Search

The aim of this work is to understand the characteristics of the pozzolanic reactivity of nanoSiO 2 from studies of its pozzolanic reaction kinetics, morphology and structure of the hydrates and the influences of these features on the properties of cement-based materials, so as to explore a more targeted way of using nanoSiO 2 in cement or concrete. It revealed that the pozzolanic reaction of nanoSiO 2 is of the first-order and the apparent reaction rate constant of nanoSiO 2 -4 nm is about one order of magnitude bigger than that of silica fume, but the specific reaction rate constant is about one half to that of silica fume. A compacter gel structure and poorer crystallinity of the hydrates of nanoSiO 2 to those of silica fume are found, as well. The rate of hydration of cement at very early ages is enhanced by nanoSiO 2 , but the rate slows down with aging due to the compact gel structure. To make the use of the high pozzolanic reactivity and ultrafine particle size of nanoSiO 2 , as well as its resulting compact gel structure, colloidal nanoSiO 2 was applied onto the hardened cement mortar by brushing technique and a less permeable surface was resulted, which shows the potential of using nanoSiO 2 as a surface treatment material for cement-based materials. [ABSTRACT FROM AUTHOR]

A reduced transport property of the superficial part of concrete structures is beneficial for enhancing their durability, due to the blocking of the migration of aggressive agents. In this study, influences of colloidal nano-SiO 2 (CNS) with a mean particle size of 20 nm and its precursor, tetraethoxysilane (TEOS), on the transport properties of hardened cement pastes with water-to-cement ratios of 0.26, 0.38, 0.60 and 1.0 by mass were investigated by measuring the water absorption ratio and water vapor transmission coefficient, after the samples were first soaked in treatment agents for 1 h. Results showed that CNS and TEOS are capable of reducing the transport properties of hardened cement pastes by altering the pore structures. CNS reduces the threshold value and volume of the pores larger than about 0.1 μm, while TEOS reduces the threshold value and volume of the pores smaller than 0.1 μm, and TEOS also reduces the ratio of the ‘ink-bottle’ pore volume to the throat pore volume to a greater extent. A linear relationship between the volume of capillary pores smaller than 0.1/0.01 μm and the transport property was found, which can be ascribed to the reduction of transport properties of hardened cement pastes after surface-treatment with CNS or TEOS. [ABSTRACT FROM AUTHOR]

Highlights: [•] Effects of colloidal nanoSiO2 (CNS) and tetraethoxysilane (TEOS) of surface-treatment of cementitious material were studied. [•] Effects of surface treatment of mortar with CNS and TEOS were attributed to the pozzolanic reactivity and filler effect. [•] CNS and TEOS were able to fill pores larger than 50nm, while TEOS was more effective in filling pores finer than 50nm. [ABSTRACT FROM AUTHOR]

Abstract: To understand the effects of colloidal nanoSiO2 (CNS) on cement hydration and gel properties in the early and later age, hydration heat, calcium morphology, hydroxide content, non-evaporable water (NEW) content and nanoscale mechanical properties were measured. Some comparison studies were conducted on silica fume (SF) paste, as well. Results revealed that the accelerating effect of CNS on hydration in the early age is achieved by the acceleration of cement dissolution and hydrate nucleation on reacted nanoSiO2 particles. Although cement hydration can be greatly accelerated by CNS in the early age, its later age hydration is hindered. The NEW content of CNS-added paste experiences a higher rate of increase initially, but gradually becomes smaller than that of the control paste due to changes in the gel structure, making NEW content an unsuitable method for monitoring the hydration of CNS-added paste. However, nanoindentation results revealed that CNS modifies the gel structure to increase the high-stiffness C–S–H gel content. [Copyright &y& Elsevier]

Abstract: The influences of colloidal nanoSiO2 (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO2 had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO2 on cement/FA hydration at later ages were proposed. [Copyright &y& Elsevier]

The Composite Centered Experimental Design (CCED) using the response surface methodology was used to optimize the synthesis of a hematite‐geopolymer composite, applied to the preparation of a Pb2+ electrochemical sensor. Two important experimental parameters in the formulation (curing time and hematite content) were considered in the optimization of the synthesis of the composite material. The electrochemical signal of Pb2+ at a carbon paste electrode modified by the synthesized composite was used as a response to identify the best formulation. After applying the generated experimental plan, the optimal composite material was obtained using 17.08% hematite and a curing time of 0.16 days. The characterization of the optimized composite material revealed the presence of a porous structure and a poly(phospho‐ferro‐siloxo) network, but with a fraction of well dispersed and unreacted hematite. Preliminary experiments confirmed that the sensor prepared with the composite showed high stability and reproducibility of the Pb (II) signal, justifying its application for accurate sensing of the metal cation in an aqueous solution. A Pb2+ calibration curve was obtained after optimizing the experimental detection parameters (composition of the carbon paste electrode, pH of the accumulation solution, accumulation time, and electrolysis potential). A detection limit of 0.2 nM (based on a signal‐to‐noise ratio of 3) was obtained for a studied concentration range between 0.04 and 0.48 nM. The designed sensor also showed remarkable selectivity and excellent performance in real media samples (spring, tap, rain, and well water). [ABSTRACT FROM AUTHOR]