Optimization of Mechanical Properties of Geopolymer-Stabilized Soil Using Central Composite Design

The aim of this work is to develop a geopolymer-stabilized soil for civil engineering, especially in applications like curbs and crash barriers. Firstly, a central composite design was used to study the impact of the input factors on both compressive and flexural strengths. The input factors were the molar ratios SiO2/Al2O3 (X), Na2O/Al2O3 (Y) and H2O/Na2O (Z), which varied from 3 to 4, 0.9 to 1.1 and 20 to 26, respectively, while metakaolin/soil ratio remained constant at 0.33. Thanks to central composite design method, fifteen experiments were conducted to establish a second-order empirical model, linking the response function (mechanical strength) to the input factors (X, Y, Z) and their interactions. Secondly, the mass ratio of metakaolin to soil was decreased from 0.33 to 0.16, in order to minimize the environmental impact of the material. The physico-chemical and mechanical properties of mix-designs were investigated. The results revealed that the molar ratio H2O/Na2O (Z) was the most significant parameter, closely followed by SiO2/Al2O3 (X) and Na2O/Al2O3 (Y). Compressive strength values range from 3.4 to 23.0 MPa, while flexural strength values vary from 0.5 to 4.1 MPa. The highest mechanical strengths (23.0 and 4.1 MPa) are obtained for optimal molar ratios X, Y, and Z corresponding to 3.8, 0.9 and 20, respectively.