Insight into the local C-S-H structure and its evolution mechanism controlled by curing regime and Ca/Si ratio

Abstract

Calcium silicate hydrate (C-S-H), as the main hydration product in hardened Portland cement, significantly affects its mechanical properties and durability. A profound insight into nanostructured C-S-H is of great significance for tailoring the microstructure and performance of cementitious materials. However, the effect of high temperature and pressure, which is widely adopted in manufacturing precast concrete components, on the microstructural evolution of C-S-H lacks a systematic study. In this study, the structural features of C-S-H with Ca/Si ratios of 0.83, 1.0, and 1.5, synthesised by calcium oxide and fumed silica curing under steam (80 °C) and autoclaving (180 °C, 1 MPa) conditions were investigated, where a room temperature (25 °C) curing was used as a reference. The chemical components, along with structural and morphological features of C-S-H, were characterised at a multi-scale level using X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, nuclear magnetic resonance spectroscopy, scanning electron microscopy, transmission electron microscopy, and N2 adsorption. The results prove that autoclave curing can improve the crystallinity of C-S-H and destroy the gel pores more than steam curing. A flaky tobermorite and fibrous xonotlite at Ca/Si ratios of 0.83 and 1.0, respectively, are characterised as the main structure. Autoclave curing significantly contributes to an increasing degree of polymerisation of C-S-H owing to the interlayer dehydration and crystalline transformation, with Q2 units directly converted to Q3. When the Ca/Si ratio is increased to 1.5, the decalcification of C-S-H caused by carbonisation contributed to a high polymerisation in the C-S-H structure. The rising temperature promotes the formation of C-S-H with a high Ca/Si ratio, thus exhibiting Q1 units. A detailed exploration of the C-S-H structure helps optimise the mixture and fabrication process according to the performance requirements of precast concrete components.