Phase Formation and Evolution in Mg(OH)2–Zeolite Cements

Atomistic Simulations of Geopolymer Models: The Impact of Disorder on Structure and Mechanics

Geopolymers are hydrated aluminosilicates with excellent binding properties. Geopolymers appeal to the construction sector as a more sustainable alternative to traditional cements, but their exploitation is limited by a poor understanding of the linkage between chemical composition and macroscopic properties. Molecular simulations can help clarify this linkage, but existing models based on amorphous or crystalline aluminosilicate structures provide only a partial explanation of experimental data on the nanoscale. This paper presents a new model for the molecular structure of geopolymers, in particular for nanoscale interfacial zones between crystalline and amorphous nanodomains, which are crucial for the overall mechanical properties of the material. For a range of Si-Al molar ratios and water contents, the proposed structures are analyzed in terms of skeletal density, ring structure, pore structure, bond-angle distribution, bond length distribution, X-ray diffraction, X-ray pair distribution function, elastic moduli, and large-strain mechanics. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with an impact on mechanical properties. This offers a new starting point for the multiscale modeling of geopolymers.

Blast furnace slag-Mg(OH)2 cements activated by sodium carbonate

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge.