Cement-silica Fume Paste Research Articles

Effect of sulfate on the compatibility between polycarboxylate superplasticizer and cement-based materials with or without silica fume

Polycarboxylate superplasticizer (PCE) is a crucial raw material used for preparing concrete. However, there has always been a compatibility issue between PCE and different types of cementitious materials due to their varying qualities. In concrete raw materials, the types and contents of sulfates have a significant impact on compatibility, and it is well-established that a competitive adsorption exists between PCE and sulfate ions solubilized in the interstitial solution of cement paste, which may cause a loss of the dispersing properties. However, previous studies focused on the effect of sulfate on the compatibility between PCE and cement, and there was a lack of research on silica fume. In this study, the workability of fresh cement and cement-silica fume paste and the mechanical properties of harden mortar were evaluated in the presence of different concentrations of five types of sulfates (Na2SO4, K2SO4, MgSO4, KAl(SO4)2, and CaSO4·2H2O) added in aqueous solution. The results show that soluble sulfates have a more adverse effect on the compatibility between PCE and cement-based materials with or without silica fume than slightly sulfate. And the effects of different sulfates on saturation points of PCE in cementitious materials and the mechanical properties of cement and cement-silica fume mortar are various. Finally, it is important to note that a high sulfate content in the solution can lead to salt precipitation of PCE, which is highly undesirable.

Effect of PCEs with different structures on hydration and properties of cementitious materials with low water-to-binder ratio

Polycarboxylate ethers (PCEs) with different structures and densities of anchor groups in the backbone were synthesized and characterized. The surface tension of their solutions, their adsorption on cementitious particles and zeta potential of cement-silica fume pastes with PCEs were measured. Their effects on hydration and properties of cement-silica fume paste with low water-to-binder ratio were examined. Results indicated that the adsorption kinetics of theses PCEs on cementitious particles are fitted the pseudo-second-order model. The introduction of long silane chain on PCE decreased its electrostatic adsorption, while PCE was adsorbed onto the surface of particles via both chemical and electrostatic adsorption, when silicon hydroxyl is directly connected to its backbone. In the presence of PCE with higher adsorbed amount, pastes exhibited lower yield stress and higher flowability, and higher strength at 7 d and 28 d. The low plastic viscosity, autogenous and drying shrinkage were related to the low surface tension of PCE solution.

Effects of polycarboxylate ether grafted silica fume on flowability, rheological behavior and mechanical properties of cement-silica fume paste with low water-binder ratio

Utilizing low water-binder ratio and high dosage of supplementary cementitious materials is the effective way to improve the performance of concrete, however, it is often accompanied by poor workability. Herein, two kinds of polycarboxylate ether grafted silica fume (VPCE-SF/KPCE-SF) were prepared by grafting comb-like PCE onto the surface of SF. The flowability, rheological behavior and mechanical properties of cement-SF paste with VPCE-SF/KPCE-SF were investigated at water-binder ratio was 0.18. The results indicated that VPCE-SF and KPCE-SF were both displayed more effective dispersibility than comb-like PCE in cement-SF paste. Rheological analysis showed that the yield stress and viscosity of cement-SF paste with KPCE-SF were lower than that of VPCE-SF. It is due to the adsorption ability of KPCE-SF on cement and SF particles were stronger than VPCE-SF. The flexural and compressive strength of cement-SF paste with KPCE-SF were lower than that of VPCE-SF at 3 d, since cement-SF paste with KPCE-SF represented stronger retardation effect on hydration than VPCE-SF. However, cement-SF paste with KPCE-SF has higher flexural and compressive strength compared with VPCE-SF at 7 d and 28 d.

