Silica Fume Replacement Research Articles

Use of pumice stone and silica fume as precursor material for the design of a geopolymer.

Geopolymers are alternative materials to cement because they require less energy in their production process; hence, they contribute to the reduction in CO 2 emissions. This study aims to evaluate the possibility of using industrial residues such as silica fume (SF) to improve the physical and mechanical properties of a pumice stone (PS)-based geopolymer. Through an experimental methodology, the process starts with the extraction, grinding, and sieving of the raw material to carry out the physical and chemical characterization of the resulting material, followed by the dosage of the geopolymer mixture considering the factors that influence the resistance mechanical strength. Finally, the physical and mechanical properties of the geopolymer were characterized. This research was carried out in four stages: characterization of the pumice stone, design of the geopolymer through laboratory tests, application according to the dosage of the concrete, and analysis of the data through a multi-criteria analysis. It was determined that the optimal percentage of SF replacement is 10%, which to improves the properties of the geopolymer allowing to reach a maximum resistance to compression and flexion of 14.10 MPa and 4.78 MPa respectively, showing that there is a direct relationship between the percentage of SF and the resistance. Geopolymer preparation involves the use of PS powder with a composition rich in silicon and aluminum. The factors influencing strength include the ratio of sodium silicate to sodium hydroxide, water content, temperature, curing time, molarity of sodium hydroxide, and binder ratio. The results showed an increase in the compression and flexural strength with 10% SF replacement. The geopolymer's maximum compressive strength indicates its non-structural use, but it can be improved by reducing the PS powder size.

Elucidating the role of recycled concrete powder in low-carbon ultra-high performance concrete (UHPC): Multi-performance evaluation

Reusing recycled powder as an alternative binder for ultra-high performance concrete (UHPC) offers a highly value-added approach to recycling concrete waste. This study shows the multi-performance evaluation of low-carbon UHPC containing recycled powder as a substitution of cement and silica fume. Recycled powder, which includes recycled concrete powder (RCP) and recycled paste powder (RPP), contains massive SiO2, CaCO3 and hydration products. Substituting recycled powder for cement reduces the cumulative hydration heat per gram binder, while increasing the cumulative hydration heat per gram cement of UHPC. Replacing high-volume cement and silica fume with recycled powder negatively impacts the micro-characteristics, increasing the drying shrinkage and transport properties. When using recycled powder as cement replacement, incorporating a moderate dosage of recycled powder has negligible impact and can even confer some beneficial effects on the mechanical strength. Substituting recycled powder for high-volume cement reduces the mechanical strength, yet the blended UHPC retains good mechanical strength. The compressive strength of UHPC containing RCP as 0 %, 30 %, 50 % and 70 % of cement is 157.6 MPa, 154.6 MPa, 145.0 MPa and 129.9 MPa, respectively. Due to its good dilution and active effects, the replacement of cement with recycled powder causes a significant increase in compressive strength per unit cement, and the mix of 70 % RCP increases the compressive strength per unit cement by 174.7 %. Besides, incorporating recycled powder as silica fume replacement causes a limited reduction in the mechanical strength of blended UHPC. The RCP-UHPC generally has better micro-macro properties than the RPP-UHPC. By optimizing recycled powder type and replacement rate, UHPC with excellent micro-macro performance can be achieved.

Unleash the potential of mechanically activated Iron ore tailings in cement mortar containing silica fume and fly ash

This paper presents the effects of mechanically activated Iron Ore Tailings (IOT) in cement mortar containing silica fume, fly ash, and Ordinary Portland Cement (OPC). Five different mixes (i.e. M1- M5) were prepared which include control mix (M1), replacement of cement with IOT (M2), replacement of cement with IOT and silica fume (M3), replacement of cement with IOT and fly ash (M4), and replacement of cement with IOT, fly ash and silica fume (M5). Results reveal that utilization of mechanically activated IOT with 30% replacement of cement, 20% replacement of fly ash, and 10% replacement of silica fume performs well in flowability and compressive strength of cement mortar.

Use of pumice stone and silica fume as precursor material for the design of a geopolymer

Background Geopolymers are alternative materials to cement because they require less energy in their production process; hence, they contribute to the reduction in CO2 emissions. This study aims to evaluate the possibility of using industrial residues such as silica fume (SF) to improve the physical and mechanical properties of a pumice stone (PS)-based geopolymer. Methods Through an experimental methodology, the process starts with the extraction, grinding, and sieving of the raw material to carry out the physical and chemical characterization of the resulting material, followed by the dosage of the geopolymer mixture considering the factors that influence the resistance mechanical strength. Finally, the physical and mechanical properties of the geopolymer were characterized. This research was carried out in four stages: characterization of the pumice stone, design of the geopolymer through laboratory tests, application according to the dosage of the concrete, and analysis of the data through a multi-criteria analysis. Results It was determined that the optimal percentage of SF replacement is 10%, which to improves the properties of the geopolymer allowing to reach a maximum resistance to compression and flexion of 14.10 MPa and 4.78 MPa respectively, showing that there is a direct relationship between the percentage of SF and the resistance. Conclusions Geopolymer preparation involves the use of PS powder with a composition rich in silicon and aluminum. The factors influencing strength include the ratio of sodium silicate to sodium hydroxide, water content, temperature, curing time, molarity of sodium hydroxide, and binder ratio. The results showed an increase in the compression and flexural strength with 10% SF replacement. The geopolymer’s maximum compressive strength indicates its non-structural use, but it can be improved by reducing the PS powder size.

Hydration kinetics and apparent activation energy of cement pastes containing high silica fume content at lower curing temperature

To promote the application of silica fume (SF) in civil engineering in cold regions, relative influences from SF content and low temperature environment on the early hydration kinetics of composite cement pastes were evaluated based on the isothermal calorimetric method. The effect of high SF content on the apparent activation energy (Ea) of Portland cement and the filler effect (via the index of area multiplier) of SF on the hydration parameters was also analyzed. The results showed that the intensity of the shoulder peak (Al peak) of the hydration kinetic curve (heat flow curve) gradually grew as the SF content increased, and the double-peak phenomenon was gradually clear. The heat flow peak is delayed and lowered with the decreasing curing temperature, while the final time of the induction stage and the duration of the acceleration stage are delayed. Moreover, the duration of the acceleration stage was slightly extended with the increase of SF content. The full-width at half-maximum (FWHM) value of the heat flow curves dropped as the SF content and curing temperature increased. The low temperature environment and the inclusion of SF inhibited the growth in early hydration degree. The higher SF content mitigated the impact of low temperature on the hydration time parameter τ. The apparent Ea results also indicated that the SF reduced the temperature sensitivity of the hydration process of the composite cement pastes, which might contribute to the development of the hydration reaction at low temperatures. The AM (Area Multiplier) factor increased approximately linearly with increasing SF replacement levels. Apparent Ea decreased linearly with increasing AM, which may be attributed to a further reduction in the diffusion-controlled hydration rate of composite cement pastes with larger AM.

Creep and fracture of UHSC — A microindentation study

Creep and fracture behaviour of normal strength concrete was investigated by various researchers whereas, research on creep and fracture of Ultra-High-Strength Concrete (UHSC), i.e., concrete with 28-day compressive strength greater than 100MPa is scanty. Also, a detailed investigation of the creep–fracture interaction of UHSC has not been conducted.This study aims at examining creep and fracture of UHSC cement paste mixes (w/b = 0.20) with varying levels of silica fume (SF) replacement (0%, 20%, and 40%) at different ages (3d, 7d, 28d, 90d) using microindentation technique. Two different holding durations, namely 5 s and 180 s, were considered in the indentation test. The findings revealed a noteworthy variation in estimated fracture toughness (Kc) between the two dwelling periods. Longer sustained loading times (180s) indicated a decrease in fracture toughness for all ages considered, suggesting progressive internal cracking during creep phase. Attempt has been made to investigate the interaction effect.

Optimal Quantity Investigation of Metakaolin and Silica Fume in Production of Durable Acid Resistance Alkali Activated Slag Concrete

The urgent requirement to reduce greenhouse gas emissions during Portland cement production and to enhance the durability of concrete in destructive environments are essential reasons to seek other alternative materials like alkaline activated binders. In this study, the feasibility of producing durable alkali-activated slag-based concrete under deteriorative environmental conditions was studied using 0, 10, 20, 30, and 40% of metakaolin (MK) and 0, 5, and 10% of silica fume (SF) instead of ground granulated blast furnace slag (GGBFS) and optimizing contents through the response surface method (RSM).To evaluate the performance of studied alkali-activated slag-based concrete in an aggressive environment, the permeability and the reduction in compressive strength of alkali-activated slag-based concrete under sulfuric acid attack have been investigated. In addition, the mass change of specimens after exposure was measured. The results indicate that replacing 40% of the slag with metakaolin and 10% with silica fume in alkali-activated concrete has reduced 9% and 34.9% of the compressive strength at 28 days, respectively. Also, increasing MK replacement up to 40% increased the water absorption to 27.8%, but 10% SF replacement reduced it to 17.7%. In addition, the alkali-activated slag-based concrete mass changes after 90 days of acid exposure were 2.3%, while the replacement of slag by 40% of MK and 10% of SF reduced this value to 1.14%. However, it improved the durability performance of alkali-activated concrete against sulfuric acid attacks.

Evaluating the physicochemical properties of pervious recycled aggregate concrete incorporating pozzolanic additives under acid rain attack

The utilization of recycled concrete aggregate (RCA) from construction and demolition wastes to produce pervious concrete show various environmental benefits in the construction of sponge city. However, its durability performance in an acid rain attack environment has received less attention. This study aims to understand the deterioration of mechanical properties and microstructure of pervious recycled aggregate concrete (PRAC) exposed to simulated acid rain attack and drying-wetting cycles. The effects of the ground granulated blast-furnace slag (GGBS) replacement ratios, the silica fume (SF) replacement ratios and the erosion times on the mass loss, compressive strength, relative dynamic elastic modulus, permeability coefficient and porosity of PRAC were evaluated. The results demonstrated that the acid rain attack caused obvious mass loss and compressive strength reduction for PRAC without pozzolanic additives, thus resulted in increase on the permeability coefficient and porosity. Moreover, the physicochemical properties of PRAC decreased strongly with the increase of acid rain exposure time. Nevertheless, the addition of 20%GGBF and 7%SF in PRAC showed better resistance to acid attack due to the synergistic effect. The microscopic characterization revealed that the added pozzolanic additives declined the content of calcium hydroxide of matrix in PRAC and thereby inhibited the formation of gypsum, which delayed the acid rain erosion process.

Enhancing Cementitious Concrete Durability and Mechanical Properties through Silica Fume and Micro-Quartz

The existing body of literature has witnessed extensive research efforts dedicated to exploring the impact of supplementary cementitious materials (SCMs) possessing pozzolanic characteristics on concrete. Nevertheless, the holistic concept of micro-scale fillers has frequently been a subject that remains insufficiently explored. This study endeavors to formulate binary cementitious systems that incorporate silica fume (SF) and micro-quartz filler (MQF) to enhance the durability and mechanical properties of cementitious concrete. We systematically investigate the effects of varying replacement levels of SF and MQF, alongside changes in the water-to-binder (w/b) ratio. With w/b ratios spanning 0.25 to 0.40, we explored replacement levels of 8, 10, and 12% (wt.) for SF, and 5, 8, 10, 15, 25, and 35% (wt.) for MQF. The findings revealed a consistent decrease in porosity and permeability as the replacement levels increase. Notably, a marked increase in compressive strength is observed with SF replacement, reaching its peak at an 8% MQF replacement level. Even as MQF replaces 15% of SF, concrete mixtures with 12% SF consistently exhibit superior strength. Importantly, MQF’s ultrafine particle size mirrors SF’s impact on enhancing compressive strength, porosity reduction, and permeability, despite its high crystalline structure. The study employs an analysis of variance (ANOVA) to rigorously assess the influence of each variable on the studied responses.

Experimental study and simulation of workability and compressive performance of SCC with high-volume mineral admixtures

Using industrial waste such as fly ash and silica fume to substitute most of the cement to prepare self-compacting concrete can not only greatly reduce the cement consumption, but also improve the resource utilization of solid waste powder, which is conducive to cost reduction and environmental protection. In this paper, the fly ash (FA) and silica fume (SF) replacement ratio of 40%, 50% and 60% was adopted to produce self-compacting concrete with high-volume mineral admixtures (HVMA-SCC). The slump flow test, L-box test and compression test were conducted, and the effect of different high replacement ratio on the workability, compressive performance and stress–strain relationship of HVMA-SCC were analyzed. The experimental results showed that when the total content of mineral admixtures was the same, an increase in SF content was beneficial for the development of early compressive strength. Under the same SF dosage, the larger the FA dosage, the faster the strength development rate, which was conducive to the sustained growth of HVMA-SCC strength in the later stage. Moreover, a stress–strain calculation model of HVMA-SCC was proposed based on damage theory. Besides, the workability and compression tests were simulated with the discrete element method (DEM). The results showed that the simulated stress–strain curves were in good agreement with experimental curves, and the mesoscopic failure characteristics of HVMA-SCC were analyzed from the development of cracks, stress conditions, and failure modes.

Mechanical behaviour of fiber-reinforced grout in rock bolt reinforcement

Grouted rock bolts subject to axial loading in the field exhibit various failure modes, among which the most predominant one is the bolt-grout interface failure. Thus, mechanical characterization of the grout is essential for understanding its performance in ground support. To date, few studies have been conducted to characterize the mechanical behaviour of fiber-reinforced grout (FRG) in rock bolt reinforcement. Here we experimentally studied the mechanical behaviour of FRG under uniaxial compression, indirect tension, and direct shear loading conditions. We also conducted a series of pullout tests of rebar bolt encapsulated with different grouts including conventional cementitious grout and FRG. FRG was developed using 15% silica fume (SF) replacement of cement (by weight) and steel fiber to achieve high-strength and crack-resistance to overcome drawbacks of the conventional grout. Two types of steel fibers including straight and wavy steel fibers were further added to enhance the grout quality. The effect of fiber shape and fiber volume proportion on the grout mechanical properties were examined. Our experimental results showed that the addition of SF and steel fiber by 1.5% fiber volume proportion could lead to the highest compressive, tensile, and shear strengths of the grout. The minimum volume of fiber that could improve the mechanical properties of grout was found at 0.5%. The scanning electron microscopy (SEM) analysis demonstrated that steel fibers act as an excellent bridge to prevent the cracks from propagating at the interfacial region and hence to aid in maintaining the integrity of the cementitious grout. Our laboratory pullout tests further confirmed that FRG could prevent the cylindrical grout annulus from radial crack and hence improve the rebar's load carrying capacity. Therefore, FRG has a potential to be utilized in civil and mining applications where high-strength and crack-resistance support is required.

Mechanical properties and service life analysis of high strength concrete using different silica fume contents in marine environment in Vietnam

This study evaluated the durability of concrete incorporating different silica fume (SF) content and also computed the service life of this concrete in marine environments. The specimens were prepared with three different SF content (i.e. 8%, 10%, and 12% by mass of cement replaced with SF). Three water/binder ratios (0.25, 0.30, and 0.35) were used for preparing concrete specimens. The mechanical properties were assessed via compression and splitting tensile tests. The rapid chloride penetration and chloride migration experiments were employed to evaluate the durability of concrete. The results showed that the mixture with 10% SF replacement showed the best performance. The inclusion of SF not only reduced the total porosity but also refined the volume fraction of harmless and less harmful pores (< 200 nm). Consequently, the chloride ion penetration resistance of concrete was improved, which in turn reduced the potential corrosion rate of reinforcement. From the results of service life prediction using Life-365 software, it is indicated that the utilization of SF for cement replacement effectively improved the corrosion resistance of steel bars in marine reinforced concrete. The service life against salt damage can be remarkably extended by substituting cement with an appropriate SF level.

Flexural Behavior of Reinforced Rubberized Reactive Powder Concrete Beams under Repeated Loads

Non-biodegradability of rubber tires contributes to pollution and fire hazards in the natural environment. In this study, the flexural behavior of the Rubberized Reactive Powder Concrete (RRPC) beams that contained various proportions and sizes of scrap tire rubber was investigated and compared to the flexural behavior of the regular RPC. Fresh properties, hardened properties, load-deflection relation, first crack load, ultimate load, and crack width are studied and analyzed. Mixes were made using micro steel fiber of the straight type, and they had an aspect ratio of 65. Thirteen beams were tested under two loading points (Repeated loading) with small-scale beams (1100 mm, 150 mm, 100 mm) size. The fine aggregate is replaced by 5, 10, and 15%, respectively, with crumb rubber. While replacement of silica fume was 10, 20, 30, and 50%, respectively, with very fine rubber. Also, chip rubber was added to the mixture as coarse aggregate with 5, 10, and 15%. Five tested beams were chosen as case studies to analyze and compare the results of the ABAQUS software with the experimental results. The results showed that the flexural behavior of RRPC beams that contains rubber was acceptable when compared with the flexural behavior of the RPC beam (depending on load-carrying capacity). The crack width was decreased by including waste rubber and steel fibers. There is a satisfactory agreement between the results of the numerical analysis and the results of the experimental testing. Slight ultimate load differences are targeted between the effects of the monotonic loading and the repeated loading.

