基于响应面法的粉煤灰-电石渣基地聚物的砂浆配比优化

Optimization of fly ash-calcium carbide slag⁃based geopolymer mortar ratio using response surface method

【目的】为改善粉煤灰-电石渣基地聚物砂浆室温养护下的力学性能,实现工业固废粉煤灰和电石渣的资源再利用。【方法】利用单因素试验确定电石渣的最优掺量,初步确定粉煤灰-电石渣基地聚物砂浆的最优取值范围,然后以NaOH溶液浓度、液固比、水玻璃与NaOH溶液质量比为自变量因素,以砂浆28 d的抗压强度和抗折强度为响应值,进行响应面法实验,对粉煤灰-电石渣基地聚物砂浆配合比进行优化,并进行微观机制解释。【结果】响应面法能较为准确地优化粉煤灰-电石渣基地聚物砂浆配合比,当粉煤灰和电石渣的质量比为7∶3、NaOH溶液浓度为10 mol/L、液固比(质量比)为0.62、水玻璃与NaOH溶液质量比为2.3时,粉煤灰-电石渣基地聚物砂浆综合性能最优。【结论】在最优配比时,粉煤灰-电石渣基地聚物砂浆的水化产物以水合硅酸钙凝胶、水化硅铝酸钙凝胶为主,并随着固化时间的延长,粉煤灰-电石渣基地聚物体系的微观结构中凝胶含量增加,表现出更高的致密性以及更好的宏观力学性能,粉煤灰-电石渣基地聚物砂浆的综合力学性能及施工性能得以提高。

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-tosolid 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.Conclusion In the paper,the optimal mix ratio of fly ash and calcium carbide slag precursor was initially determined through one-way design tests.Then,by analyzing the effects of varying calcium carbide slag substitution rates on the compressive strength and fluidity of the fly ash-calcium carbide slag-based geopolymer mortar,the optimal mix ratio was 7∶3.Then,utilizing the response surface methodology,a quadratic polynomial regression model was constructed to predict the compressive and flexural strength of the fly ash-calcium carbide slag-based geopolymer mortar at 28 days.The following optimal parameters were identified:a c(NaOH)concentration of 10 mol/L,a liquid-solid ratio of 0.62,and a mass ratio of water glass to c(NaOH)of 2.3.With those parameters,the compressive strength at 28 days was 33.24 MPa and the flexural strength at 28 days was 4.65 MPa,providing a reference to the proportional design of the fly ash-calcium carbide slag-based geopolymer mortar.Finally,microanalysis tests,including XRD,TG-DTG,and SEM analysis,showed that the hydration products of the geopolymer mortar were dominated by C-S-H and C-A-S-H gels at the optimal proportion.With the extension of the curing time,the gel content in the microstructure increased,which exhibited higher densification as well as improved macroscopic mechanical properties.The comprehensive mechanical properties and construction performance of fly ash-calcium carbide slag geopolymer mortar were improved.