High strength concrete containing natural pozzolan and silica fume

Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Long-term chloride diffusion in silica fume concrete in harsh marine climates

Supplementary cementitious materials such as silica fume are typically necessary for producing high performance concrete for marine environments in hot regions, such as the Persian Gulf. Silica fume use generally improves the strength and/or durability properties of the concrete. This paper investigates the effects of silica fume on various properties of concrete specimens that were exposed to Persian Gulf conditions. Samples were taken at the ages of 3, 9 and 36 months and analyzed to determine the chloride diffusion coefficient. The results show that partial cement replacement with up to 7.5% silica fume reduces the diffusion coefficient, whereas for higher replacement rates the diffusion coefficient does not decrease significantly. Also time-dependent chloride diffusion and compressive strength of concrete containing silica fume are investigated.     

Prediction model of carbonation depth for recycled aggregate concrete

The prediction of carbonation depth for recycled aggregate concrete (RAC) is investigated in this paper. The existing prediction models were evaluated, and it showed that the coefficient of variation (COV) of model error for the existing models is high. By introducing the weighed water absorption of aggregates, the COV of model error can be effectively decreased. Compared with the existing models, the proposed model can predict more accurate carbonation depths. For RAC specimens, compared with the fib model and Xiao and Lei's model-a, the COV of model error of the proposed model is 0.36 which is decreased by 33.3%, and when compared with Xiao and Lei's model-b and Silva et al.’s model, the corresponding decreases are 55.2% and 16.2%. Finally, the proposed model is validated by a 10-year-old carbonation experiment, which indicates that the proposed model is reasonable and can be applied to predict the carbonation depth of RAC.     

Preparation of high silica microporous zeolite SSZ-13 using solid waste silica fume as silica source

Silica fume (SF) is a kind of solid waste that produced in the process of industrial silicon smelting. The disposal of SF for environmental problem is of great urgency. Here, a facile and novel one-step approach of high silica microporous materials SSZ-13 (SF-SSZ-13) were hydrothermally synthesized using silica fume (SF) as silica source. This method requires significantly shorter reaction times (48?h) compared to conventional SSZ-13. The as-synthesized SF-SSZ-13 exhibited high purity structure, popcorn-like morphology, and a large BET surface area of 545.74?m²?g^?1. Additionally, on the basis of controlled growth under different hydrothermal times, the formation mechanism of the SF-SSZ-13 outlined for further extension to other materials. The results on time- and energy-efficient of SF-based preparation of SSZ-13 pave the way for the reducing the cost of production of raw materials and decreasing environment load of solid waste, and also extend the application of silica fume.     

Biphasic carbonation behaviour of structural lightweight aggregate concrete produced with different types of binder

This study aims to characterise the carbonation behaviour of structural lightweight aggregate concrete (SLWAC) produced with different types of aggregate, w/c ratios and types and content of mineral admixtures (silica fume, fly ash, lime filler) by means of accelerated carbonation tests. A new biphasic model to describe the carbonation behaviour of SLWAC is suggested. Based on this model, the carbonation coefficient of SLWAC is estimated as a function of the w/c ratio, for different density classes of lightweight aggregates. It is concluded that the carbonation resistance of SLWAC cannot be properly predicted by its compressive strength and that the carbonation rate is little affected by the type of binder, being appropriate to neglect the contribution of mineral admixtures in carbonation resistance of SLWAC. It is also concluded that even with pastes of moderate quality, the carbonation mechanism should not be relevant to the durability of SLWAC.     

Prediction of long-term chloride diffusion in silica fume concrete in a marine environment

Chloride-induced corrosion is the main factor in determining the durability and service life of the reinforced concrete structures exposed to marine environments. Recognition of chloride diffusion phenomenon in concrete and developing a prediction model that can estimate the service life of the concrete structures subject to long-term exposure is vital for aggressive marine environments. The present study focuses on developing such a prediction model of chloride diffusion coefficient for silica fume concrete under long-term exposure to a durability site located in the southern region of Iran. All investigations are based on 16 concrete mix designs containing silica fume with variable water-to-binder ratios exposed to sea water for maximum period of 60 months. This empirical model is developed by applying regression analysis based on Fick’s second law on the experimental results and is compared with previous studies in this area. This comparison indicates that the predicted chloride diffusion coefficient level is within a ±25% error margin in the specimens. The results indicate that reducing the water-to-binder ratio and adding the silica fume to a dosage of 10% reduces the chloride diffusion coefficient in concrete. This study also confirms that the chloride diffusion coefficient increases with temperature and decreases over time.     

Property investigation of individual phases in cementitious composites containing silica fume and fly ash

In this study, nanoindentation was used to investigate the microstructures of cementitious composites containing silica fume and fly ash. With the help of scanning electron microscope, the mechanical properties (elastic modulus and hardness) of individual phases (like outer product, inner product, calcium hydroxide, remained fly ash particles, residual cement grains) in cementitious composites containing silica fume and fly ash were investigated and analyzed. Additionally, this study examined the differences between the ‘C–S–H’ phases in the different cementitious composites and provided an insight into the influence of mineral admixtures (silica fume and fly ash) on the properties of the ‘C–S–H’ phase.     

