Partial replacement of ordinary portland cement by silica fume (SF) accelerates its rate of hydration reactions. This acceleration is attributed to the enhanced heterogeneous nucleation of the main hydration product, i.e., calcium−silicate−hydrate (C−S−H), on the extra surfaces provided by SF. However, such enhancement of C−S−H nucleation is suppressed in the presence of polycarboxylate ether (PCE) dispersant, which is added to regulate the fluidity and rheological properties of fresh paste. A generalized phase boundary nucleation and growth model with time-dependent growth of C−S−H is used to fit the hydration rates of plain and binary (10% to 30% SF) cement pastes prepared with and without PCE. The results show that while SF accelerates cement hydration, increments in hydration rates are significantly smaller in relation to the extra surface area provided by SF. This is because of the agglomeration of SF particles which renders up to 96% of their surface area unavailable for C−S−H nucleation. Furthermore, it is shown that the hydration of cement, in both plain and binary pastes, is suppressed in relation to the PCE dosage. This is because of (a) adsorption of PCE molecules onto cement and SF surfaces resulting in inhibition of sites for product nucleation and (b) interaction of PCE with C− S−H, which suppresses growth of C−S−H throughout the hydration process. It is shown that the effects of nucleation site inhibition by PCE are more pronounced in SF as compared to cement. The outcomes of this study improve our understanding of the mechanisms that drive the hydration of cement in the presence of SF and PCEs.

No abstract

No abstract

Nanomaterials have attracted much interest in cement-based materials during the past 9 decade. In this study, the effects of different nano-CaCO 3 and nano-SiO 2 contents on flowability, heat 10 of hydration, mechanical properties, phase change, and pore structure of ultra-high strength concrete 11 (UHSC) were investigated. The dosages of nano-CaCO 3 were 0, 1.6%, 3.2%, 4.8%, and 6.4%, by the 12 mass of cementitious materials, while the dosages of nano-SiO 2 were 0, 0.5%, 1.0%, 1.5%, and 2%. 13 The results indicated that both nano-CaCO 3 and nano-SiO 2 decreased the flowability and increased 14 the heat of hydration with the increase of their contents. The optimal dosages to enhance 15 compressive and flexural strengths were 1.6% to 4.8% for the nano-CaCO 3 and 0.5% to 1.5% for the 16 nano-SiO 2 . Although compressive and flexural strengths were comparable for the two nanomaterials 17 after 28 d, their strength development tendencies with age were different. UHSC mixtures with 18 nano-SiO 2 showed continuous and sharp increase in strength with age up to 7 d, while those with 19 nano-CaCO 3 showed almost constant strength between 3 and 7 d, but sharp increase thereafter. 20 Thermal gravimetry (TG) analysis demonstrated that the calcium hydroxide (CH) content in UHSC 21 samples decreased significantly with the increase of nano-SiO 2 content, but remained almost 22 constant for those with nano-CaCO 3 . Mercury intrusion porosimetry (MIP) results showed that both 23 porosity and critical pore size decreased with the increase of hydration time as well as the increase of 24 nanoparticles content to an optimal threshold, beyond which porosity decreased. The difference 25 between them was that nano-CaCO 3 mainly reacted with C 3 A to form carboaluminates, while 26 *Corresponding author. Tel./2 nano-SiO 2 reacted with Ca(OH) 2 to form C-S-H. Both nano-CaCO 3 and nano-SiO 2 demonstrated 27 nucleation and filling effects and resulted in less porous and more homogeneous structure. 28 29

The use of silica fume can significantly enhance mechanical properties of concrete given its beneficial filling and pozzolanic effects. In this study, a simple and effective double-side pullout testing method was adopted to characterize the interfacial bond properties, which include pullout load-slip relationship, bond strength, and pullout energy, of steel fiber-matrix in ultra-high strength cement-based material (UHSC) with 0-25% silica fume by the mass of binder. The effects of silica fume content on flowability, heat of hydration, compressive and flexural strengths, hydration products, and pore structure of matrix at different curing time were evaluated as well. Backscatter scanning electron microscopy (BSEM) and micro-hardness measurement were used to examine the quality of interfacial transition zone (ITZ) around the fiber. In terms of the results, the optimal silica fume content could be in the range of 15%-25%. UHSC mixtures with these dosages of silica fume showed significant improvement in pullout behavior. Its bond strength and pullout energy at 28 d could increase by 170% and 250% compared to the reference samples without any silica fume. The microstructural observation verified the findings on the macro-properties development. Formation of more and higher strength of hydration products and refinement of ITZ around the fiber ensured higher micro-hardness, and thus increased the bond to fiber.