Thermally Treated Cutter Soil Mixing Residue (CSMR) as Construction and Building Materials

Concrete with ordinary Portland cement (OPC) as a primary binder has become the most produced building material worldwide over the last decades with yearly production levels reaching approximately 30 billion tons. However, the cement industry is annually responsible for 5-8% of the global anthropogenic CO2 emissions, and the number is predicted to be increased by 12-23% in 2050 because of the rapid development of the infrastructural construction. In this context, there has been an increasing need to explore alternative cementitious materials, especially those from the waste stream. 

This thesis focuses on the feasibility of using cutter soil mixing residue (CSMR), a solid waste consisting of a soil-cement mixture, to develop more sustainable cementitious materials. Firstly, CSMR was firstly thermally activated at 800 °C. The chemical and mineral compositions of the raw and calcined CSMR were characterized. Then, the calcined CSMR was used as 1) a supplementary cementitious material (CSM) to develop blended cement and 2) a precursor to synthesize alkali-activated cement (AAC). The performances of those new cement binders were studied in terms of the fresh properties, hydration kinetics and products, and mechanical properties. Finally, those new binders were used to synthesize strain-hardening cementitious composites (SHCCs), which are a special type of high-performance concrete with ultra-high ductility. The binder properties, mechanical properties, and micromechanics parameters of the SHCCs were investigated. Moreover, the embodied energy and carbon emissions of the cement-based materials prepared with the calcined were analyzed. 

The calcination allows the formation of Ca-rich amorphous aluminosilicates with pozzolanic activity and C2S with hydraulic properties. The former is derived from the synergy between clay minerals and Ca-bearing phases (e.g., calcite and hydrated cement) during the calcination, while the latter is generated from the dehydrated cement. The formation of those phases with pozzolanic and/or hydraulic properties suggests the potential of the calcined CSMR as an SCM in blended cement or a precursor in AAC. 

The specific surface area (SSA) of the calcined CSMR plays a dominant role in the fresh properties of the blended cement. A calcined CSMR sample with a higher (or lower) SSA than OPC is prone to increasing (or decreasing) the cement paste's yielding stress and plastic viscosity. The calcined CSMR can contribute to the strength development of the blended cement through increased hydration degree, accelerated hydration rate, pozzolanic reaction, and the creation of additional hydrates. An almost linear correlation exists between the reactive phase content in the calcined CSMR (from XRD characterization) and the compressive strength of blended cement pastes. The incorporation of up to 20% calcined CSMR has no detrimental effect on the compressive strength of the cement mortar. The addition of the calcined cement is conducive to limiting the dry shrinkage and improving the durability (e.g., sulfate and chloride resistances) of the cement mortar. Using the calcined CSMR in SHCCs as a partial cement replacement increases the tensile strain capacity by modifying the fiber-matrix interfacial properties, while maintaining the compressive strength. The new SHCCs has reduced embodied energy and CO2 emissions by 13-39% and 17-50%, respectively.

The alkali-activated calcined CSMR (AA-CSMR) cement shows rapid early strength development and can achieve a maximum 28-day compressive strength of 33.2 MPa, which can be increased to 50 MPa or even higher by either incorporating 10% slag or curing at 70 °C for 1 day. The primary alkaline reaction products are C-(A)-S-H gels, which could verify the formation of the reactive calcium-rich amorphous phases in the calcined CSMR. Compared with the OPC-based SHCCs, the SHCCs prepared with AA-CSMR cement exhibit comparable or even better mechanical properties, while reducing around 40 % embodied energy and about 60% CO2 emissions. 

The results of this research support the use of calcined CSMR as greener cementitious materials. Considering the high complexity of the composition in CSMR, systematic research on more CSMR samples from various sources is needed in the follow-up research.