Strength Optimization of Reactive Powder Concrete Notes

Introduction

  • Reactive Powder Concrete (RPC) development achieved by:
    • Eliminating coarse aggregates.
    • Granular optimization.
    • Pressure application during casting.
    • Thermal treatment after hardening.
    • Incorporation of metallic microfibers.
  • Granular optimization strengthens RCP mechanical performance and consistency.
  • Fine aggregates in RCP include quartz and metallic aggregates.
  • Basaltic, granitic, and calcareous aggregates can also be used.
  • Aggregate is not a limiting factor in compressive strength for RCP; >200 MPa can be achieved with low strength aggregates like limestone (83 MPa).
  • RCP has higher supplementary cementitious materials (SCM) content than standard high-performance concretes.
  • Silica fume is the most used SCM in RCP.
  • Other SCMs used: fly ash, blast furnace slag, pulverized phosphorus slag, glass powder, and recycled powder of ceramic bricks to increase granular density.
  • These SCMs improve mixture performance in the fresh state, reduce superplasticizer and cement consumption.
  • Research explores nanoparticles like nanosilica and titanium dioxide to improve RPC performance.
  • RPC demands high energy for mixing due to fine powders.
  • Mixing method influences RPC characteristics.
  • Hiremath and Yaragal [15] investigated mixing speed (25-150 rpm) and time (10-30 min) effects on RPC.
    • Mixture speed should be between 50 and 100 rpm; low speeds compromise performance and high speeds entrain air.
    • Mixing time of 15 min showed better fluidity and strength.
  • Conventional concretes (Class I) improve strength at early ages with thermal cure, but strength reduces at 28+ days; this is not observed in RPC.
  • RPC with 200 MPa compressive strength can be mixed with fine quartz sand, quartz powder, and thermal treatment at 90°C.
  • Steel microfibers increase material ductility and tensile strength.
  • RPC with >600 MPa strength requires casting with ~50 MPa compression loads and thermal treatment at 250-400°C in an autoclave.
  • Metallic aggregates replacing quartz aggregates can further improve mechanical performance.
  • Steel fibers increase RPC ductility, flexural strength, and fracture energy.
  • Han et al. [17] found that 0.5% stainless-steel fibers (8 μm and 20 μm diameters) significantly increased flexural strength and fracture energy.
  • Ji et al. [18] investigated steel fiber content (1-4%) on RPC crack resistance; fiber content significantly affects cracking behavior; 3% fiber content provided best performance.
  • Pressure application in casting increases compressive strength.
  • Ipek et al. [19] studied densification strain effect using a piston, with strains of 25-125 MPa; higher compressive strength reached was 475 MPa at 100 MPa casting strain, plus thermal treatment at 300°C for 12 hours.
  • Aydin et al. [3] showed strength increase could reach 130 MPa comparing autoclave cured concrete (270 MPa) with pressed and autoclave cured concrete (400 MPa).
  • RPC properties, like flexural tensile strength and fracture energy, improve with densification strain.
  • Ipek et al. [20] studied strain application in RPC densification with six levels (0-25 MPa) assessing flexural tensile strength and fracture energy. Flexural tensile strength increased 34% for lowest densification strain; fracture energy increased up to 3 times for the highest strain level.
  • Autoclave curing improves RPC mechanical performance.
  • Yazıcı et al. [21] determined the influence of pressure-temperature (0MPa-20°C; 1 MPa-180°C; 2 MPa-210°C and 3MPa-235°C) and time of autoclave curing (0h; 4h; 6h; 12h and 24h).
    • RPC compressive strength increases after autoclave curing because many reactive components remain unreacted under room temperature cure conditions (0MPa-20°C).
    • There is a critical time (optimum) for each temperature-pressure level.
  • Mostofinejad et al. [22] reached a 174% increase in RPC strength using autoclave curing with temperatures between 125 and 220°C.
  • Zerb [23] investigated autoclave curing time and heating/cooling ramp, verifying curing time is most critical for compressive strength; autoclave treatment refines pores smaller than 100 μm.
  • Research investigates RPC in structural elements; in pillars, it improves impact and shear strength [24].
  • Ni et al. [25] used RPC with fibers in plates for high ballistic performance.
  • Low porosity of RPC increases its durability, making it suitable for nuclear waste storage.
  • Matte and Moranville [26] simulated leaching in radioactive waste recipients over 300-500 years; high active silica content avoids calcium hydroxide leaching, reducing porosity and increasing durability.
  • This work aims to optimize an RPC composition using Brazilian aggregates, nanosilica, and inorganic pigments to maximize mechanical performance without fibers, densification loads, and autoclave curing.

Materials and Experimental Program

  • Research development phases:
    • Phase 1: Influence of aggregate particle size in RPC resistance and elasticity modulus.
    • Phase 2: Feasibility of incorporating nanosilica to increase compressive strength; specimens tested for flexural strength.
    • Phase 3: Influence of pigment type in RPC strength.

