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A Cost-Effective Slag-based Mix Activated with Soda Ash and Hydrated Lime: A Pilot Study

This pilot study demonstrates that a cost-effective, structural-grade geopolymer concrete can be produced by activating ground granulated blast furnace slag and silica fume with industrial-grade soda ash and hydrated lime, achieving a compressive strength of 41.33 MPa at 120 days while reducing activator costs by approximately 94.5% compared to analytical-grade reagents.

Original authors: Jayashree Sengupta, Nirjhar Dhang, Arghya Deb

Published 2026-02-19

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Jayashree Sengupta, Nirjhar Dhang, Arghya Deb

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to bake the perfect cake, but instead of using expensive, high-end flour and sugar, you want to use cheap, industrial-grade ingredients to save money. That is essentially what this research paper is about, but instead of cake, they are making concrete.

Here is the story of how they turned a "cheap" recipe into a structural powerhouse, explained simply.

The Problem: Concrete is Too Expensive and "Green" is Hard

Traditional concrete relies on cement, which creates a lot of pollution. To fix this, scientists have been trying to use "green" ingredients like slag (a waste product from making steel) and silica fume (a byproduct of making silicon).

However, to make these waste materials harden into concrete, you usually need to "wake them up" with a chemical activator. The problem? The best activators are usually expensive, dangerous liquids (like strong lye) that require special training to handle and cost a fortune. It's like trying to bake a cake but needing a professional chemist to mix the batter.

The Solution: The "Industrial-Grade" Shortcut

The researchers asked: Can we use cheaper, less pure, solid powders instead of expensive liquid chemicals?

They decided to use two common, industrial-grade powders:

Soda Ash: The stuff used in washing soda and glass making.
Hydrated Lime: A common building material.

The Magic Trick:
Instead of buying pure, 99% chemical-grade ingredients (which cost a lot), they bought the "industrial grade" versions (which are only about 80–90% pure).

The Result: They saved a massive 94.5% on the cost of the chemicals! It's like buying generic brand baking soda instead of a fancy, lab-grade version.

The Experiment: Two Ways to Mix

They tested two ways to mix these ingredients, like trying two different recipes:

The "Dry Mix" (Set A): They threw the dry slag, silica fume, lime, and soda ash into a mixer, added water, and stirred.
The "Wet Mix" (Set C): They dissolved the soda ash in water first, then added it to the dry mix.

What Happened?

The Wet Mix: It acted like a "false start." The soda ash dissolved too quickly, creating a crust that stopped the other ingredients from reacting properly. It set fast but didn't get very strong later on.
The Dry Mix: This was the winner. When the dry soda ash and lime met the water, they played a game of "chemical tag." The lime grabbed onto the soda ash's carbon, freeing up the sodium to wake up the slag. This created a slow, steady, and strong reaction.

The Secret Sauce: How Strong is it?

They found the "Goldilocks" recipe:

10% Silica Fume (The filler that makes it dense).
90% Slag (The main ingredient).
Just the right amount of Soda Ash and Lime.
Water ratio: Not too wet, not too dry.

The Results:

After 28 days, the concrete was strong enough to hold up a building (35 MPa).
After 120 days, it got even stronger (41 MPa).
Verdict: It is strong enough for real structural use (like bridges or buildings), not just for sidewalks.

The Science Behind the Strength (The "Micro-Party")

To understand why it worked, they looked at the concrete under a microscope and with special heat scanners. They found that the ingredients threw a party and formed new "guests" (crystals):

The Main Guests: They formed a glue called C-S-H (Calcium-Silicate-Hydrate), which is the same glue found in regular cement.
The Special Guests: Because they used soda ash, they also got NASH (Sodium-based glue) and Hydrotalcite (a magnesium-based crystal).
The Analogy: Think of the concrete as a brick wall. The slag is the bricks. The chemical reaction creates the mortar. In this study, the "mortar" wasn't just one type; it was a mix of different super-strong glues that filled in all the tiny gaps, making the wall incredibly dense and hard.

Why This Matters

This study proves that you don't need expensive, high-tech chemicals to build green, strong concrete.

Cost: You can save nearly 95% on the activator cost.
Safety: You are using solid powders (like baking soda) instead of dangerous, corrosive liquids.
Environment: You are using industrial waste (slag) and avoiding the carbon footprint of making traditional cement.

In a nutshell: The researchers found a way to make "super-concrete" using cheap, everyday industrial powders instead of expensive lab chemicals. It's a cheaper, safer, and greener way to build the future.

1. Problem Statement

Alkali-activated binders (AABs) using industrial byproducts like Ground Granulated Blast Furnace Slag (GGBFS) offer a sustainable alternative to Ordinary Portland Cement (OPC) by reducing CO₂ emissions. However, widespread adoption is hindered by:

High Costs: Traditional activators often require expensive analytical-grade chemicals (e.g., NaOH, Na₂CO₃, Ca(OH)₂) or silicate solutions.
Safety and Handling: Liquid alkali solutions are highly corrosive, requiring trained supervision and specialized storage.
Complexity: Two-part mixing systems (liquid activator + dry binder) are less compatible with standard construction practices compared to one-part mixing.
Purity vs. Cost: Previous studies often rely on high-purity (99%) reagents, ignoring the potential of lower-purity, industrial-grade chemicals which are significantly cheaper but less studied.

