A Solidification and Stabilization Process in Cementitious Materials: A Review

The solidification/stabilization (S/S) process is a widely used technology for the treatment of hazardous waste, especially those containing heavy metals. This method aims to reduce the mobility, toxicity, and solubility of contaminants by promoting their incorporation into a solid and stable matrix. The cementitious matrix, typically based on portland cement, is the most employed due to its physicochemical properties, such as high alkalinity, low permeability, good mechanical strength, and its ability to form hydration products capable of interacting with metals. This review seeks to explain the main immobilization mechanisms: (1) chemical interactions with hydration products (C–S–H, ettringite, portlandite), (2) physical adsorption on the surfaces of hydrates, and (3) physical encapsulation within a dense, low-porosity microstructure. It also aims to summarize the various factors on which the effectiveness of the S/S process depends, such as the type and concentration of the metal/waste, the pH of the mixture, the degree of hydration, and the porosity of the matrix. Some metals, such as lead and zinc, can interfere with hydration reactions, delaying them or altering the products formed. Chromium can be immobilized with increasing amounts of FeSO4 in the solidified product; both Cr3+ and Cr6+ can be retained by ettringite. Cr6+ substitutes for sulfate, while Cr3+ can substitute for Al in most calcium aluminate hydrate phases or precipitate as chromium hydroxide, Cr(OH)3. When Cu(NO3)2, Pb(NO3)2, Cd(NO3)2, or Zn(NO3)2 are incorporated into cement pastes, a delay in hydration occurs due to the influence of these metals on the hydration of C3S. Lead and copper are fixed within the silicon tetrahedra of C–S–H, while zinc and cadmium precipitate as hydroxides or oxides. These hydrates appear to be responsible for the delayed effect on cement hydration, temporarily or permanently blocking the hydration of anhydrous grains. Furthermore, the main criteria for verifying the efficiency of the S/S process will be discussed. In the future, predictive leaching models that integrate variables such as pH, porosity, curing time, and type of metal; long-term performance assessments; the impact of organic waste on the cementitious matrix in the S/S process; and the study of the S/S process in more sustainable matrices deserve to be explored by researchers.