Effects of Layered Double Hydroxide Nanosheets with Different Cations on the Compressive Strength of Cement Composites

Two types of layered double-hydroxide (LDH) nanosheets with different cationic laminates were prepared to increase the early mechanical strength of cement to ensure the smooth progress of a cementing project. Different effects of nanosized MgAl-LDH and ZnAl-LDH on the mechanical properties of hardened cement were compared. Results showed that the compressive strength of the hardened cement modified with 0.1 wt % MgAl-LDH or 0.1 wt % ZnAl-LDH increased by 43.2% or 100.4%, respectively, compared to that of plain cement in a 3-day curing period. Meanwhile, the late compressive strength of cement with MgAl-LDH or ZnAl-LDH also increased. The reinforcing mechanism of LDH was analyzed via scanning electron microscopy (SEM), X-ray diffraction, Raman spectroscopy, and Si-29 magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Results indicate that MgAl-LDH and ZnAl-LDH exhibit nanoscale lamellar structures. ZnAl-LDH improved the hydration degree of cement because of its smaller size than MgAl-LDH, resulting in a higher compressive strength of cement composites. Calcium silicate hydrate (CSH) is a key factor affecting the early compressive strength of cement. It was synthesized using the coprecipitation method, and the effect of LDH on CSH was determined. The morphology results and the structure determined via SEM and Si-29 MAS NMR spectroscopy indicated that LDH was responsible for a closer arrangement of the CSH. When ZnAl-LDH was added, CSH was found to be fibrous, the platelets were observed to be smaller, and CSH was more closely packed with smaller pores. Additionally, the mean chain length of CSH containing ZnAl-LDH showed a maximum value, verifying that the introduction of LDH in cement increased the chain length of CSH.