Effect of initial cure time on toughness of geopolymer matrix composites

The toughness of geopolymer matrix composites (GMC) has been identified as a limiting factor to their use in structural applications. Advanced ceramic matrix composites (CMC), which also are limited by brittle behavior, have shown gains in toughness through careful tailoring of the interface between fiber and matrix. This can create various crack dissipating mechanisms and prevent premature composite failure. Such interface modification has already been applied to a fiber reinforced geopolymer and while the resulting composite showed a reduction in brittle behavior, the modified interface produced an unacceptable loss in modulus without any other well-defined quantitative gains. Information gathered from other studies suggests the large decrease in modulus observed in the GMCs with the weakened interface may have been the result of poor matrix properties stemming from an inadequate cure. Therefore, this current study explores the effects of initial cure time on composite performance by measuring the mechanical properties GMCs with a modified interface. GMCs containing unidirectional Nextel 610 fiber were cured under two different sets of process conditions to better understand the influence of matrix properties. Additionally, specimens consisted of cleaned and carbon coated fiber surfaces, in an attempt to evaluate extremes of interfacial strength. Mechanical properties were then evaluated for comparison to determine if improved geopolymer matrix properties would allow a weakened interface to yield performance gains more in keeping with expectations based on CMC's. The results of the study indicate that specimens with carbon coating benefited from the longer initial cure time. The average increase in flexural modulus and strength over samples with one hour initial cure time was ~65% and ~170% respectively. Stress-strain behavior of the carbon-coated specimens with an extended cure time also indicated a greater degree of damage tolerance as compared to those without interphase.