Influence of silica fume on the microstructure of cement pastes: New insights from 1H NMR relaxometry

a r t i c l e i n f o Keywords: 1 H NMR (B) Silica fume (D) C–S–H composition (B) Density (B) 1 H NMR has been used to characterise white Portland cement paste incorporating 10 wt.% of silica fume. Samples were measured sealed throughout the hydration without sample drying. Paste compositions and C–S–H characteristics are calculated based on 1 H NMR signal intensities and relaxation analysis. The results are compared with a similar study of plain white cement paste. While the presence of silica fume has little influence on C–S–H densities, the chemical composition is impacted. After 28 days of sealed hydration, the Ca/(Si + Al) ratio of the C–S–H is 1.33 and the H 2 O/(Si + Al) ratio is 1.10 when 10% of silica fume is added to the white cement. A densification of the C–S–H with time is observed. There are no major changes in capillary, C–S–H gel and interlayer pore sizes for the paste containing silica fume compared to the plain white cement paste. However, the gel/interlayer water ratio increases in the silica fume blend. Basic cement hydration involves the dissolution of alite (C 3 S) and belite (C 2 S) with the formation, by precipitation, of calcium silicate hydrates (C–S–H) and calcium hydroxide (CH, also called " portlandite "). While calcium hydroxide is a very well known crystalline phase [1,2], the C–S–H structure, chemical composition, density and morphology remain uncertain [3]. The primary reasons are twofold: first, C–S–H is a highly disordered nanoscale material incorporating a significant amount of water that is difficult to probe experimentally. Only a few techniques are able to adequately characterise as-prepared C–S–H without removing the water, a procedure that damages the nanoscale structures that are of interest. Secondly, the way C–S–H precipitates is highly dependent on the chemical and physical conditions in which the hydration takes place [4–6]. The different phenomena leading to C–S–H precipitation become even more complex when supplementary cementitious materials (SCMs) are used. Some SCMs act only as nucleation sites for the C–S–H to grow: the so-called filler effect [7]; others cause pozzolanic reactions, i.e., the consumption of CH to form more C–S–H. Silica fume is a by-product from the production of elemental silicon and its alloys. Silica fume is typically used at replacement levels of 5–10% in high performance concrete to improve strength and durability. It not only can react with calcium hydroxide …