Sources: Concrete Sustainability Hub at Massachusetts Institute of Technology, Cambridge; Corning Inc., New York; CP staff
Borrowing from a ‘rigidity theory’ that has led Corning Inc. engineers to the high performance Gorilla Glass—its toughness in thin sections equal to smartphone and laptop computer screens—the Concrete Sustainability Hub is studying potential portland cement reformulation aimed at higher fracture resistance than current ASTM C 150 product.
Rigidity and fracture properties can hinge on volume adjustments of three common salts in conjunction with pyroprocessing. Gorilla Glass production requires replacement of small sodium ions with larger potassium ions in the molten phase, imparting high compressive stress deep into the finished product. In the portland cement parallel, Concrete Sustainability Hub researchers are focusing on clinker phase chemistry changes netting higher silica content in calcium-silicate-hydrate (C-S-H), the principal binder in concrete.
“Tougher cement would allow using less material while achieving comparable mechanical properties. Increased resistance to fracture would improve cement’s longevity, making it even more sustainable. As such, it is of primary importance to understand how composition affects the resistance to fracture of calcium-silicate-hydrates, starting from the atomic scale,” Hub researchers note in “Gorilla cement: Tougher, yet greener,” their May Research Brief.
“Gorilla Glass reduces complex molecular networks to simple mechanical trusses. A network can be flexible, stressed-rigid, or isostatic, if, respectively, the number of chemical constraints is lower, higher, or equal to the number of degrees of freedom of the atoms.” C-S-H grains’ fracture toughness is optimal for isostatic compositions, at a Ca/Si molar ratio around 1.5, while Hub researchers characterize flexible and stressed-rigid compositions, respectively, at 1.7 and 1.3 ratios—the former most representative of typical portland cement.
““Isostatic materials are able to deform and feature relatively high surface energy… We predict that decreasing the Ca/Si of C-S-H from 1.7 to 1.5 would increase its toughness by 70 percent, thus allowing use of less material without compromising performance,” Hub researchers explain. “Decreasing the relative amount of calcium in cement can be achieved by replacing clinker with silica-rich byproduct materials such as fly ash. Tougher and greener, this ‘Gorilla cement’ would help improve the sustainability of our built environment.”
Gorilla cement research continues under Concrete Science, which along with Buildings and Pavements, encompasses areas the Ready Mixed Concrete Research and Education Foundation and Portland Cement Association prioritize as Hub sponsors.