Investigation on the Effect of Silica Fume on Shrinkage Characteristics of Portland Cement-Silica Fume Paste

The ring-test was used to evaluate the plastic shrinkage crack tendency of paste specimen added with silica fume in this investigation. Silica fume was used as a partial replacement by weight of cement from 0% to 30%. The compressive strength and drying shrinkage were also examined. The test results show that the addition of SF (up to 30%) increases the compressive strength of paste. No significant influence of silica fume was found on the drying shrinkage of paste. The crack index is significantly increased in the paste specimen added with 10% and 20% replacement. This shows that the addition of SF increases the plastic shrinkage cracking tendency of paste. Using the ring-test method to detect the plastic shrinkage cracking of SF paste can provide more accurate measurements, which is helpful to understand the plastic shrinkage crack tendency of paste added with SF.

Effectiveness of Polycarboxylate Superplasticizers in Ultra-High Strength Concrete: The Importance of PCE Compatibility with Silica Fume

Methacrylate ester as well as allylether based polycarboxylates (PCEs) were synthesized to plasticize pastes of cement and silica fume having a water/cement ratio of 0.22. Methacrylate ester copolymers were found to disperse cement well, whereas allylether copolymers are more effective with silica fume. Mechanistic investigations revealed that in cement pore solution, the surface charge of silica fume becomes positive by adsorption of Ca 2+ onto negatively charged silanolate groups present on the silica surface. This way, polycarboxylate copolymers adsorb to and disperse silica fume grains. Thus, mixtures of both copolymers were tested in cement-silica fume pastes. These blends provide significantly better dispersion than using only one polymer. Apparently, the surfaces of hydrating cement (here mainly ettringite) and silica fume are quite different with respect to their chemical composition. Therefore, PCEs with different molecular architectures are required to provide maximum coordination with calcium atoms present on these surfaces.

Size and Volume Relationship of Pore for Construction Materials

A general relationship between cumulative pore volume and pore sizes as measured through mercury intrusion porosimetry (MIP) and BSE image analysis technique is presented in this paper. From the results of the MIP study conducted on hardened cement paste, cement silica fume paste, cement silica fume sand mortar, brick, soil cement, and concrete, it is observed that a general relationship exists between applied pressure and intruded volume of mercury. This relationship closely follows the MMF (Morgan, Mercer and Flodin) model. The general relationship between pore sizes and intrusion volume in MIP is then derived from this relationship using Washburn's equation. A similar model is also observed to describe the relationship between cumulative volume and pore sizes obtained through BSE image analysis. The variations of the parameters namely median pore radius and dispersion coefficient (a measure of the spread of pore sizes), with silica fume content, ratio of water-to cementitious material and age are then discussed to demonstrate the utility of the model.

Enhancing Water Resistance of Cement and Gypsum-Cement Materials

In recent years silica fume, a by-product in the production of metallic silicon or ferrosilicon, has gained acceptance for use as a mineral admixture in cementitious materials. The principal areas of application of silica fume include producing high strength concretes and improving concrete resistance to aggressive chemicals. A new application of silica fume for enhancing water resistance of cement and gypsum-cement materials is reported. As a characteristic of water resistance the wet/dry strength ratio of the material was used. The values of strength and the wet/dry strength ratio for hardened cement pastes of various ages, with water to cementitious material ratio varying from 0.33 to 0.50, were obtained. It is shown that, despite the low value at 3 days, the wet/dry strength ratio of cement-silica fume paste rises significantly after 28 days of water curing, achieving the value of pure portland cement paste having the same water to cementitious material ratio, and at 200 days exceeds it, approaching a value of 1. However, the most pronounced improvement of water resistance was fixed for the gypsum-cement materials with 5% addition of silica fume, where the value of the wet/dry strength ratio after 200 days of water immersion increased twice as much, compared with the ratio after 3 days of water immersion. The physical and chemical mechanisms of this improvement are discussed.

