Exploring and modifying the C-S-H structure at a micro–nano level is an effective solution to improve the performance of Portland cement. Compared with organics inserting C-S-H, the research on the performance of a polymer-binding C-S-H structure from nanoscale to macroscale is limited. In this work, the mechanical properties of a modified C-S-H, using hydroxyl-terminated polydimethylsiloxane (PDMS) as the binders, are evaluated. The PDMS-modified C-S-H structures are introduced into macro-defect-free cement to obtain stress–strain curves changes at a macro scale. The AFM–FM was adopted to measure the morphology and elastic modulus of C-S-H at a nano scale. The molecular dynamics (MD) simulation was performed to assess the toughness, tensile properties, and failure mechanism. The results show that the PDMS-modified C-S-H powders change the break process and enhance ductility of MDF cement. The elastic modulus of PDMS-modified C-S-H is lower than pure C-S-H. When PDMS molecules are located between the stacking crystal units, it can enhance the toughness of C-S-H aggregates. The PDMS-modified C-S-H stacking structure has better plasticity, and its tensile strains are higher than the pure C-S-H. PDMS molecules hinder the initial crack expansion, leading to the branching of the initial crack. In addition, the measurement of AFM–FM can identify and obtain the mechanical properties of basic units of C-S-H. This paper enhances the understanding of cement strength sources and modification methods.

Formwork lateral pressures are critical with respect to engineering safety, and laboratory tests are often limited by time, height, etc. Formwork lateral pressures are related to early concrete fluid behavior (e.g., thixotropy). In this paper, we propose the use of ultra-deep underground diaphragm walls instead of formwork for conventional lateral pressure testing. During the construction process, three measurement points were set up in an ultra-deep diaphragm wall at −40 m, −80 m and −100 m. The concrete was divided into 22 casts, and the development of lateral pressure and the effect of time on lateral pressure were observed under each of the casts. We found that the characteristic height of all three measurement points was approximately 21 m. The average casting speed for this test was 16.846 m/h, with the fastest speed of 32.148 m/h. A time-dependent phenomenon of rapid rise and fall in lateral pressure with each casting was observed. This method provides a new way to conceptualize formwork lateral pressure, with the advantages of long testing time, high casting height and multiple tests, not only for formwork lateral pressure but also for early age thixotropy measurement of concrete.