (χ, Vproducts/Vslag-reacted), while the capacity decreases with increasing Na2O content and curing age.

Pore Structure Characterization of Hardened Cement Paste by Multiple Methods

“…And the peak size of the three curves all concentrates around 30 nm. is result is in agreement with that of previous literature studies [13,26]. Although NS can Advances in Materials Science and Engineering observe gel pores, the PSD curves show no evident concentration in the range of 0-10 nm.…”

“…is is opposite to the results of NS, which implies that the indirect method may induce misunderstanding the pore morphology of hardened cement paste. As presented in previous literature [1,14], four series of porous volumes are observed in FIB/SEM images (Figure 3(e)): air holes, microcracks, large capillary pores, and small capillary pores [8,13]. 3D images show that the porous volume is largely connected by large capillary pores and small capillary pores.…”

“…To date, few researchers have applied this method to observe the 3D porous volume of cement-based materials [1,14]. For finer observation, transmission electron microscopy (TEM) can observe the gel pores with the resolution down to less than 1 nm [1,13]. Previous literature studies show that TEM images provide a similar pore width distribution with the dominant pore size of around 5 nm, which is similar to the results of argon adsorption [15].…”

“…SEM can observe images with resolution down to around 3 nm and thus is applied to characterize the pore morphology of capillary pores in hardened cement paste [12]. Based on the SEM images, one can analyse porosity, PSD, pore shape, and representative elementary area (REA) of porous volume, which helps to understand the material transport in porous volume of hardened cement paste [13]. However, SEM can only scan the sample's surface, while the inner porous volume is still largely unknown.…”

Preliminary Interpretation of the Induction Period in Hydration of Sodium Hydroxide/Silicate Activated Slag

“…The obtained SEM images with a magnification of 500× were used to determine the degree of reaction of slag through image analysis. More details on the image analysis method are provided in [32]. Moreover, the SEM images with a magnification of 1000× were also recorded for more clearly observing the microstructure morphology of alkali-activated slag pastes.…”