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.
Highlights
When ground granulated blast furnace slag is brought into contact with an alkaline activator, the slag grains start to dissolve and a set of reactions commence, resulting in various solid reaction products
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 Na2 O
It is noted that two-tone layers of reaction products are found surrounding the unreacted slag grains for the sample with longer curing time and higher Na2 O
Summary
When ground granulated blast furnace slag is brought into contact with an alkaline activator, the slag grains start to dissolve and a set of reactions commence, resulting in various solid reaction products. In alkali-activated slag, the reaction products are usually categorized into primary reaction products and secondary reaction products [1]. The primary reaction product is a type of calcium-sodium aluminosilicate hydrates (C–(N–)A–S–H) [2,3]. The secondary reaction products are crystalline phases, such as hydrotalcite [3], tetracalcium aluminate hydrate [4], katoite [5], and stratlingite [6], etc. With continuous reaction of slag, capillary pores are gradually filled by the reaction products. Capillary pores are gradually blocked and the sizes are refined with time. The sizes of capillary pores are in the range from several nanometers (nm) to several micrometers (μm). Similar to calcium silicate hydrates (C–S–H) in Portland cement-based materials, C–(N–)A–S–H in alkali-activated slag is a type
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