Development of alkali-activated binder using hwangtoh without calcination

•Compressive strength of alkali activated natural hwangtoh reached 20 MPa by using sodium hydroxide and water glass solution.•The strength development was attributed to pore refinement based on mercury intrusion porosimetry analysis.•XRD analysis revealed kaolinite and halloysite in hwangtoh transform to amorphous aluminosilicate and faujasite-Na.•29Si NMR showed local environments of the Si in hwangtoh transformed from Q3(0Al) to Q3(1Al) and Q4 in the amorphous phase.

In terms of flow and compressive strength, we examined natural hwangtoh, a readily available eco-friendly material mineralogically similar to kaolin, as a possible alkali-activated binding material. The phase and molecular structures of hwangtoh before and after alkali activation were analyzed by X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (NMR). The sample prepared by mixing hwangtoh with 10 M sodium hydroxide and sodium silicate solutions at 0.45 and 0.10 by weight of hwangtoh, respectively, gave the highest compressive strength of 25.4 MPa and a flow value of 50%. The results of the XRD analysis were consistent with the results of 29Si solid-state NMR analysis, which showed that local environments of the 29Si nucleus in hwangtoh transformed from Q3(0Al) to Q3(1Al) and Q4 in the amorphous phase during the alkali-activation process. The newly formed reaction products filled the pores and densified the matrix, improving the compressive strength of the alkali-activated hwangtoh paste.