Pore structure characterization of hardened cement paste is important to concrete mechanical and transport properties. Hence, we apply indirect methods (NS, MIP, and NMR) and direct methods (XCT, FIB/SEM, and HIM) to provide a general view of the pore structure of hardened cement paste. The results show that the 3D pore network of hardened cement paste is isolated in microscale yet is largely connected by large capillary pores, small capillary pores, and microcracks in nanoscale. In indirect methods, MIP and NMR have a wide measurement range and permit observing most of the porous volume with an average porosity of 8.4–9.9% of hardened cement paste. In contrast, direct methods have a relatively narrow measurement range and thus lead to a large porosity scattering of 1.8–16.4%. The pore size distribution (PSD) curves by indirect methods show that the pore structure is mainly concentrated in three sections of 1–10 nm, 10–100 nm, and around 10 μm, which correspond to the imaging range of HIM, FIB/SEM, and XCT. However, FIB/SEM implies that the most porous part of hardened cement paste (10–100 nm) is underestimated by MIP due to the “ink-bottle” effect. Another “underestimation” of NMR is because C-S-H gels swell during the wetting, and the swelling separates the capillary pores into smaller ones. The microcracks induced by sample preparation contribute 0.1-0.2% (XCT) and 4.5–8.1% (FIB/SEM) of porosity to the porous volume of cement paste, but they have limited influence on the pore size distribution and pore connectivity.

Waste sediment generated during tunnel construction is applied to prepare synchronous grouting material, where the influences of fly ash, slag powder, and bentonite on the rheological properties (such as consistency, fluidity, setting time, drainage rate, and stone rate) are studied. The results show that adding fly ash content increases the initial consistency, setting time, and fluidity of grouting material, but also increases its drainage rate and decreases its stone rate. The addition of slag powder results in a slight increase in the setting time and fluidity of the grouting material, yet a decrease in the initial consistency value. In contrast, with the addition of bentonite, both the initial consistency and fluidity of the grouting material decrease. Finally, the optimal mix ratio of high-performance and low-cost grouting materials is fixed to be 30% fly ash, 50% slag powder, and 10% bentonite. Therefore, the fluidity of grouting material can be 170 mm, with an initial consistency of 122 mm, setting time of 1050 min, stone rate of 96.2%, drainage rate of 1.5%, and 28-day compressive strength of 8.3 MPa.

In order to alleviate the increasing serious urban waterlogging problem, the rainstorm resistance of a new self-compacting recycled pervious concrete (NSRPC) under the coupling of freeze–thaw (F-T) and fatigue is studied. The once-in-a-century rainfall was simulated, and the rainstorm resistance of NSRPC was evaluated mainly through the ponding depth and drainage time. In addition, the mechanical properties (compressive strength and flexural strength), mass loss rate and relative dynamic elastic modulus of NSRPC during F-T and fatigue coupling were measured. The microstructure of NSRPC was observed by scanning electron microscopy, and its deterioration mechanism was analyzed. The results show that the fatigue load aggravates the F-T damage of NSRPC in the later stage. With the increase in the number of fatigue cycles, the loss rate of compressive strength and flexural strength of NSRPC increases continuously, and the permeability coefficient decreases first and then increases. With the increase in the number of freeze–thaw and fatigue cycles, the mass loss rate increases gradually, and the relative dynamic elastic modulus decreases gradually. After the coupling of fatigue and F-T cycles, the minimum mass loss of NSRPC is only 2.14%, and the relative dynamic elastic modulus can reach 86.2%. The increase in the number of fatigue cycles promotes the generation and expansion of micro-cracks and provides more channels for water to invade the matrix. Under the action of rainstorm in the 100-year return period, the maximum ponding depth of NSRPC with steel fiber content is 84 mm, and the drainage time is 7.1 min, which meets the needs of secondary highway. This study will provide theoretical basis for improving the service life and drainage capacity of urban drainage pavement in cold areas.