"The team also developed button-size capacitors with different ratios of cement to carbon black but found that while adding more carbon black (above 10 percent

"The team also developed button-size capacitors with different ratios of cement to carbon black but found that while adding more carbon black (above 10 percent by volume) to the mixture increased its storage capacity, this came at the cost of the cement’s structural integrity. And for the use cases that Ulm and colleagues have in mind, the structural strength of the cement is essential." From this it seems that below a certain amount of carbon black being used, the structural strength is not affected.