Silica Fume Cement Pastes Research Articles

Experimental investigation on the seismic behavior of TRAC frame column with RCA treated by the silica fume cement paste

AbstractThe physical properties of recycled coarse aggregate (RCA) can be strengthened by pre‐soaking them in silica fume cement paste, thereby improving the mechanical properties of recycled concrete. To evaluate whether treated recycled coarse aggregate (TRCA), which is obtained by pre‐soaking in silica fume cement paste, can be applied to structural elements, considering the axial compression ratio and volumetric stirrup ratio, TRCA concrete (TRAC) frame column specimens and natural aggregate concrete (NAC) column specimens were prepared. Subsequently, a cyclic loading test was conducted on the specimens. The failure mode, hysteresis loop, skeleton curve, ductility, and energy dissipation capacity of the specimens were analyzed and compared. The results showed that compared with NAC specimens, the bearing capacities of the TRAC specimens were slightly higher. The seismic behavior of the TRAC frame columns achieved performance level similar to that of the NAC structural members at low axial compression ratios. This indicates that the RCA treatment of pre‐soaking in silica fume cement paste is effective, and TRAC can be used as loading‐bearing structural concrete.

Influence of slurried silica fume on microstructure and tritiated water diffusivity of cement pastes

Silica fume cement based materials have long been used in nuclear industry particularly for low-level waste (LLW) and intermediate-level waste (ILW) management. The durability of these materials is therefore of a great importance for the safety of nuclear waste storage.This study investigates the durability in the aspect of tritiated water (HTO) diffusion in relation with the porosity and the microstructure within silica fume cement pastes. Silica fume (SF) was used as a slurry; a liquid mixture of undensified powder and water. Due to its better dispersion, it was found that slurried silica fume improves the pore structure and diffusion properties of SF pastes compared to the use of a densified form of SF.The effect of silica fume and water content were also explored. Silica fume (SF) replacement varying from 0 to 20% by weight of cement was used to prepare cement pastes at water-to-binder ratios (w/b) ranging from 0.3 to 0.5. The manufactured pastes were tested by through-out diffusion and their microstructure characterized by water and mercury porosimetry (MIP), thermogravimetric analysis (TGA) and by scanning electron microscopy (SEM) associated to (EDS) analysis.Test results showed that despite an increase in total porosity, the addition of slurried silica fume has a positive impact on pore refinement and on reducing the effective diffusion coefficients.Finally, HTO diffusion coefficients seem to increase linearly with w/b ratio especially for cement pastes without SF where the increase is more important compared to SF pastes.

Diffusivity in Cement-Silica Fume Based Materials: Experimental and Computer Modeling Results on Mortars

Silica fume cement (SFC) based materials are largely used as a containment barrier for nuclear waste management. The safety of this storage mode depends on the knowledge of the effective diffusion coefficients of such materials. This work proposes a combination of computer models able to estimate the diffusion coefficients of SFC pastes and mortars, from a single investigation of the microstructure by nitrogen adsorption. The approach used consists firstly in manufacturing SFC mortars by varying sand volume fraction from 30 to 65% while silica fume replacement and water to binder ratio were respectively set at 10% and 0.4. Nitrogen adsorption tests were then performed and collected data on C-S-H nature are introduced into a SFC pastes hydration model. The latter provides the mineral composition which is an input parameter in the multilayer transport model that estimates the effective diffusion coefficient (De) of cement pastes. For mortars, a 3D biphasic model (sand and cement matrix) was used to compute the (De) of mortars at different inclusion volume fractions. The numerical results were approved by comparison to experimental data obtained from tritiated water (HTO) diffusion tests performed on manufactured mortars.

Physico-mechanical properties of composite cement pastes containing silica fume and fly ash

This works aims to study the effect of partial substitution of ordinary Portland cement (OPC) by silica fume (SF) and fly ash (FA) on the physico-mechanical properties of the hardened OPC–FA–SF composite cement pastes. The OPC was partially replaced by 20% and 30% fly ash along with 5% and 10% silica fume. The phase composition of the hydration products was investigated using XRD and DTA techniques. It was found that, the increase of FA content in OPC–FA–SF composite cement decreases the water consistency values and increases the setting times. On the other hand, the increase of SF content leads to increase the water of consistency and decrease the setting times. The partial substitution of OPC by FA and SF leads to higher porosity values with a consequent decrease in the compressive strength values especially during the early ages of hydration. At the later ages of hydration, however, the OPC–FA–SF cement pastes possess total porosity and compressive strength values close to those of the neat OPC paste. The lower of free lime contents were obtained for OPC–FA–SF composite cement pastes with the formation of further additional amounts of CSH as a result of the pozzolanic reaction. The results showed also that, the physico-mechanical properties of composite cement paste [OPC (65%)–FA (30%)–SF (5%)] were improved at later ages.