Materials

Portland Cement

  • CPV ARI (high early strength Portland cement) used in phases 1 and 2.
  • CPB 40 Portland cement used in phase 3, as pigments present better results with white cement.
  • Table 1: Physical and chemical properties of cements.
  • Figure 2: Particle size distributions of cements; surface areas and strength at 28 days are equal.

Silica Fume

  • Specific mass: 2.20 kg/dm^3.
  • Figure 2: Particle size distribution; average diameter (d50) = 5.86 µm.

Quartz powder

  • Two types:
    • 50% of grains below 11.08 µm and 100% passing through the 45 µm sieve (325 mesh).
    • 50% of grains below 22.87 µm and 100% passing through the 75 µm sieve (200 mesh).
  • Figure 2: Particle size distribution of both quartz powders.
  • Specific mass: 2.61 g/cm^3 for both types.

Fine aggregate

  • Two natural sands with specific mass of 2.63 g/cm^3.
  • Figure 2: Particle size distribution for both sands.
  • Maximum characteristic dimensions: 1.20 mm and 0.60 mm.
  • Fineness modules: 2.64 and 1.23, respectively.

Superplasticizer and superplasticizer with nanosilica

  • Two chemical admixtures:
    • Polycarboxylate-based superplasticizer (PCE): density from 1.067 to 1.107 g/cm^3, pH between 5 and 7, solid content between 38 and 40%.
    • Polycarboxylate-based superplasticizer with nanosilica (PCE_N).
  • Superplasticizer efficiency assessed using Part 2 of NBR 7681 [28] for melt flow index and ASTM C939 [29] for saturation point.
  • Saturation point not directly used in RPC mix proportion; higher contents used due to high powder amount.
  • Chemical admixture content defined for the mixture to reach the capillary state, following Formagini [30].
  • Superplasticizer content adopted: 4.3% of the binder mass.
  • PCE_N: liquid state with dispersed nanosilica; specific mass is 1.06 g/cm^3.
  • Nanosilica manufacturer recommends up to 2% admixture for high strength concretes.
  • Marsh funnel test used to verify PCE and PCE_N compatibility with materials, using cement (2,000 g), silica fume (500 g), and water/cement ratio of 0.23.
  • Figure 3: PCE saturation point determination for w/c ratio of 0.23.
  • Table 2: Flowing times measured for different PCEN and PCE combinations; flowing time increases with increasing PCEN percent, indicating PCE is more efficient in the dispersion of fine particles.

Inorganic pigments

  • Pigments used presented high fineness, superior to the 45 µm sieve.
  • Yellow pigment (Color Index 77492): iron oxide (Fe2O3) with a 4.7 g/cm^3 specific mass.
  • Orange pigment (Color Index 77491): iron oxide (Fe2O3) with a 4.7 g/cm^3 specific mass.
  • Blue pigment (Color Index 74160): cobalt oxide (Co(Al,Cr)2O4).
  • Green pigment (Color Index 77492): chromium oxide (Cr2O3).

Mix proportion evaluated

  • Table 3: Mix proportion used in the research.
  • Initial determination by trial and error.
  • Phase 1: Influence of aggregate particle size in RPC.
    • Eight compositions with water/binder ratios of 0.18 and 0.25.
    • Aggregate compositions included sand with a maximum dimension of 1.2 mm and quartz powder; only sand with 0.66 mm dimension; only quartz powder with maximum 0.075 mm diameter; and only quartz powder with maximum dimensions characteristic of 0.045 mm.
  • Phase 2: PCEN incorporation instead of PCE; contents of 0%, 2%, 3% and 4% tested of substitution of PCE for PCEN.
  • Phase 3: Use of white cement (CPB 40) and different color pigments assessed in RPC compressive strength.
    • Eight composition from phase 1 were tested in phase 3.
    • Correction in mass was made for the composition to give the same proportions in absolute volume because CPB 40 presents a specific mass smaller than that of cement CPVARI,
  • Yellow, blue, green, and orange pigments were tested.
  • All compositions mixed in a planetary mixer in 5 L container.
  • Order of placement: water, cement, and superplasticizer (65 rpm); then aggregate (125 rpm).
  • After the mixture, 3 test specimens (5x10cm) of each composition were molded to determine compressive strength.
  • The test specimens were demolded 24 hours later and were taken to 28 days of thermal cure at 90°C.
  • Axial compression tests were performed at a loading speed of 0.45 MPa/s. The determination of the elasticity modulus of the molded concretes in phase 1 was made after the application of three pre-loading cycles, and a strain of 30% of the breaking strain was adopted for their calculation.
  • In phase 2, the flexural tensile strength test was performed on three points in prismatic specimens (40mm x 40 mm x 160 mm). The test was performed with three points of support and loading speed of 50 N/s.

