Effects of a Water-Glass Module on Compressive Strength, Size Effect and Stress–Strain Behavior of Geopolymer Recycled Aggregate Concrete

Abstract

Geopolymer recycled aggregate concrete (GRAC) was prepared by replacing cement with geopolymer and natural aggregate with waste concrete. The effect of the water-glass module on the mechanical properties of GRAC was studied. It was found that water-glass has a double-layer structure. The low module water-glass leads to a thicker diffusion layer and more Na+ and OH− in the solution, which activates more CaO, SiO2, and Al2O3 in the raw material, and improves the strength of GRAC. Moreover, two kinds of gel structures, namely layered C-A-S-H (calcium silicate hydrate) and networked N-A-S-H (zeolite), were found in the products of geopolymer. As the water-glass module changed, the phase of zeolite changed significantly, whereas the calcium silicate hydrate did not change, indicating that the decrease in the water-glass module contributes to the formation of more N-A-S-H gel. The compressive strengths of GRAC with the sizes of 200, 150, and 100 mm3 were in line with Bazant’s size effect theoretical curve. Through the segmented fitting method, the relationship of the size conversion coefficient of GRAC (α), the critical strength (fcr), the critical dimension (Dcr), and the water-glass module (ε) were determined. It was found that ε = 1.5 is the segmented point of the three equations. The elastic modulus and peak stress of GRAC are inversely proportional to the water-glass module, and the peak strain is proportional to the water-glass module, indicating that by reducing the water-glass module, the strength of GRAC can be improved, but the brittleness is increased. The constitutive equation of GRAC with only the water-glass module as a variable was also established. It was found that the polynomial mathematical model and rational fraction mathematical model are optimal for the rising-stage and falling-stage, respectively, and the relationship between the parameters of the rising-stage (a) and the falling-stage (b), and the water-glass module, is given.

Time-Varying Pattern and Prediction Model for Geopolymer Mortar Performance under Seawater Immersion

“…All of their studies all indicated that the lower modulus of the sodium silicate solution was more favorable to obtain higher strength. The reason for this analysis may be related, on the one hand, to the bilayer structure of the waterglass particle ( Figure 7 ) [ 36 ], where a change in the modulus causes a change in the bilayer. The bilayer structure of the waterglass particle is an amorphous (SiO 2 ) m as the cores, the outer layer of the cores adsorbs a large amount of negatively valent H 3 SiO 4 − , the negatively valent H 3 SiO 4 − and part of the Na + are distributed within the dense layer, and another part of Na + is free in the diffusion layer, which finally forms the bilayer.…”

Mechanical Performance Optimization and Microstructural Mechanism Study of Alkali-Activated Steel Slag–Slag Cementitious Materials

“…Water glass exhibits a double-layered structure comprising an amorphous mSiO 2 core, as depicted in Figure 5 [27]. The surface of the core is characterized by the adsorption of H 3 SiO 4− ions, with a certain amount of Na + being adsorbed in the compact layer and another portion distributed within the diffusion layer.…”

Mechanical Properties of Geo Polymer Concrete (GPC) by Using Steel, Polypropylene and Glass Fibers

“…This enhancement, in turn, bolsters the strength of GRAC. Geopolymer products display two distinct gel structures, namely N-A-S-H and C-A-S-H. Reducing the water-glass module is linked to an increased formation of N-A-S-H gel, contributing to this shift in gel composition (15,16) .…”