Department of Mechanics: Seminar: Bazant 2025
The shard test and nanoporomechanics reverse classical paradigm of cement hydration being contractive
Zdeněk P. Bažant
McCormick School of Engineering, Northwestern University, Evanston, Illinois, USA
17 April 2025, 11:00-12:00 CET, Room B-168 @ Thákurova 7, 166 29 Prague 6
Abstract: Le Chatelier in 1887 and Powers in 1947 demonstrated that the volume of nanoscale C-S-H (calcium silicate hydrate) particles formed during hydration is smaller than the combined volume of the reactants—the anhydrous Portland cement and water. Hydration has thus been considered as contractive. An experiment shows that the opposite is true above the nanoscale. The porous skeleton of cement paste expands as the growing C-S-H particles push each other apart, similar to crystal growth pressure. This is significant for high-performance concretes (HPC) with low water-cement ratios (w/c = 0.3), where chemical self-desiccation lowers pore relative humidity by 40%, compared to just 1% in traditional concretes (w/c = 0.5).
Standard American Society for Testing and Materials (ASTM) C1608 tests, using 10 mm thick water-immersed specimens, show large shrinkage because the half-time of water ingress is many decades, unable to offset shrinkage-causing self-desiccation. The present experiment, using a laser confocal microscopy-topography technique, proves the opposite—expansion, evidenced by measuring the length changes of water-immersed HPC shards 0.5 mm thick in which the diffusion halftime, only about one hour, allows continuous resaturation of pores, canceling self-desiccation. The faster diffusion (half-time of one hour) enables continuous pore resaturation, preventing shrinkage. When sealed with paraffin oil, the shards self-desiccate and shrink. These findings align with studies since 2015, showing that models excluding hydration expansion cannot fit test data across various specimen sizes and sealing conditions. The results suggest that standardized ASTM tests for the so-called chemical shrinkage in modern concretes with very low water-cement ratios are misleading and need revision.