Influence of mix proportioning parameters and curing regimes on the properties of ultra-high strength alkali-activated concrete

This study investigates the influence of mix proportioning parameters and curing regimes on the material properties of alkali-activated binder-based ultra-high strength concrete (AAB-UHSC). The effects of five groups of mix proportioning parameters (i.e., fly ash replacement ratio, silica fume replacement ratio, alkali to binder ratio, water glass modulus, and steel fiber content) and three curing regimes(i.e., ambient curing, standard curing, and hybrid curing) on the workability, compressive strength, flexural strength and microstructure development of AAB-UHSC were systematically evaluated. The results indicated that increasing the steel fiber and silica fume dosage negatively affected the flowability of freshly mixed AAB-UHSC. There existed an optimal alkali-to-binder ratio and water glass modulus in terms of flowability, compressive strength, and flexural strength, but the fly ash replacement ratio had a complex effect on the material performance of the AAB-UHSC. The microstructures of the specimens cured in hybrid curing regimes were denser, and fewer cracks were observed, which resulted in higher strength compared to specimens cured under ambient curing and standard curing conditions. The three types of curing regimes adopted barely alter the phases and molecular bonds of hydration products based on the XRD and FTIR results. In addition, the correlations between the compressive strength of the AAB-UHSC obtained under ambient curing conditions, standard curing conditions, and obtained under hybrid curing and standard curing conditions were analyzed, and the fitted results had higher correlation coefficients (R2).

Exploring interpretable ensemble learning to predict mechanical strength and thermal conductivity of aerogel-incorporated concrete

As a new type of cementitious composite material with low carbon and environmental protection, the mechanical strength and thermal conductivity of aerogel-incorporated concrete (AIC) are important performance indicators. However, due to the complexity of its impacting components, it is challenging to forecast material performance only from the mix proportion. In this paper, 660 sets of test data obtained by laboratory investigation were adopted to explore a prediction method for mechanical strength and thermal conductivity of AIC based on interpretable ensemble learning. Important variables such as water-binder ratio, aerogel replacement rate, silica fume replacement rate, age, and dry/saturated state were selected as input parameters. The ensemble learning model was compared with four traditional machine learning algorithms. Cross-validation and grid search were applied to the hyperparameter optimization. The results showed that integrated learning was superior to traditional machine learning methods, and the MAPE of compressive strength, flexural strength and thermal conductivity decreased by 69.8%, 63.6% and 53.7% on average, respectively. In ensemble learning, the prediction accuracy of the Boosting algorithm was better than that of the Bagging algorithm, and the R2 of all prediction models was higher than 0.97. The LightGBM was the best in the prediction of compressive strength and thermal conductivity, while the XGBoost was in the prediction of flexural strength. The most important factor affecting mechanical strength of AIC was water-binder ratio and the replacement rate of aerogel, and the SHAP values could all reach above 20.0 MPa. For the thermal conductivity, only the replacement rate of aerogel had an obvious effect, and the SHAP value could achieve more than 1.0 W/m∙K.

Effect of rice husk ash as partial replacement of ordinary Portland cement in ultra-high-performance glass concrete

Ultra-high-performance concrete (UHPC) is a high-tech concrete whose excellent mechanical and durability characteristics are ascribed to the homogeneity and high packing density of its matrix. However, due to the high contents of cement and silica fume usually necessaries for achieving this packing density, UHPC's final cost and carbon-footprint are far higher than standard concretes. Therefore, over the last few years, the spotlight of UHPC’s research has focused sharply on the analysis of locally available supplementary cementitious materials as partial replacement of cement and silica fume. This article presents an investigation to analyze the effect of rice husk ash (RHA) as partial substitution of cement in a previously optimized mixture of recycled-glass-UHPC by means of several statistical tools. Cementitious materials employed in this research involved silica fume, limestone powder, recycled glass powder and ordinary Portland cement (OPC). Based on the results, it can be concluded that RHA addition in the optimized glass UPHC yields to a significant decrease in the workability of concrete and a slight decrease of compressive strength. Results also demonstrated the high interaction between the RHA and water contents, which could be ascribed to the water absorption and internal curing provided by RHA particles.

