Valorization of marble powder wastes using rice husk ash to yield enhanced-performance inorganic polymer cements: Phase evolution, microstructure, and micromechanics analyses

The challenge for sustainable development is now constraining scientists, and policymakers to consider alternative materials and processes requiring low energy consumption and low emissions while prioritizing local materials and industrial waste recycling. This study investigates ways to valorize marble waste powders to design a new class of inorganic polymer cement. These powders are among the most important solid wastes globally, and in the Italian island of Sardinia they create a disposal problem. When these powders, which are rich in calcium silicate hydrate (C–S–H) phases, are incorporated into the geopolymer synthesis, this gives rise to a mix between C–S–H and alkali metal aluminosilicate hydrate (M-A-S-H) systems that are the focus of this study. In detail, C–S–H systems are prepared with various Ca/Si ratios, including CS, C2S, and C3S. The microstructure and mechanical properties are investigated using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy, along with density methods (mercury intrusion porosimetry), macroscopy mechanical testing methods (flexural strength tests) and nanoscale mechanical testing methods (indentation testing and scratch testing). The resulting inorganic polymer cements exhibit both an amorphous and a crystalline phase containing sodium-calcium-aluminum-silicate hydrate N–C-A-S-H. The fraction of the crystalline phase increases when the silica content decreases, from CS to C3S, and when the C–S–H fraction increases. In turn, the crystalline phase dictates the pore structure and mechanical properties. Specifically, a homogeneous and compact microstructure with low porosity is observed. The flexural strength ranges from 5.8 to 8.7 MPa, the elastic modulus ranges from 10 to 17 GPa, and the fracture toughness ranges from 0.23 to 0.48 MPa m1/2. In general, the lower the silica percentage of the calcium-silicate hydrates and the higher the fraction of calcium silicate hydrate are, the higher the flexural strength, elastic modulus, and fracture toughness. The underlying mechanism for the observed stiffening, strengthening, and toughening is the insertion of aluminum in the C–S–H defect sites to yield C-A-S-H, along with the formation of sodium silicate gel from excess colloidal silica, which contributes to the reduction in gel pores and matrix densification. The values of porosity, density, and mechanical properties suggest that inorganic polymer cements derived from marble wastes are promising candidates for innovative low-temperature and low-density binders with low carbon footprint satisfying both the requirements of sustainability and the local material concept.