Rheological behaviour of ultra-high performance cementitious composites containing high amounts of silica fume

This study investigates the rheological behaviour of ultra-high performance cementitious composite mortars containing 15–25 % of silica fume. The utilization of two Portland cements with different mineralogical compositions and their influence on yield stress of mortar was monitored. The coaxial rheometer was used for determination of flow curves of tested samples. It was found that besides the relation between flow and water-to-binder ratio, there is also a substantial relationship with the mortar composition, in particular with the content of silica fume. The yield stress can be described by an exponential function of volume content of solids in the mortar. Such a function can describe not only the influence of granulometry but also the impact of structure formation on early age Portland cement hydration. It was found that the estimation of yield stress can be done even by a simple modular in-field technique such as a spread flow test.     

Combined effect of silica fume and steel fibers on the impact resistance and mechanical properties of concrete

This study investigated the impact resistance and mechanical properties of steel fiber-reinforced concrete with water–cement ratios of 0.46 and 0.36, with and without the addition of silica fume. Hooked steel fibers with 60-mm length and an aspect ratio of 80, with three volume fractions of 0%, 0.5%, and 1% were used as the reinforcing material. In pre-determined mixtures, silica fume is used as a cement replacement material at 8% weight of cement. The experimental results show that incorporation steel fibers improve the strength performance of concrete, particularly the splitting tensile and the flexural strengths. A remarkable improvement was observed in impact resistance of the fibrous concretes, as compared with the reference materials. The results demonstrate that when steel fiber is introduced into the specimens including silica fume, the impact resistance and the ductility of the resulting concrete are considerably increased.     

Evaluation of a carbonation model for existing concrete facades and balconies by consecutive field measurements

The square root model is widely used to predict the initiation phase of reinforcement corrosion induced by carbonation of the concrete cover. The model is based on diffusion laws which makes its validity arguable. The model has been accused of not being able to model accurately carbonation in structures exposed to drying/wetting cycles. The model was evaluated by field measurements on 18 existing concrete buildings conducted twice at an average interval of 8 years. Data from individual parallel samples as well as averaged measurement data were produced. The propagation of carbonation over a certain period of time and variation in the carbonation coefficient were studied using the data. Individual measurements indicated high variation and even inconsistency in carbonation depth. Thus, the carbonation coefficient calculated for the square root model also varied widely. Despite the high scatter, the averaged carbonation of many buildings was found to be closely in line with the prediction of the square root model.     

Reactive transport modelling of long-term carbonation

Concrete specimens of sulphate resistant Portland cement (SRPC) exposed to natural carbonation in a controlled environment for 13 years revealed that the carbonation was significantly underestimated by accelerated tests and extrapolation by a linear diffusion equation. The results confirmed that carbonation as a phenomenon is too complex to be predicted by a conventional diffusion equation for periods considerably exceeding the experimentally covered period. A comprehensive approach consisting of a thermodynamic model and a statistical methodology to simulate long-term carbonation verified by versatile experimental data was presented. As an application of the method, long-term carbonation was simulated for the SRPC concretes.     

Pore structure and chloride ion permeability of mortars containing silica fume

The addition of silica fume in concrete causes a remarkable increase in strength and a drastic reduction in chloride ion permeability. These effects may be due primarily to microstructural changes both in the cement paste phase and in the interfacial zone around aggregates. The standard method of test for rapid determination of the chloride permeability of concrete, AASHTO T 277–831, has increasingly been used to evaluate the permeability of concrete. However, for the concrete containing silica fume, the results of the AASHTO T 277–831 test, which is expressed in terms of electrical charge passed, do not necessarily reflect the real diffusion index of chloride ion through the concrete. There seems to be factors other than the pore structure which govern the results of the AASHTO T 277–831 test in the concrete containing silica fume. In this study, the effects of silica fume to reduce the chloride ion permeability of the mortar were investigated based on the results of pore size distribution measurements, X-ray diffraction analysis, SEM observations and pore solution extraction. The application of the AASHTO T 277–831 test to the evaluation of the chloride ion permeability of the concrete containing silica fume was discussed.     

Investigation into the synergistic effects in ternary cementitious systems containing portland cement, fly ash and silica fume

This research was primarily conducted to verify the presence of synergistic effects in ternary cementitious systems containing portland cement (OPC), class C fly ash (FA) and silica fume (SF). A subsequent objective of the study was to quantify the magnitude of the synergy and to determine its source. For a ternary mixture containing 20% FA and 5% SF by mass, the synergistic effect was observed mostly at later ages (7 days onward) and it resulted in an increased compressive strength and resistance to chloride ion penetration as well as a reduced rate of water absorption (sorptivity) compared to predictions based on individual effects of FA and SF in respective binary systems. The observed synergy was attributed to both chemical and physical effects. The chemical effect manifested itself in the form of an increased amount of hydration products. The physical effect associated with packing density was, somewhat contrary to general belief, not due to an optimized particle size distribution of the binder components of the ternary cementitious system. Instead, it was the result of smaller initial inter-particle spacing caused by lower specific gravities of both FA and SF which, in turn, led to a lower volumetric w/cm. If the mixture design was adjusted to account for these differences, the physical effect would be diminished.     