Multi-Response Optimization of Semi-Lightweight Concrete Incorporating Expanded Polystyrene Beads

The utilization of expanded polystyrene (EPS) beads in semi-lightweight concrete (SLC) intended for repair and building applications has gained great attention in recent years. This study examines the effect of mix design parameters including binder content, water-to-binder ratio (w/b), EPS content, and silica fume (SF) additions on the mechanical properties and durability of SLC mixtures. The experimental program was carried out following the Taguchi approach for four parameters, each having three levels, to produce an L9 orthogonal array. The performance criteria under investigation were the superplasticizer demand, density, compressive strength, splitting tensile strength, ultrasonic pulse velocity, water absorption, sorptivity, and abrasion resistance. Test results showed that the w/b and EPS content were the most contributing parameters that altered the SLCs performance. The multi-response optimization method (TOPSIS) revealed that superior performance could be achieved using a binder content of 375 kg/m3, a w/b of 0.45, an EPS content of 3 kg/m3, and a SF replacement rate of 8%. The mix design parameters were utilized to create multivariate regression models to predict the SLCs mechanical and durability properties. Such data can be of particular benefit to engineers seeking the use of lightweight materials for sustainable construction with optimized durability and a reduced cement carbon footprint.

A study on the hydration of calcium aluminate cement pastes containing silica fume using non-contact electrical resistivity measurement

In this study, a non-contact electrical resistivity method is employed to investigate the hydration process of calcium aluminate cement (CAC) pastes incorporating silica fume (SF) at an early age. The CAC-SF pastes have a water-to-binder ratio (w/b) of 0.6 and a replacement of SF for CAC at 0, 5, 10, and 20% (by weight of the binder). The pastes are tested for electrical resistivity, thermal analysis (internal temperature), setting time, and compressive strength, and the hydration products of the pastes are also examined. The results indicate that the evolution of electrical resistivity is closely related to the hydration process of the CAC pastes, and the conversion stage is closely associated with the setting time of pastes. As the increase of SF replacement, the hydration process of CAC pastes was accelerated within 24 h, and the development of compressive strength was hampered at an early age but increased later. SF replacement inhibits the conversion of metastable CAC hydrates (CAH10 and C2AH8) to C3AH6 through filling, seeding, and reacting with CAC, thus reducing the heat of hydration.

Mechanical Characteristics of Slurry Infiltrated Fiber Concrete

This paper presents and evaluates a new method for producing Slurry Infiltrated Fiber Concrete (SIFCON). A study was carried out to investigate the impact on the mechanical properties of SIFCON, where several cubes, cylinders, and prisms were cast to compare the proposed gradual and the traditional multi-layer method using the same mixing proportions: 6% straight steel fiber, cement, sand, 10% silica fume replacement, and 2.4% superplasticizer. The results showed that when using the proposed gradual mix method, compressive strength, splitting tensile strength, modulus of elasticity, and modulus of rupture improved by 15.21%, 8.06%, 9.07%, and 5.26% compared to the traditional multi-layer method.

Hydration characteristics, hydration products and microstructure of reactive powder concrete

Reactive Powder Concrete (RPC) is a new type of cementitious materials with a complex hydration mechanism, and active admixtures greatly influence the hydration reaction, formation of hydration products, and evolution of microstructure. In order to comprehensively study the quantitative effects of active admixtures contents, namely silica fume, slag and fly ash, on hydration characteristics, hydration products, and microstructure of RPCs, tests of workability, setting time, electrical conductivity, bound water and mechanical properties were conducted. Furthermore, a series of properties including morphology and micro-structure characteristics of RPCs were analyzed by thermogravimetric (TG) analysis, X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), mercury intrusion porosimetry (MIP), Brunauer-Emmet-Teller (BET), and scanning electron microscope (SEM). The results indicate that the initial hydration reaction rate of RPCs is reduced by partly replacing cement with active admixtures. The pozzolanic effect created by the active admixtures enhances hydration and improves RPC's compressive and flexural strength. RPCs made of cement-silica fume mixture exhibit the best macroscopic properties. The adoption of silica fume promotes the production of C–S–H gel during hydration and exerts pozzolanic and crystal nucleation effects to promote cement hydration. RPCs made of pure cement exhibit 15.3% porosity after 28 days of hydration, with the largest proportion of less harmful pores in the microstructure. The porosity is reduced to 5.2% when cement is partially replaced with silica fume, and the microstructure is dominated by harmless pores. When replacement of silica fume is kept at 25%, using slag powder or fly ash substitute part of cement also reduces the number of less harmful pores. It is beneficial to add slag powder to increase the number of gel pores, whereas fly ash reduces the number of gel pores. The investigation presented in this paper would contribute to the production of low cost and environmentally-friendly RPCs, and accelerate the wider applications of ultra-high performance concrete (UHPC) in engineering structures.