Results and Discussions

Compressive strength

  • Table 4: Compressive strength results in the three phases.
  • Phase 1: Maximum characteristic dimension of the aggregate impacts RPC compressive strength.
  • Table 5: Phase 1 compressive strength variance analysis.
    • Probability of average strengths belonging to the same population is much inferior to 5% (p = 0.05).
    • Maximum characteristic dimension significantly influences RCP compressive strength (95% reliability).
  • Figure 4: Average compressive strength results in phase 1 (w/c ratio of 0.25).
    • Representative reduction of 61% in RPC axial compression strength when quartz powder with grains passing through the sieve with a mesh opening of 0.045 mm is replaced by sand with a maximum dimension of 1.2 mm.
  • De Larrard e Sedran (1994) proposed a model to explain the increase in compressive strength in concretes; maximum thickness of the paste (Equation 1) is the second most important factor (first is w/c ratio).
    • (e_M = D * (g^* / g - 1)^{1/3}) - Equation 1
    • e_M is the maximum paste thickness, D is the maximum aggregate size, g^* is the aggregate packaging density, and g is the total volume of aggregate used per cubic meter of concrete.
    • Experimental results of this work can be explained by the model proposed by de Larrard and Sedran [16] because the smaller the aggregate grain size, the lower the maximum thickness of the paste. The more confined is the paste, the higher is the RPC compressive strength.
  • Nanosilica content did not significantly impact compressive strength.
  • Table 6: Variance analysis for phase 2; probability (value-p) of strengths belonging to one single distribution is superior to 0.05 (5%); content of chemical admixture with nanosilica did not significantly change RPC compressive strength.
  • Figure 5: Average strengths determined in phase 2.
  • Objective of phase 3 was to assess inorganic pigment incorporation in RPC.
  • Similar to phase 2, eight compositions of phase 1 were used and CP V-ARI was replaced by CP-B-40 to improve the concrete aspect.
  • The results of phase 3 can lead us to two conclusions:
    • There is no significant difference across the different pigments studied in the RPC compressive strength. As can be observed in Table 7, the variation across the averages is not significant for 95% confidence (value-p is higher than 0.05).
    • Figure 6 presents the average strengths for compositions of phase 3.
    • The pigmented concretes exhibited compressive strength significantly superior to that of the RPC with CPV ARI.
    • The difference in average strength of composition 8 (253.9 MPa) when compared to the composition 13 (310.7 MPa) was 56.8 MPa, which corresponds to a 22% increase in strength.
    • The authors understand that the superior fineness (Table 1) of white cement improved the packaging of particles, as well as the larger surface area of the particle, which provides larger contact surface among particle, favoring the formation of hydration products.
    • Another representative factor that contributed to strength increase was the presence of pigment, composed of highly fine particles. According to Richard and Cheyrezy [1], the increase in granular packaging improves RPC mechanical performance.
  • Table 7: ANOVA Phase 3 – influence of the type of pigment in concrete strength
  • Among the different RPC research using natural aggregate, without fibers, those that obtained the highest compressive strengths reached strengths superior or close to 300 MPa [1], [3], [11], [12].
    • However, to surpass the 300 MPa strength, the authors use pre-densification stress like Yazıcı et al. [11] which get 324 MPa, pre-densification stress with autoclave curing like Aydin et al. [3] which get 325 MPa. Thus, the results presented in phase 3 of this research are relevant due the high strength achieved without any pre-densification or autoclave curing.

Tensile strength

  • Figure 7: Average values determined for flexural tensile strength.
  • Flexural tensile strength was significantly changed by the incorporation of the superplasticizer containing nanosilica.
  • Table 8: ANOVA for phase 2.
    • Probability of results belonging to one single population is lower than 5% (value-P lower than 0.05).
  • Presence of nanosilica provided an average increase of 50% in flexural tensile strength.
  • Though there is an increase in flexural tensile strength with an increase in nanosilica content, the difference determined across contents (2%; 3% and 4%) is not significantly different.

Elasticity modulus

  • Figure 8: Average values for phase 1 compositions’ modulus of elasticity.
  • Average elasticity modulus was 44.4 GPa.
  • The results found in this study demonstrate that the maximum aggregate size did not have a significant influence on the elasticity modulus.

Conclusions

  • RCP strength can be significantly increased by reducing the maximum aggregate size; reducing from 1.2 mm to 0.045 mm increased strength by 156 MPa (61%).
  • Maximum aggregate size does not significantly change RPC elasticity modulus.
  • Addition of nanosilica starting from 2% provided an average increase of 50% in RPC flexural tensile strength, but no significant change in compressive strength.
  • The study showed the possibility of producing a colored RPC with compressive strength superior to 300 MPa only with the thermal cure, without the need of pre-densification procedures or even autoclave curing.