Microanalysis and optimization-based estimation of C-S-H contents of cementitious systems containing fly ash and silica fume

In this study, a new approach to characterize hardened pastes of pure portland cement as well as those containing cement with supplementary cementitious materials (SCM) was adopted using scanning electron microscopy (SEM) and energy dispersive X-ray spectra (EDS) microanalyses. The volume stoichiometry of the hydration reactions was used to estimate the quantities of the primary and secondary calcium silicate hydrate (C-S-H) and the calcium hydroxide produced by these reactions. The 3D plots of Si/Ca, Al/Ca and S/Ca atom ratios given by the microanalyses were compared with the estimated quantities of C-S-H to successfully determine the Ca/Si ratio of eleven different cementitious systems at four different ages using a constrained nonlinear least squares optimization formulation by General Algebraic Modeling System (GAMS). The estimated mass fraction of calcium hydroxide from the above method agreed well with the calcium hydroxide content determined from the thermogravimetric analyses (TGA).     

Influence of mix design on the carbonation,mechanical properties and microstructure of reactive MgO cement-based concrete

This study assesses the influence of mix design on the hydration and carbonation of reactive MgO cement (RMC)-based concrete formulations by varying the water and cement contents. Samples were subjected to accelerated carbonation under 10% CO₂ for up to 28 days and compared with corresponding PC-based samples. Their performance was analyzed by compressive strength, porosity, density, water sorptivity and thermal conductivity measurements. XRD, TGA/DSC and FESEM/SEM analyses were employed to investigate the formation of hydration and carbonation products and microstructural development. RMC samples achieved 28-day strengths of 62 MPa, which was comparable with PC samples. Strength gain of RMC samples was accompanied with a substantial decrease in porosity, determined by the amount and morphology of carbonates. The initial water content was more influential on final performance and thermal conductivity than cement content. Lower water contents led to higher strengths due to lower porosities and faster CO₂ diffusion within dry mediums.     

Relation between carbonation resistance,mix design and exposure of mortar and concrete

When cement with mineral additions is employed, the carbonation resistance of mortar and concrete may be decreased. In this study, mortars containing mineral additions are exposed both to accelerated carbonation (1% and 4% CO₂) and to natural carbonation. Additionally, concrete mixtures produced with different cements, water-to-cement ratios and paste volumes are exposed to natural carbonation. The comparison of the carbonation coefficients determined in the different exposure conditions indicates that mortar and concrete containing slag and microsilica underperform in the accelerated carbonation test compared to field conditions. The carbonation resistance in mortar and concrete is mainly governed by the CO₂ buffer capacity per volume of cement paste. It can be expressed by the ratio between water added during production and the amount of reactive CaO present in the binder (w/CaO_reactive) resulting in a novel parameter to assess carbonation resistance of mortar and concrete containing mineral additions.     

The influence of metakaolin and silica fume on the chemistry of alkali–silica reaction products

This investigation studies the influence of two mineral admixtures, silica fume (SF) and high-reactivity metakaolin (HRM), on the chemistry of alkali–silica reaction (ASR) products. Four different mortar bar mixes containing different combinations of high-alkali cement, alkali–inert dolomitic limestone, reactive Beltane opal, HRM, and SF were prepared and stored in a 1 N NaOH solution at 80°C (ASTM C 1260) for 21 days. Expansion of bar specimens was measured, and chemical analysis was performed at different ages using X-ray spectra and maps. Test results confirmed that HRM and SF significantly reduce expansion due to ASR. In addition, X-ray microanalysis showed that calcium content increases with time in ASR products. Furthermore, it was found that as ASR proceeded the calcium content of reaction products increased proportionally as the silica content decreased.     

Development of the geopolymer-based zeolite from powdery silica fume and its application in methanol to dimethyl ether

A large number of non-biodegradable powdery silica fume waste produced in industry increase the severity of environmental problems due to their potential harmfulness. The synthesis of geopolymers or zeolitic materials using powdery silica fume as silicon source has become an attractive sustainable solution to remedy this crisis. In this paper, a cost-effective porous ferrierite was synthesized by a facile hydrothermal strategy using silica fume-based geopolymer as precursor and applied to methyl dimethyl ether reaction (MTD). The specific surface area of geopolymer increased from 30.5 to 258.4 m²/g of ferrierite, and the methanol conversion rate correspondingly improved from 16.04 % to 78.5 %. Moreover, rare earth (RE) metal ions were introduced to optimize the performance, and the maximum conversions of RE-modified ferrierite were higher than 90 %. The superior catalytic performance of RE-modified ferrierite was related to the synergy of abundant porosity, acidic active sites and metal functions, which could be realized by regulating the crystallization degrees and ion exchange concentrations. Moreover, the abundant active amorphous silica in silica fume is conducive to the preparation of eco-friendly geopolymers and the further development of zeolite materials, realizing the high value-added utilization of solid wastes.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.