Experimental Investigation of Strength and Durability Characteristics of Alkaline Activated Slag Concrete Replacing the Fine Aggregate with Bottom Ash

Technical Summary: Experimental Investigation of Strength and Durability Characteristics of Alkaline Activated Slag Concrete Replacing the Fine Aggregate with Bottom Ash

Problem Statement
The global construction industry faces significant environmental challenges due to its reliance on Ordinary Portland Cement (OPC) and natural river sand. Cement production is a major contributor to global CO₂ emissions (approximately 4–8%), with 1 tonne of cement releasing 1 tonne of CO₂. Concurrently, the disposal of industrial byproducts, specifically coal combustion bottom ash, poses environmental risks through land degradation and potential heavy metal leaching. The study addresses the dual need to reduce the carbon footprint of concrete and manage industrial waste by developing a sustainable Geopolymer Concrete (GPC) that utilizes Ground Granulated Blast Furnace Slag (GGBFS) as a cement substitute and bottom ash as a partial replacement for natural fine aggregate (river sand).

Methodology
The research employed a systematic experimental approach divided into three phases:

1. Mix Preparation: GPC specimens were prepared using GGBFS as the binder. Bottom ash was used to replace natural river sand at varying levels: 0% (control), 10%, 20%, 30%, 40%, and 50%. The mixes were activated using alkaline solutions of Sodium Hydroxide (NaOH) and Sodium Silicate (Na₂SiO₃).
2. Engineering Properties Evaluation: Fresh concrete workability was assessed via the slump test (IS 1199). Hardened properties were evaluated through compressive strength and split tensile strength tests at various curing ages (3, 7, 28, and 90 days) under both ambient and chemical curing conditions.
3. Durability Assessment: The resistance of the concrete to aggressive environments was tested by exposing specimens to 5% Sulphuric Acid (H₂SO₄) and 5% Magnesium Sulphate (MgSO₄) solutions. Residual compressive strength and mass loss were measured after 28, 56, and 90 days of exposure.

Key variables investigated included the Alkaline Activator-to-Binder ratio (ranging from 0.35 to 0.55) and NaOH molarity (8M to 16M).

Key Results

• Workability: The control mix (0% bottom ash) exhibited a slump of 108 mm. As the bottom ash content increased, workability decreased due to the porous and uneven texture of the ash particles. The 50% replacement mix showed a slump of 76 mm, representing a 29.63% reduction. However, increasing the Alkaline/Binder ratio from 0.35 to 0.55 improved workability, with slump values rising from 65 mm to 145 mm.
• Compressive Strength:
  • Optimal Ratios: An Alkaline/Binder ratio of 0.45 yielded the best balance, achieving 42.5 MPa at 28 days and 49.2 MPa at 90 days.
  • NaOH Concentration: Higher molarity significantly enhanced strength. The 16M NaOH mix achieved 32.9 MPa (7 days), 44.6 MPa (28 days), and 51.8 MPa (90 days), compared to lower strengths in the 8M mix.
  • Bottom Ash Impact: Increasing bottom ash content generally reduced compressive strength. However, mixes with up to 40% replacement maintained structurally satisfactory strengths (35.7–42.5 MPa at 28 days; 43.1–49.1 MPa at 90 days under ambient curing). All mixes exceeded the 40 MPa target design strength at 90 days.
• Split Tensile Strength: Tensile strength decreased with higher bottom ash content, dropping from 4.75 MPa (control) to 4.23 MPa (50% replacement) at 28 days. Despite this reduction, all mixes remained within structurally sound limits.
• Durability:
  • Acid Resistance: After 90 days in 5% H₂SO₄, all mixes retained over 69% of their original compressive strength. The control mix retained 76.5%, while the 50% bottom ash mix retained 69.2%.
  • Sulphate Resistance: Exposure to 5% MgSO₄ for 90 days resulted in all mixes retaining over 82% of their compressive strength (Control: 88.5%; 50% replacement: 82.1%).

Key Contributions
The study demonstrates that bottom ash can effectively replace up to 40% of natural fine aggregate in alkali-activated GGBFS concrete without compromising structural integrity or durability. It identifies the optimal mix parameters for this sustainable concrete: an Alkaline/Binder ratio of 0.45 and a NaOH concentration of 16M. The research confirms that the dense geopolymeric matrix formed by GGBFS activation provides high resistance to acid and sulphate attacks, even with the inclusion of porous industrial waste.

Significance
The paper claims that this approach offers a viable pathway for sustainable construction by simultaneously reducing the consumption of natural resources (river sand) and mitigating the environmental impact of industrial waste (bottom ash) and cement production. The findings suggest that geopolymer concrete utilizing these materials can meet standard engineering requirements for strength and durability, thereby promoting the adoption of eco-friendly building materials in the construction sector.