Introduction Alkali-activated slag (AAS) cementitious material has attracted much attention due to its high early strength, good corrosion resistance, and low carbon emission. The microstructure of AAS has a decisive influence on its properties, which depends on the quantity and spatial distribution of the reaction products. It is thus of great significance for understanding the formation process of its microstructure to explore the distribution of reaction products in AAS system. In previous studies, the type of activator and the dosage of sodium oxide both have an effect on the microstructure of AAS. However, the underlying mechanism of sodium oxide content affecting the microstructure of AAS system via the product distribution is still unclear. In this paper, the dissolution behavior and product distribution characteristics of slag in sodium hydroxide solution with different concentrations were in-situ observed by a polarizing microscope. The dissolved ions, phase composition, microscopic morphology, distribution characteristics of reaction products, and pore structure of hardened AAS were also analyzed to elucidate the difference of reaction process of slag at different concentrations of sodium hydroxide solution.Methods Sodium hydroxide solutions with molar concentrations of 0.5, 2.0 mol/L and 4.0 mol./L were prepared. Slag was uniformly mixed with sodium hydroxide solution at a solid/liquid ratio of 1/100. 1-2 drops of the mixture were added into a single concave slide by a dropper. The excessive solution was removed with absorbent paper and the edges of the cover slip were sealed with paraffin. The prepared slide sample was placed on the stage of the polarizing microscope and independent slag particles were selected for the coming characterization. The area change rate of slag particles or reaction products was analyzed by a software named Image J. The surface morphology of slag and the distribution of reaction products were characterized by scanning electron microscopy, and the elemental analysis of different morphological substances was determined by energy-dispersive X-ray spectroscopy.After the mixed samples were stayed for 1, 6 h and 48 h, pore solutions were obtained through centrifugation and filtration, and the reaction products in the precipitate were dissolved by a methanol-salicylic acid solution. The ion quantity was determined by inductively coupled plasma optical emission spectroscopy. After the mixture was stayed for 6 h and 48 h, residual solids were obtained after centrifugation. In the hydration of residual solids, the phase composition was analyzed by X-ray diffraction, and the reaction degree was determined by a selective dissolution method.The alkali-activated slag pastes with different sodium oxide contents (i.e., 4%-8%) and a water-solid ratio of 0.4 were prepared. After standard curing for 3 d, the samples were broken, and then the hydration was terminated and dried. The dried samples were placed into the dilatometer, and the pore size distribution was determined by a model Auto pore IV 9500 mercury injection instrument.Results and discussion At a low concentration of sodium hydroxide (sample N1), the ion dissolves slowly because of the slow depolymerization of slag. Since the saturation concentration is not reached, the dissolved ion continues to diffuse outwards, forming a new gel phase zone containing a great amount of water, which makes the initial dissolved precipitate products distributed in a wide range. After 48 h reaction, the area of this region is increased by 257.8%, compared to the initial area of slag. The main reaction products are C-(A)-S-H gel and hydrotalcite. At a high concentration of sodium hydroxide (samples N2 and N3), [SiO4]4- and Ca2+ are dissolved in large quantities due to the rapid depolymerization of slag. Calcium hydroxide crystallizes out because its saturation index is greater than 0. Also, ion concentration rapidly reaches the solubility product (Ksp) of calcium silicate hydrate, forming a layer of dissolve-precipitate reaction products on the surface of slag particle. Therefore, the product distribution range of AAS system with a high concentration of sodium hydroxide is narrow. The area growth rates after 48-h reaction for samples N2 and N3 are 15.6% and 4.0%, respectively. The reaction products mainly consist of C-(A)-S-H gel, hydrotalcite, katoite and portlandite.At the same curing age, AAS with a high sodium oxide content has a higher reaction degree and a lower porosity, compared to AAS with a low sodium oxide content. However, AAS with a high sodium oxide content has more macro-pores due to the difference in the distribution characteristics of reaction products.Conclusions The concentration of sodium hydroxide had an influence on the product distribution in AAS system. For the AAS system with a low concentration of sodium hydroxide, the ions dissolved slowly and spread outward continuously, forming a new gel phase around the slag, and then polycondensation and dehydration with the area increment rate of 257.8%. For the AAS system with a high concentration of sodium hydroxide, the slag quickly dissolved a large number of silicate and calcium ions, resulting in the saturation index of calcium hydroxide of greater than 0 and the formation of portlandite crystals. The distribution range of reaction products was relatively narrow with the area growth rate of less than 15.6% because the surface of the slag particles was quickly covered by reaction products. Although the hardened AAS paste with a high Na2O content had a higher reaction degree and a smaller porosity, compared to AAS with a low Na2O content, the size of pore was larger.