Drying Shrinkage Of Hardened Cement Paste And Its Relationship To The Microstructure

Drying shrinkage is a typical source of distress in concrete structures in more ways than one. Drying shrinkage is typically non-uniform throughout a sample and this leads to cracking and warping, which in turn cause durability problems including mechanical or esthetic failure, and pathways for the ingress of corrosive ions.The shrinking component in concrete is the cement paste. The microstructural features of cement paste that control drying shrinkage have not yet been determined. The aim of this paper is to relate microstructure to the drying shrinkage of hardened cement paste. The strategy was to vary the microstructure via processing (water to cement ratio and curing time) and relate the changes to the deformation behavior.Calcium silicate hydrate, calcium hydroxide and pore structure were the microstructural features chosen for study. Because calcium silicate hydrate cannot be measured by direct techniques, a method for estimating the amount of C-S-H based on the combination of TG-DSC and single plus 29Si solid state MAS NMR was developed. Drying shrinkage behavior of pastes are studied by drying specimens through successive steps of RH to 7% RH and re-saturating the specimens to determine the reversible and irreversible component of shrinkage strains. Some specimens were chosen for the second drying and re-saturating process in order to determine the effects of drying on the pore structure and deformation behavior of hardened cement paste.The following conclusions can be drawn:1) With the same w/c, prolonged curing increased the hydration degree and the amount of main hydration products, CH and C-S-H in hardened cement paste, at the meanwhile, the amount of gel pores increases and the average pore size decreases. Moreover, extended curing increases the content of Q2 in C-S-H, indicating a stiffer and stronger structure with enhanced ability to resist deformation. The total shrinkage of cement paste in the 100%-7%RH range increases with curing time and then decreases.2) With the same curing time, the hydration degree and the amount of main hydration products increases with increasing w/c. at the meanwhile, the volume of different types of pores and the average pore size increases. The total shrinkage of cement paste in the 100%-7%RH range increases with increasing w/c and most part of the growth occurs in the 100%-54%RH range. 3) During the drying process, the weight loss and liner shrinkage increases as RH decreases. When re-saturating, weight loss can be almost regained while a large portion of the drying shrinkage is irreversible. There was a significant hysteresis between the weight loss and weight regaining and the case is the same for shrinkage. Most part of the irreversible drying shrinkage occurs in the 84%-7%RH range.4) The amount and size of pores and the amount and polymerization of C-S-H are the dominant microstructure feature that control drying shrinkage.5) The capillary pores play an important part in the 100%-54%RH range, with more capillary pores comes more meniscus while the increase of the capillary pore size brings less effective stress for the meniscus. These two effects are often cooperative in resulting a final shrinkage strain in the 100%-54% RH range.6) The gel pores play an important part below 54% RH, however, as hydration proceeds, the amount and polymerization of C-S-H overweighed the effect of gel pores.7) In the 2nd drying process of 28-day-old cement pastes, weight loss in the 100%-84% and 84%-54% RH ranges increase while weight loss in the 54%-7%RH range decreases. Irreversible drying shrinkage is relatively small in the 2nd drying.In this paper, three microstructural features of hardened cement paste, especially calcium silicate hydrate, were analyzed quantitatively accompanying by an extensive study of the drying shrinkage behavior of cement paste using a modified testing technique. The resulting findings provide a more in-depth understanding of the relationship between drying shrinkage and microstructure of hardened cement paste and how drying shrinkage was affected by calcium silicate hydrate.