Effects and mechanisms of reactive nanoparticles and triethanolamine on the properties of limestone calcined clay cement

Abstract

The large-scale production of cement worldwide contributes to 6-8% of global anthropogenic CO2 emissions. Among supplementary cementitious materials (SCMs), limestone and calcined clay are abundant and widely available yet remain underutilised. Limestone Calcined Clay Cement (LC3) enables up to 60% clinker replacement, achieving approximately 40% lower CO2 emissions compared with ordinary Portland cement (OPC).

LC3 containing 40% calcined kaolinite develops higher strength than OPC from 7 days onwards. However, LC3 shows lower early-age strength and a slower rate of strength gain compared with OPC. To address this limitation, this thesis focuses on enhancing the early strength of LC3 through the incorporation of nanomaterials and triethanolamine (TEA). In addition, the effects of nano-SiO2 (NS) and calcined clay content on the durability and rheological properties of LC3 are also investigated.

Chapter 4 mainly investigates the effects of nano-Al2O3 (NA) and nano-SiO2 (NS) on the hydration and mechanical properties of LC3. Gypsum optimisation is carried out first with a moderate dosage of 2% by weight, yielding the highest cumulative heat release. Results suggest that NA exhibits both pozzolanic and nucleation effects, whereas nano-TiO2 (NT) functions only as a nucleation site. However, the beneficial contribution of NA is largely restricted to the very early stages of hydration (within the first day) and even leads to reduced strength in LC3 at later ages. When NS is incorporated into LC3, it is observed to accelerate hydration, reduce workability, and enhance strength at all ages, with particularly significant gains at early ages. Furthermore, this chapter examines the influence of calcined clay (Cc) content on LC3 hydration. A higher Cc content enhances the pozzolanic reaction and improves compressive strength across all curing ages. While increasing Cc raises the total porosity of LC3, it simultaneously refines the pore structure by increasing the proportion of fine pores, despite a concurrent rise in large pores.

Chapter 5 evaluates how NS and Cc influence the sulfate resistance of the LC3 system. The incorporation of NS improves dimensional stability during sulfate exposure, as evidenced by reduced relative mass and length changes. While NS has minimal influence on ettringite formation under standard curing, it significantly inhibits ettringite generation when the system is subjected to sulfate attack.

Increasing the Cc content also enhances sulfate resistance, reflected in lower mass and length variations. Under standard curing, LC3 mixes with Cc achieve higher long-term strength than those without Cc. However, in Na2SO4 solution, only the blend containing 30% Cc shows a clear early age strength improvement, and the long-term strength benefit remains limited.

Sulfate exposure promotes the formation of mono-carboaluminate (Mc), but its overall content decreases as Cc levels rise. Although adding Cc increases ettringite formation across all blends, its influence differs between curing conditions: under standard curing, higher Cc has little effect on ettringite content, whereas under sulfate attack, it distinctly suppresses ettringite formation. Chapter 6 studies the effects of nano-SiO2 (NS) on the rheological behaviour of the LC3 system. Results reveal that increasing NS dosage leads to higher plastic viscosity as well as greater static and dynamic yield stress. Similarly, extending the resting time increases viscosity and both yield stresses (static and dynamic). All mixes exhibit a gradual decrease in phase angle over time, indicating a transition from viscous (fluid-like) to elastic (solid-like) behaviour. Chapter 7 explores the individual and synergistic effects of NS and TEA on the hydration and hardened properties of the LC3 system. The findings show that NS primarily enhances the hydration of the silicate phase, whereas TEA regulates the timing of the silicate peak. For the aluminate peak, both NS and TEA advance its occurrence, with NS exerting a stronger influence. Either additive alone increases the intensity of the aluminate peak, while their combination produces an even stronger synergistic effect. TEA enhances LC3 strength at all ages, whereas NS contributes mainly to early age strength. The synergy of NS and TEA produces more pronounced improvements than either additive alone. Furthermore, TEA primarily alters pore size distribution rather than total porosity, driving a shift from larger to finer pores. The synergistic blend of NS and TEA achieves the most favourable pore size distribution.