The Effects of Diatomite as an Additive on the Macroscopic Properties and Microstructure of Concrete.

Diatomite is a siliceous sedimentary rock containing amorphous silica, which can be used as a green mineral admixture to improve the properties of concrete. This study investigates the affecting mechanism of diatomite on concrete performance by macro and micro tests. The results indicate that diatomite can reduce the fluidity of concrete mixture and change its water absorption, compressive strength, resistance to chloride penetration (RCP), porosity, and microstructure. The low fluidity of concrete mixture containing diatomite can reduce workability. With increasing diatomite as partial replacement for cement in concrete, water absorption of concrete decreases before increasing, while compressive strength and RCP rise first and then drop. When diatomite is added to the cement at a content of 5% by weight, the concrete has the lowest water absorption and the highest compressive strength and RCP. Through the mercury intrusion porosimetry (MIP) test, we determined that the addition of 5% diatomite reduces the porosity of concrete from 12.68% to 10.82% and changes the proportion of pores with different sizes in concrete, the proportion of harmless and less harmful pores increases, and the proportion of harmful pores reduces. Based on the microstructure analysis, the SiO2 in diatomite can react with CH and produce C-S-H. C-S-H is responsible for developing concrete because it fills pores and cracks, forms a platy structure, and makes the concrete much denser, thereby improving its macroscopic performance and microstructure.

Evolution of pore structure and radon exhalation characterization of porous media grouting.

Cracks and pores are considered as major sources of radon. Cement is widely used as a grouting material in mines, tunnels, and other projects for reinforcement, seepage prevention, and water plugging. This paper mainly experimentally studied the correlation between the radon exhalation rate of the porous medium after grouting and the sand grain diameter, grouting pressure, and slurry water-cement ratio. The pore characteristics of the samples before and after grouting were also studied based on the low field nuclear magnetic resonance (LF-NMR). The findings of the study show that the porosity of samples increases after the superfine cement solidification with an increase in the water-cement ratio, and the radon exhalation rate is proportional to porosity, the radon exhalation rate increases by 0.0005 Bq·m-2/s at W/C = 1.5, and by 0.0017 Bq·m-2/s at W/C = 2 increases, in comparison to the W/C = 1.The radon exhalation rate of porous media gradually increased after grouting in response to an increase in grouting pressure and the water-cement ratio. The radon exhalation rate of the porous media with larger pores was relatively higher and exhibited a positive correlation with the volume of micropores in porous media，the correlations of coarse, medium and fine media are 0.815, 0.826, and 0.859. The change in pore structure has an influence on radon exhalation. Although grouting changes the pore structure and reduces the connectivity between internal pores, the micropores generated after cement slurry solidification improves the radon exhalation rate by providing new channels, When the water-cement ratio is 1.5 and the grouting pressure is 1.5 MPa, the radon exhalation rate of porous media is 0.00273 Bq·m-2/s. The research results serve as a reference basis for the evaluation of the impact of rock masses on grouting reinforcement and pore sealing.

Study on the pore structure and radon release characteristics of coal in northern China.

Identifying the release characteristics of radon (Rn-222) in coal mines is critical preventing cancer risks for coal miners and coal fires. The present investigates the pore structure characteristics of coal samples from eleven coal mines in northern China, using low-temperature nitrogen adsorption (LTNA) test, combined with the radon exhalation rate in coal. The findings of the study reveal that the N2 adsorption isotherms of all the coal samples fall under the inverse S type, with micropores dominating in low-rank coals and mesopores dominating in the medium and high-rank coals, due to the separation of organic matter and quartz by shrinkage of micro-components and the orderly arrangement of aromatic rings as a result of ring condensation and thermal cleavage. The pore diameters of coal samples show similar distribution characteristics for sizes >2 nm, represented by a single peak near the pore diameter of 3 nm. Ash yield controls the mesopore and micropore volumes of medium and high-rank coal samples. The radon emission rate displays positive linear correlation (r2 = 0.87) with micropore volumes of analyzed coal samples due to the infillings of free radon in micropores. The radon element is derived by uranium decay, which causes a greater radon exhalation rate of coal mines in areas near the uranium mines. The results of the present study could be helpful to understand the influence mechanism of radon emission processes in coal, which provides an important basis for reducing cancer risks for coal miners and predicting coal fires.

Effect of the pore structure of granite and gabbro after heat treatment on the radon emission rate.

The pore structure of rock changes significantly during the heat treatment process, which affects the internal emission rate of radon. This study introduces the results of radon emission rate of rock after high temperature and analyzes the effect of the pore structure in rock mass on radon emission. The results show that there is a good positive correlation between temperature and radon emission in rock mass within a certain temperature range, and the higher the temperature, the higher the radon concentration emission. Two igneous rocks have the rate which is highest after the treatment in 400 °C. The absorption and sealing of radon in the microcapillary are the main occurrences of radon in rock mass. This is reflected in the increase in microcapillary porosity and the decrease in DP, which changes the microstructure of the rock and increases the connectivity of the internal pore channels of the rock, thereby increasing the volume and length of the migration channels for radon. These results are helpful to understand the influencing mechanism of radon emission process in rocks.

Frost Damage in Tight Sandstone: Experimental Evaluation and Interpretation of Damage Mechanisms.

Low-porosity tight rocks are widely used as building and engineering materials. The freeze-thaw cycle is a common weathering effect that damages building materials in cold climates. Tight rocks are generally supposed to be highly frost-resistant; thus, studies on frost damage in tight sandstone are rare. In this study, we investigated the deterioration in mechanical properties and changes in P-wave velocity with freeze-thaw cycles in a tight sandstone. We also studied changes to its pore structure using nuclear magnetic resonance (NMR) technology. The results demonstrate that, with increasing freeze-thaw cycles, (1) the mechanical strength (uniaxial compressive, tensile, shear strengths) exhibits a similar decreasing trend, while (2) the P-wave velocity and total pore volume do not obviously increase or decrease. (3) Nanopores account for >70% of the pores in tight sandstone but do not change greatly with freeze-thaw cycles; however, the micropore volume has a continuously increasing trend that corresponds to the decay in mechanical properties. We calculated the pressure-dependent freezing points in pores of different diameters, finding that water in nanopores (diameter <5.9 nm) remains unfrozen at -20 °C, and micropores >5.9 nm control the evolution of frost damage in tight sandstone. We suggest that pore ice grows from larger pores into smaller ones, generating excess pressure that causes frost damage in micropores and then nanopores, which is manifested in the decrease in mechanical properties.

Effects of Cyclic Freeze-Thaw on the Steel Bar Reinforced New-To-Old Concrete Interface.

Frost damage of concrete has significant effects on the safety and durability of concrete structures in cold regions, and the concrete structures after repair and reinforcement are still threatened by cyclic freezing and thawing. In this study, the new-to-old concrete interface was reinforced by steel bar. The shear strength of the new-to-old concrete interface was tested after the new-to-old combination was subjected to cyclic freeze-thaw. The effects of the diameter of the steel bar, the compressive strength of new concrete, the number of freeze-thaw cycles and the freezing temperatures on the shear properties of new-to-old concrete interface were studied. The results showed that, in a certain range, the shear strength of the interface was proportional to the diameter of the steel bar and the strength of the new concrete. Meanwhile, the shear strength of the reinforced interface decreased with the decreasing of the freezing temperature and the increasing of the number of freeze-thaw cycles.