2. Methodology

The study investigated a one-part alkali-activated system using industrial-grade Soda Ash (SA) and Hydrated Lime (HL) as activators for a binder composed of GGBFS and Silica Fume (SF).

Materials:
- Precursors: GGBFS (basic modulus $B=1.14$ ) and Silica Fume (SF).
- Activators: Industrial-grade SA (Sodium Carbonate, ~80-90% purity) and HL (Calcium Hydroxide, ~75-96% purity).
- Control: Analytical-grade chemicals and traditional NaOH solutions.
Mix Design:
- Optimal Composition: 10% SF, 90% GGBFS, 9.26% HL, and 13.25% SA (stoichiometrically equivalent to 10% NaOH).
- Water-to-Solid (w/s) Ratio: 0.45 (optimized for flow and strength).
Mixing Techniques Compared:
1. Set A (Direct Solid): SA and HL added directly as powders to the dry binder.
2. Set B (Control): Traditional two-part mixing using NaOH solution.
3. Set C (Premixed): SA premixed with water before adding to the dry binder mixture.
Characterization Techniques:
- Rheology: Yield stress and plastic viscosity analysis using a rheometer.
- Mechanical Testing: Compressive strength at 7, 28, and 120 days.
- Microstructural Analysis: X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM-EDS), and Thermogravimetric Analysis (TGA/DTA).

3. Key Contributions

Cost Reduction: Demonstrated that using industrial-grade SA and HL reduces activator costs by approximately 94.5% compared to analytical-grade counterparts and 91% compared to stoichiometrically equivalent NaOH solutions.
One-Part Feasibility: Validated that solid industrial-grade chemicals can effectively activate slag without the safety hazards of liquid alkalis, aligning better with standard concrete mixing practices.
Mechanistic Insight: Elucidated the chemical interaction between SA and HL, showing how they generate NaOH in situ while managing carbonate ions to prevent premature setting barriers.
SF Optimization: Identified that while SF improves density, exceeding 10% content negatively impacts flow and strength due to increased water demand and particle interference.

4. Key Results

A. Rheology and Flow

Set A (Direct Solid): Showed optimal flow. The reaction between solid SA and HL released ions ( $Na^+$ , $OH^-$ , $Ca^{2+}$ ) that lowered yield stress.
Set C (Premixed): Exhibited "false set" or rapid stiffening. Premixing SA with water created a high concentration of $CO_3^{2-}$ ions, which reacted immediately with $Ca^{2+}$ to form carbonates, hindering slag dissolution and reducing fluidity.
SF Impact: Increasing SF from 10% to 20% increased yield stress and reduced flow due to the "ball-bearing" effect diminishing and interparticle friction increasing.

B. Compressive Strength

Optimal Mix (SF10NH10): Achieved 35.1 MPa at 28 days and 41.33 MPa at 120 days, confirming suitability for structural applications.
SF Content: 10% SF yielded higher strength than 20% SF. Excess SF (20%) led to insufficient wetting of slag particles and delayed dissolution, reducing strength.
Comparison: While analytical-grade activators produced slightly higher early strength in some cases, the industrial-grade mix maintained superior long-term performance and cost-efficiency.

C. Microstructural and Phase Analysis

Hydration Products:
- 7 Days: Dominated by Calcium-Sodium-Aluminate-Silicate-Hydrate (CNASH), Calcium-Aluminate-Silicate-Hydrate (CASH), and NASH gels. Calcite formation was prominent.
- 28 Days: A coexistence of CSH, CASH, and NASH gels drove strength.
- Secondary Phases: Hydrotalcite (formed from Mg in GGBFS) and Calcite contributed to strength. Magnesite and Dolomite formed at later ages.
Reaction Mechanism:
- In Set A, $Ca^{2+}$ from HL consumed $CO_3^{2-}$ from SA to form $CaCO_3$ , freeing $Na^+$ and $OH^-$ to activate the slag effectively.
- In Set C, excess $CO_3^{2-}$ in solution reacted with $Ca^{2+}$ prematurely, forming barriers (Gaylussite/Calcite) that slowed slag dissolution.
Bound Water: TGA analysis confirmed that the SF10NH10 mix had the highest bound water content, correlating with the highest gel formation and strength.

5. Significance

This study provides a practical, economically viable pathway for the construction industry to adopt alkali-activated binders. By utilizing widely available, low-purity industrial chemicals (Soda Ash and Hydrated Lime) in a one-part mixing system, the research:

Eliminates Safety Hazards: Removes the need for handling corrosive liquid alkalis on-site.
Reduces Carbon Footprint: Offers a low-cost alternative to OPC and expensive activators, facilitating the use of industrial waste (slag).
Validates Structural Use: Proves that mixes activated with industrial-grade chemicals can achieve compressive strengths (>40 MPa at 120 days) suitable for structural concrete, challenging the notion that only high-purity reagents yield high-performance materials.

The research concludes that the SF10NH10 mix (10% SF, 10% NaOH equivalent, w/s 0.45) represents the optimal balance of cost, workability, and mechanical performance for structural applications.