The role of silica fume in the kinetics and mechanisms during the early stage of cement hydration

The role of silica fume in the reaction kinetics and mechanisms of the early stage hydration of Portland-slag cement–silica fume pastes (W/S=0.5 at 20°C) has been studied. Using differential scanning calorimetry (DSC), the rate of calcium hydroxide (CH) formed during hydration has been determined quantitatively and the percentage of reaction of hydration has been calculated. The kinetic analysis was used to obtain parameters, which were employed to find out the influence of the silica fume content on the reaction rate constant value. The reaction stages have been analysed and explained by kinetic terms. The delay in CH formed during hydration between 8 and 10 h from the beginning of hydration has been noticed and a mechanism has also been discussed. The investigations have shown that the addition of silica fume of more than 8 mass% reduces the duration of the phase boundary interaction as a rate-determined process resulting in the fast diffusion rate-determining process. The results of this study have also revealed evidence of the accelerator effect of silica fume during the first 8 h of hydration when it still exists as chemically inert filler. The pozzolanic reaction between silica fume and CH formed during hydration is occurring after three days of hydration.

Oxygen and water vapour transport in cement–silica fume pastes

Partial replacement of cement with Silica Fume (SF) has been proposed as a method of reducing the corrosion rate of embedded reinforcing steel in concrete. Experimental work has been carried out to determine the effect of the SF on the transport rates of oxygen and water in cementitious samples. By analysing the effect of different pressure drops across the samples it is concluded that the flow of oxygen is described by the Darcy equation, but the flow of water vapour is not. The different mechanisms of transmission cause the transmission rates for oxygen to be spread over a far greater range than those for water vapour with some of the SF samples almost impermeable to oxygen.

Application of A.C. impedance techniques in studies of porous cementitious materials (III): ACIS behavior of very low porosity cementitious systems

The microstructure of very low porosity cement and cement-silica fume paste systems was investigated by A.C. impedance spectroscopy (ACIS). The theoretical relationship between ACIS parameters and the structure of porous materials previously proposed by authors has been confirmed for these systems. Results indicate that a delayed appearance of the high frequency semicircle, present in spectra of normal cement pastes at high water/cement ratios, does not occur in the present cases due to their lower initial porosity. The high frequency arc diameter, R 2, increases significantly with hydration time and compacting pressure; this is compatible with results from other measurements, e.g. mercury intrusion porosimetry, helium pycnometry, etc. The densification effect of silica fume in very low porosity systems has also been demonstrated. It was found that there is an optimum addition of silica fume at which R 2 has a maximum value. It is apparent that ACIS techniques are applicable to studies of the microstructure of very low porosity cementitious systems.

An a.c. impedance spectroscopy study of micro-cracking in cement-based composites during compressive loading

An a.c. impedance technique was applied to the investigation of crack growth mechanisms in portland cement and cement-silica fume pastes, mortars and cement-wollastonite micro-fiber reinforced systems during compressive loading. It was demonstrated that the high frequency arc in the complex plane is very sensitive to microstructural change and micro-cracking in the specimens subjected to compressive load. The experimental results indicate that micro-cracking in different cement-based materials depends on their intrinsic microstructural characteristics.

Mechanism of plastic shrinkage cracking in portland cement and portland cement-silica fume paste and mortar

The objective of this paper is to discuss the mechanism of plastic shrinkage in portland cement and portland cement-silica fume paste and mortar. The effects of the three delivery forms of silica fume; asreceived powder, densified powder, and slurry, on plastic shrinkage are presented. In paste, plastic shrinkage is primarily related to development of tensile capillary pressure during drying. The higher the surface area of particles, the higher the tensile capillary pressure, and consequently, the more vulnerable the system would be to plastic shrinkage cracking. In mortar, plastic shrinkage is controlled by both capillary pressure in the paste and presence of fine aggregate particles. These particles serve to reduce cracking by (a) arresting cracks, and (b) refining the size and distribution of capillary pores. Data of rate of evaporation of water can only indicate degree of drying. Charts developed by Portland Cement Association to calculate the rate of evaporation can not predict whether or not plastic shrinkage cracks occur. A quantitative method could perhaps be developed to predict plastic shrinkage cracking.

Durability of Portland Cement-Silica Fume Pastes in Magnesium and Sodium Sulfate Solutions

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