Development of alkali-activated binder using hwangtoh without calcination

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.