Sea sand exhibits typical features of loose structure, large compressibility and poor consolidation, causing severe challenges for the construction and emergency repair of coastal engineering. A new Hybrid Non-Isocyanate Polyurethane (HNIPU) was utilized as a curing agent to prepare HNIPU-sea sand mixtures (HSM) in this study. A series of uniaxial compression and scanning electron microscopy (SEM) tests were conducted to explore the strength property and microscopic curing mechanism of HSM. As the mass ratio of polyurethane to sea sand increased from 0.1 to 0.7, the uniaxial compressive strength of HSM first increased from 8.75 MPa to 20.09 MPa then decreased to 17.90 MPa. The maximum strength was achieved at a mass ratio of 0.5. The uniaxial compressive strength increased by 79.46% and 58.41% within a temperature range of - 10-60 degrees C and a curing time range of 10-90 min, respectively. Nevertheless, it decreased by 31.06% with an increase from nil to 10% in moisture content. The prediction formula for the uniaxial compressive strength of HSM can be expressed as a function of multiple influencing factors using regression analysis. The main factors affecting the uniaxial compressive strength of HSM are mass ratio, moisture content, curing time, temperature and gradation in sequence. Furthermore, the progressive failure of HSM were identified based on the complete stress-strain curves in conjunction with the failure mode (X-shaped conjugated shear). It is well documented that HNIPU can rapidly fill, encapsulate and cement sea sand and remarkably enhance its strength, ductility and toughness. These research results may provide an effective method for the rapid reinforcement of the beach sand foundation.

Microbially induced carbonate precipitation (MICP) is a potential method for ground improvement. It can enhance the physical and mechanical properties of soils through soil particle cementation and pore filling. However, the excessive number of injections of cementing solution required for MICP is a major limitation for practical engineering application. To reduce the number of injections and improve the efficiency of the cementing process, an enhancement method was investigated in this study in which an aluminum ion flocculant (AIF) was added to the cementing solution. It can enhance the curing rate and effect of MICP. Experiments were carried out on MICP-treated sand columns, MICP-treated glass beads and aqueous solutions, and the influence of AIF on the production of calcium carbonate and unconfined compressive strength (UCS) was studied. The effect of AIF on the composition and morphology of the deposited calcium carbonate was evaluated from SEM micrographs and other microscopic observations. The results of the study showed that the addition of AIF to the cementing solution significantly reduced the number of injections required and increased the UCS of the reinforcement compared to those of a control group without adding AIF. The proposed method resulted in the experimental sand column being reinforced after 3 treatments of the cementing solution with an UCS of 323.3 kPa. The UCS reaches 1692 kPa after 5 treatments of the cementing solution, compared to 9 treatments using the conventional method. The UCS was 3.2-12.7 times higher than that of the control group for the same calcium carbonate content.