Revisiting the effects of carbon nanotube agglomerates in cement

Agglomerates of carbon nanotube (CNT) are typically considered as defects in cement matrix, yet without a clear physiochemical picture regarding why and how. By representing CNT agglomerates with CNT sponge powders (CNTSPPs), we discovered that hydrates can precipitate in the pores of CNTSPPs to form a CNT/hydrates nanocomposite shell, the thickness and compactness of which are dependent on the curing temperature. In contrast to the detrimental effects of CNTSPPs for the samples cured under standard condition, the addition of 0.05 wt % CNTSPPs increased the 28d compressive strength by ∼16 % compared to the reference sample under steam curing condition. This is because steam curing condition significantly improves the hydrophilicity of CNTSPPs, resulting in its absorption of pore solution and calcium ions, which eventually increase the CNT/hydrates nanocomposites shell thickness and microhardness. Such nanocomposites shell with high mechanical properties can screen the negative effects of the porous inner CNTSPPs.