Synergistic Effects between Carbonation and Cracks in the Hardened Cement Paste

Abstract

This paper investigates the synergistic interaction between carbonation and microcracks in hardened cement pastes. Ordinary Portland cement pastes with three different water/cement ratios of 0.4, 0.5 and 0.6, hydrated for 28 days with crack apertures ranging from 10 to 150 μm were subjected to accelerated carbonation in a climate chamber that controls the targeted relative humidity (65 and 75%), CO2 concentration (0.3 vol.% and 1 vol.%) and temperature (20 ℃). Mercury intrusion porosimetry, N2-adsorption and thermogravimetry analysis were used to quantify the carbonation-induced changes in pore size distribution, porosity and phase assemblages. Additionally, the changes in crack apertures were followed during carbonation. The results indicated that cracks within the investigated range facilitate the carbonation along the crack surface due to a faster gas diffusion process. The cracks with apertures below 50 μm increase the carbonation depth at least by a factor of two for all studied w/c and environmental conditions. We observed a constant increase in crack openings during carbonation and its linear relationship with the amount of precipitated calcium carbonate. Due to depletion of CH and decalcification of C-S-H close to the sample surface, the crack aperture increase becomes limited. Therefore, the crack apertures do not increase further after the material around the crack is carbonated. It was also found that the densification of the carbonated cement matrix coexists with large capillary pores (>50 nm) or cracks, especially for the low w/c samples. The shift in pore size distribution from about 100 nm towards smaller pores (4.5–50 nm) and a decrease in gel pore fractions after 28 days of carbonation also indicate a simultaneous calcium carbonate precipitation in meso/capillary pores and decalcification of C-S-H at the nanoscale.

Comparative Analysis of Engineering Carbonation Model Extensions to Account for Pre-Existing Cracks

“…Many experimental studies report carbonation depths in cracked concrete [ 10 , 12 , 13 , 14 , 15 , 16 , 17 , 19 , 35 , 36 , 37 ]. Studies with missing information regarding crack properties or mix design [ 15 , 17 , 19 , 35 , 36 ] and studies with specialized mix designs such as [ 12 , 16 ] were excluded from the dataset.…”

“…Many experimental studies report carbonation depths in cracked concrete [ 10 , 12 , 13 , 14 , 15 , 16 , 17 , 19 , 35 , 36 , 37 ]. Studies with missing information regarding crack properties or mix design [ 15 , 17 , 19 , 35 , 36 ] and studies with specialized mix designs such as [ 12 , 16 ] were excluded from the dataset. The carbonation depths at the location of the cracks were collected from [ 10 , 13 , 14 , 37 ], which includes studies on both natural cracks and artificial cracks created as notches.…”

“…Many experimental studies [ 9 , 12 , 13 , 14 , 15 , 16 , 17 ] have reported that carbonation rates in cracks increase with increasing crack width. Two limiting crack widths exist (see Figure 2 ).…”

The Investigation of Compacting Cement Systems for Studying the Fundamental Process of Cement Gel Formation

“…The study of the microstructure of calcium silicate hydrate (CSH) gels from theoretical and computational points of view, as well as the fractality of CSH and the evaluation of the mechanical properties and durability of CSH—with particular reference to carbonization and chloride/sulfate exposure—are also important in influencing the mechanical properties and durability of cement-based materials [ 30 , 31 ]. The most recent works of authors who studied the formation of the structure of cement pastes from the point of view of the mechanisms of gel formation and hardening are primarily aimed at the microstructural component of these processes and mechanisms [ 32 , 33 , 34 , 35 , 36 ]. Today, a large role is given to the study of the microstructure of cement pastes, in particular, microcracking [ 32 ], the characteristics of the pore structure [ 33 ], the effect of nanoadditives [ 34 ], and the mechanisms of carbonization and hydration, along with their products: calcium carbonate, aluminosilicate gel, and CSH phase [ 35 ].…”

“…The most recent works of authors who studied the formation of the structure of cement pastes from the point of view of the mechanisms of gel formation and hardening are primarily aimed at the microstructural component of these processes and mechanisms [ 32 , 33 , 34 , 35 , 36 ]. Today, a large role is given to the study of the microstructure of cement pastes, in particular, microcracking [ 32 ], the characteristics of the pore structure [ 33 ], the effect of nanoadditives [ 34 ], and the mechanisms of carbonization and hydration, along with their products: calcium carbonate, aluminosilicate gel, and CSH phase [ 35 ]. The study of these processes and mechanisms at the micro level contributes greatly to developing an understanding of the gel-type contributions to various materials that form next-generation concretes [ 36 ].…”