Energy Systems Division

Geothermal Materials Group

• Geothermal Materials Home

Multifunctional, self-healing, self-re-adhering cementitious composite

In recent years, with the support of DOE GTO, BNL developed a superheat- and thermal shock resistant cement (TSRC) as a non-Ordinary Portland Cement system (non-OPC). Unlike OPC, the hydrated TSRC primarily consists of crystalline hydro-ceramic phase and amorphous Na2O-aluminosilicate (-Al2O3-SiO2-)n- inorganic polymer phase withstanding a heat temperature of 600^oC (not supercritical environment). There was no compressive strength change of the cement matrix in the 5 repeated thermal-shock tests (600^oC heat to 25^oC water cycles). In contrast, conventional well cements disintegrated and cracked in the first 3 cycles. The reason for this great thermal shock resistance of TSRC was formation of dense hydro-ceramic [hydroxysodalite, Na4Al3Si3O12(OH)] nano-scale crystals and amorphous aluminosilicate-backbone inorganic polymer. As seen from chemical composition, the key to success in assembling TSRC was formation of M (metal oxides: Na2O, ZnO, ZrO2, and TiO2)-aluminosilicate structures with minimum or none CaO. One of the important characteristics of this cement is its moderate brittleness (Young’s modulus (YM) < 300x103 psi) after high-temperature (300°C) curing in water, alkali carbonate or model geothermal brine.

The moderately brittle nature of the composite allows better withstanding of shocks and easier recovery after cracks’ generation. In addition to excellent performance under thermal shock conditions, the proposed composite addressed several short-comings of other cementitious blends for applications at temperatures above 250^oC: 1) insufficient self-healing performance at such elevated temperatures, 2) lack of hot acid resistance, and 3) poor self-re-adhering behavior of cement sheath debonded from metal casing.

The first shortcoming is usually the result of high brittleness of cement, causing the formation of wider perpendicular cracks under thermophysical and chemical stresses; the second, is related to calcium-rich chemistry of their hydrates; the last one is due to inadequate chemical and mechanical interactions with the metal casing. To formulate self-healing, self-re-adhering, and corrosion-mitigating cements with the advanced properties including flexibility, acid tolerance, and chemical affinity to metal surfaces, the new cement was designed to have the combination of amorphous and crystal hydration products.

Unlike CaO-SiO2-H2O-based OPC, the amorphous hydrate product [(CaO, Na2O)-Al2O3-SiO2-H2O] played an important role in providing flexible properties of cement, while the crystal products involved silica [SiO2], analcime [Na8Al8Si16O48(H2O)8], cancrinite [Na8(AlSiO4)6(CO2)(H2O)2], hydrogrossular [Ca3Al2(SiO4)2(OH)4] and dmisteinbergite/anorthite feldspar [Ca(Al2Si2)O8], as the major crystalline compounds and boehmite (Υ-AlOOH) as the minor one. Among these crystalline phases, the silica, analcime, cancrinite, and boehmite favorably precipitated and grew in cracks, leading to the sealing and plugging effects of the damaged composite.

On the other hand, hydrogrossular and dmisteinbergite/anorthite feldspar were responsible for strengthening the matrix. Far more importantly, amorphous aluminum-rich phase significantly contributed to improving cement’s adhesion to carbon steel casing’s surface, thereby resulting in the self-re-adhering capability of debonded cement sheath to the casing.