Temperature dependence of autogenous shrinkage of silica fume cement pastes with a very low water–binder ratio

Ultra-high-strength concrete with a large unit cement content undergoes considerable temperature increase inside members due to hydration heat, leading to a higher risk of internal cracking. Hence, the temperature dependence of autogenous shrinkage of cement pastes made with silica fume premixed cement with a water–binder ratio of 0.15 was studied extensively. Development of autogenous shrinkage showed different behaviors before and after the inflection point, and dependence on the temperature after mixing and subsequent temperature histories. The difference in autogenous shrinkage behavior poses problems for winter construction because autogenous shrinkage may increase with decrease in temperature after mixing before the inflection point and with increase in temperature inside concrete members with large cross sections.

Uniaxial compression in carbon fiber-reinforced cement, sensed by electrical resistivity measurement in longitudinal and transverse directions

Uniaxial compression of carbon fiber-reinforced cement pastes in the elastic regime caused reversible decreases in both longitudinal and transverse electrical resistivities. In contrast, uniaxial tension had been previously reported to cause reversible increases in both resistivities. The fractional change in resistivity per unit strain is higher in magnitude for carbon fiber silica fume cement paste than carbon fiber latex cement paste.

Effect of stress on the electric polarization in cement

Compressive stress was found to diminish the extent of electric polarization in the transverse direction in cement pastes with and without carbon fibers, as shown by electrical resistivity measurement conducted over time. In addition, the stress decreased the time for polarization to essentially reach completion. The extent of polarization was much smaller when carbon fibers were present. It was smaller for carbon fiber cement paste containing silica fume than that containing latex. The time for polarization to reach completion was less than 5 s for carbon fiber silica fume cement paste at a compressive stress of 6.74 MPa. Polarization reversal was hastened by stress.

Carbon fiber-reinforced cement as a thermistor

Carbon fiber (∼5 mm long, 0.5% by mass of cement)-reinforced silica fume (15% by mass of cement) cement paste was found to be an effective thermistor. Its electrical resistivity decreased reversibly with increasing temperature (1–45°C), with an activation energy of electrical conduction (electron hopping) of 0.4 eV. This is comparable to those of semiconductors (typical thermistor materials) and higher than that of carbon fiber polymer-matrix composites. Without carbon fibers, or with latex in place of silica fume, the activation energy was much lower and the resistivity was higher. The voltage range for a linear current-voltage characteristic was wider in the absence of fibers. The current-voltage characteristic of carbon fiber-reinforced silica fume cement paste was linear up to 8 V at 20°C.

The role of silica fume in the direct tensile strength of cement-based materials

The direct tensile strength of silica fume cement paste and mortar were evaluated at various water-cementitious content ratios. Four different water-cementitious content ratios of 0.22, 0.25, 0.28, and 0.31 were used, and three contents of silica fume, 8%, 16%, and 25% by mass of cement. Superplasticizer content was adjusted for each mix to ensure that no segregation would occur.

The influence of silica fume on the compressive strength of cement paste and mortar

The compressive strengths of silica fume cement paste and mortar were evaluated at various water-cementitious ratios. Five different water-cementitious ratios were used including, 0.22, 0.25, 0.28, 0.31, and 0.34 and two contents of silica fume, 16% and 25% by weight of cement. Superplasiticizer content was adjusted for each mix to ensure that no segregation would occur. The results show that the increase in compressive strength of mortar containing silica fume, as a partial replacement for cement, greatly contributes to strengthening the bond between the cement paste and aggregate. Partial replacement of cement by silica fume and the addition of superplasticizer increases the strength of mortar but has no influence on the strength of cement paste. Results were verified by statistical analysis using hypothesis testing at a 95% confidence level. It was also demonstrated that superplasticizer in combination with silica fume plays a more effective role in mortar mixes than in paste mixes. This can be attributed to a more efficient utilization of superplasticizer in the mortar mixes due to the better dispersion of the silica fume particles. The paper also reviews some of the available literature on the influence of silica fume on cementitious composites and unsettled questions associated with this topic.

Subcritical crack growth in the cement paste-steel transition zone

The shape of the stress-strain curve for most concretes may be explained in terms of subcritical crack growth in the transition zone between coarse aggregate particles and the cement paste matrix. This paper discusses an investigation of subcritical crack growth in composite double torsion specimens made of steel and cement paste containing silica fume, carbon black, or plain portland cement. At age 7 days, the specimens containing carbon black proved to be less resistant to crack propagation in the transition zone than the other specimens. This result is consistent with the stress-strain behavior of concretes containing carbon black. An examination of the microstructure showed that the carbon black cement paste contained larger and more numerous voids than a comparable silica fume cement paste.