WANG Jinsong^1a，CHEN Jin^1a，DAN Li^1a，OUYANG Gaoshang²，DU Kejie^1b，YUE Xixiang^1a

Abstract

Objective Fly ash-based geopolymers prepared at room temperature usually exhibit disadvantages such as low early strength，long setting time， and high segregation levels. The study aims to improve the mechanical properties of fly ash-calcium carbide slag-based geopolymer mortar maintained at room temperature， promoting the reuse of industrial solid wastes such as fly ash and calcium carbide slag.

Methods Using the one-factor tests， the study initially fixed the binder-to-sand ratio at 1：3（mass ratio； the following ratios are the same）； the calcium carbide slag replacement rate at 30%； sodium hydroxide solution concentration at 10 mol/L； liquid-to-solid ratio （mass ratio of alkali activation solution to the sum of the fly ash and calcium carbide slag powder） at 0. 64； and the mass ratio of alkali activation solution （mass ratio of water-glass solution to sodium hydroxide solution） at 2. These served as the basic material parameters. The study respectively investigated the effects of different calcium carbide slag replacement rates （0%，10%，20%，30%，40%）， NaOH solution concentrations （4，6，8，10，12 mol/L）， liquid-solid ratios （0. 60，0. 62，0. 64，0. 66，0. 68）， mass ratios of alkali activation solution （1，1. 5，2，2. 5，3） on the mechanical properties of the geopolymer mortar. Then， based on the one-factor tests， the Box-BehnKen model in response surface methodology was used to design a three-factor， three-level test with NaOH solution concentration， liquid-solid ratio， and alkali activation solution mass ratio as independent variables. The compressive and flexural strength of the mortar at 28 days were taken as response values.

Results and Discussion Fig. 1 showed the effects of calcium carbide slag substitution rate on the compressive strength and fluidity of fly ash-based geopolymer at 28 days. With the increase in the replacement rate of calcium carbide slag， the compressive strength initially increased and then decreased. When the dosage of calcium carbide slag increased to 40%， the compressive strength decreased and the fluidity decreased sharply， indicating that the optimal dosage of calcium carbide slag was 30%. Fig. 2 showed the comparison between the predicted and actual values after the response surface optimization design， and it was found that all the points were located near the straight line y=x. The verification test of the optimal mix ratio was carried out， as shown in Tab. 6， and it was found that the absolute value of the relative error was less than 5%. This indicated that the model has high accuracy and provides a valuable reference for the optimization of fly ash-calcium carbide slag mortar proportion parameters. The effects of the interaction of the two factors on the mechanical properties were shown in Figs. 3 and 4， indicating that the interaction of c（NaOH） and liquid-solid ratio was most significant for both 28-day compressive strength and flexural strength of fly ash-calcium carbide slag-based mortar. Figs. 5，6， and 7 showed the mechanistic analysis of the specimens under the optimal mix ratio determined by response surface tests. During the hydration reaction， fly ash and calcium carbide slag consumed Ca（OH）₂ and formed more C-（A）-S-H gels. SEM images of specimens maintained for 7 and 28 days under the optimal mix ratio and determined by response surface tests were shown in Fig. 8. From the figure， it was found that at 7 days， the reaction of fly ash in the gel system was incomplete， and unevenly distributed voids were formed in the fly ash-calcium carbide slag geopolymer matrix. As maintenance period increased， parts of the surface of the fly ash particles were dissolved by the NaOH solution， and the gel particles were crosslinked and fused to form a dense mesh structure. SEM images showed that at 28 days， the fly ash glass particles in the gel system were almost completely wrapped by the flocculent gel， with only a few glass particles partially exposed. This resulted in a denser gel structure， which was conducive to its further enhancement in strength.

DOI：10.13732/j.issn.1008-5548.2024.04.007