TITLE:
Toughness Improvement of Geothermal Well Cement at up to 300°C: Using Carbon Microfiber
AUTHORS:
Toshifumi Sugama, Tatiana Pyatina
KEYWORDS:
High Temperature, Alkali Activation, Carbon Fibers, Fly Ash, Calcium Aluminate Cement
JOURNAL NAME:
Open Journal of Composite Materials,
Vol.4 No.4,
September
25,
2014
ABSTRACT: This study aimed at
assessing the usefulness of carbon microfiber (CMF) in improving the
compressive-toughness of sodium metasilicate-activated calcium aluminate/Class
F fly ash foamed cement at hydrothermal temperatures of up to 300°C. When the
CMFs came in contact with a pore solution of cement, their surfaces underwent
alkali-caused oxidation, leading to the formation of metal (Na, Ca, Al)-complexed
carboxylate groups. The extent of this oxidation was enhanced by the
temperature increase, corresponding to the incorporation of more oxidation
derivatives at higher temperatures. Although micro-probe examinations did not
show any defects in the fibers, the enhanced oxidation engendered shrinkage of
the interlayer spacing between the C-basal planes in CMFs, and a decline in
their thermal stability. On the other hand, the complexed carboxylate groups
present on the surfaces of oxidized fibers played a pivotal role in improving
the adherence of fibers to the cement matrix. Such fiber/cement interfacial
bonds contributed significantly to the excellent bridging effect of fibers,
resistance to the cracks development and propagation, and to improvement of the
post-crack material ductility. Consequently, the compressive toughness of the
85°-, 200°-, and 300°C-autoclaved foamed cements reinforced with 10 wt% CMF was
2.4-, 2.9-, and 3.1-fold higher than for cement without the reinforcement.