Performance Assessment of Concrete Incorporating Nano Titanium Dioxide and Coal Bottom Ash as a Sustainable Fine Aggregate Substitute

Technical Summary: Performance Assessment of Concrete Incorporating Nano Titanium Dioxide and Coal Bottom Ash

Problem Statement
The construction industry faces a dual challenge: the high environmental footprint of cement manufacturing, which accounts for significant carbon dioxide emissions, and the increasing scarcity and cost of natural fine aggregates (river sand). Traditional reliance on Ordinary Portland Cement (OPC) and natural sand is unsustainable due to resource depletion and ecological damage from quarrying and dredging. Furthermore, the disposal of industrial byproducts like coal bottom ash (CBA) remains a waste management issue. While supplementary cementitious materials (SCMs) have been explored, there is a need to optimize mix designs that simultaneously utilize industrial waste as a fine aggregate substitute and incorporate nanomaterials to enhance microstructural properties without compromising workability or long-term durability.

Methodology
The study focused on M35 grade concrete, designed according to IS 10262:2009 and IS 456:2000 standards. The experimental program involved three primary variables:

1. Fine Aggregate Substitution: Natural fine aggregate was partially or fully replaced with Coal Bottom Ash (CBA) in increments from 10% to 100%.
2. Nanomaterial Addition: Nano Titanium Dioxide (NTi) was added to the cement matrix at varying dosages (1% to 7% by weight) to evaluate its effect on hydration and microstructure.
3. Control Mix: A conventional M35 mix served as the baseline for comparison.

The research employed a comprehensive testing regime:

• Fresh Properties: Workability was assessed via slump cone, compaction factor, and flow table tests.
• Mechanical Properties: Compressive strength was measured at 3, 7, 28, 56, and 90 days. Split-tensile and flexural strengths were evaluated at 28, 56, and 90 days.
• Durability: Resistance to acid attack (5% HCl immersion) was tested by measuring mass loss and strength reduction over time. Rapid Chloride Penetration Tests (RCPT) were conducted at 90 days to assess permeability.
• Non-Destructive Testing (NDT): Rebound hammer tests and Ultrasonic Pulse Velocity (UPV) measurements were used to evaluate internal quality and density.
• Microstructural Analysis: X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were utilized to identify crystalline phases, hydration products, and pore structure refinement.

Key Contributions and Results

• Workability: The incorporation of 60% CBA and 40% natural fine aggregate generally maintained medium workability. The addition of NTi improved workability, with a 5% dosage yielding the highest slump (128 mm) and flow (93%), classified as "High" workability. However, dosages exceeding 5% showed diminishing returns or reduced performance.
• Mechanical Strength:
  • CBA Substitution: A mix with 60% CBA and 40% natural fine aggregate outperformed the control mix. At 90 days, this blend achieved a compressive strength of 47.1 N/mm² (compared to 46.3 N/mm² for the control), along with improved split-tensile (3.04 N/mm²) and flexural (3.54 N/mm²) strengths. Complete replacement of fine aggregate with CBA resulted in reduced strength.
  • NTi Optimization: The addition of 5% NTi to the 60% CBA mix yielded the optimal mechanical performance. This combination achieved a peak compressive strength of 47.9 N/mm² at 90 days, surpassing both the control and the CBA-only modified mixes. Lower dosages (1-2%) of NTi initially resulted in lower strength compared to the CBA-only mix.
• Durability:
  • Acid Resistance: The 5% NTi mix demonstrated superior resistance to 5% HCl exposure. At 28 days, it exhibited only a 12.07% strength reduction, significantly lower than the control (27.01%) and the 60% CBA mix (23.50%). While strength loss increased over time, the NTi-modified mix remained more resistant than the other variants at 90 days.
  • Chloride Permeability: The RCPT results indicated that the 5% NTi mix had the lowest chloride permeability (379 Coulombs), classified as "Very Low," compared to 756 Coulombs for the 60% CBA mix and 886 Coulombs for the control.
  • NDT Results: The UPV test confirmed the enhanced quality of the NTi-modified mix, recording a pulse velocity of 5.36 km/sec ("Excellent"), compared to 3.85 km/sec for the CBA mix and 3.66 km/sec for the control.
• Microstructural Insights:
  • XRD Analysis: The presence of NTi (Anatase phase) was confirmed. The 5% NTi mix showed a reduction in Calcium Hydroxide (Portlandite) peaks and an intensification of the Calcium Silicate Hydrate (C-S-H) region, suggesting that NTi acted as a nucleation site, promoting denser hydration.
  • SEM Analysis: Micrographs revealed that the NTi-modified mix had a refined pore structure with reduced voids. The CBA particles acted as micro-aggregates, filling voids, while NTi enhanced the interfacial transition zone, leading to a denser matrix.

Significance and Claims
The paper concludes that the synergistic use of Coal Bottom Ash as a sustainable fine aggregate substitute and Nano Titanium Dioxide as a performance-enhancing additive offers a viable pathway for sustainable concrete development. The study claims that the specific blend of 60% CBA and 5% NTi represents an optimal compromise, delivering:

1. Enhanced Mechanical Performance: Higher compressive, tensile, and flexural strengths compared to conventional concrete.
2. Improved Durability: Significantly reduced chloride permeability and improved resistance to acid attack, suggesting a longer service life, particularly in aggressive environments.
3. Microstructural Refinement: A denser matrix with reduced porosity and optimized hydration products.
4. Practical Feasibility: The mixes maintained or improved workability, indicating that these sustainable modifications do not require complex handling procedures on-site.

The authors position this approach not as a "silver bullet" but as a strong candidate for applications where long-term structural integrity and environmental sustainability are prioritized. The research validates that industrial waste can be valorized effectively when paired with advanced nanomaterials to address the limitations of traditional concrete.