Characterization and adaptability of layered double hydroxides in cement paste

Abstract

Layered double hydroxides (LDHs) improve the durability of concrete, whereas theirs characterization and adaptability in cement-based matrix has not been systematically investigated. Setting time, chemical shrinkage, rheology, compressive strength, hydration degree, pore structure as well as micro-mechanisms of high sulphate-resistant Portland cement (HSRPC) incorporating different LDHs, namely, CaFe-NO3 LDHs (LDHs-CF), CaAl-NO3 LDHs (LDHs-CA), and CaFeAl-NO3 LDHs (LDHs-CFA), were explored. At any types of LDHs, the trend of compressive strength of HSRPC pastes increased firstly and then decreased afterward with the addition of LDHs dosage, while the addition of LDHs increased the chemical shrinkage values of pastes during the early stage, but 28 d chemical shrinkage values were not particularly notable. Furthermore, radar map showed that the shortening of setting time, the increase of dynamic yield stress of samples presented great change with the increase of LDHs dosage, whereas the 28 d chemical shrinkage value changed little in all samples. LDHs, especially LDHs-CF and LDHs-CFA, reduced the volume fractions of harmful pores, refined on pore structure and increased tortuosity of samples since Fe-based LDHs have smaller nanoscale dimension and lower crystal transformation. The acceleration of cement hydration, decrease of workability and improvement of mechanical property at early stages were due to the interstitial filling and crystal seed effects, crystal transformation, or/and absorption of free water by LDHs 2D nano-platelets. In conclusion, LDHs shows good adaptability in cement paste, and as an effective hardening accelerator for HSRPC hydration.