⚠️ Why steel slag is difficult to use

• High free lime (f-CaO) and free magnesia (f-MgO) content causes volume instability (expansion, cracking) during hydration
• Lower amorphous glass content than GGBS - less reactive surface area for hydraulic reaction
• Variable composition between heats and steel grades
• Slow strength development without chemical activation
• Result: majority is landfilled or used as low-value road sub-base fill

✅ The alkanolamine opportunity

• Alkanolamines accelerate slag dissolution and C-S-H gel formation, unlocking latent hydraulic reactivity
• The –OH groups complex free calcium, reducing expansion tendency from f-CaO
• Works synergistically with supplementary activators (gypsum, NaOH) to give rapid strength gain
• Enables 30–60% slag content in soil binder formulations
• Dramatically reduces embodied CO₂ vs Portland cement-only stabilization

🔗 Mechanism 1: Calcium complexation - disrupting the passivating layer

The hydroxyl groups of the alkanolamine form soluble complexes with Ca²⁺ ions in the pore solution adjacent to the slag surface. By chelating free calcium, they prevent the immediate re-precipitation of C-S-H on the slag surface - keeping the surface "open" to continued dissolution. This effect is particularly strong for alkanolamines with two –OH groups (BDEA) but also significant for single-hydroxyl grades (NBEA, DMEA) at sufficient concentration. The result is a sustained, higher dissolution rate that translates to more rapid and complete pozzolanic reaction.

🔗 Mechanism 2: C₃A and C₄AF phase activation

Steel slag contains significant quantities of calcium aluminate (C₃A) and calcium aluminoferrite (C₄AF) phases that are more reactive than the calcium silicate phases but often under-utilized without activation. Alkanolamines - particularly tertiary grades DMEA and DEAE - selectively accelerate the hydration of these aluminate phases, promoting ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) and calcium aluminate hydrate (CAH) formation. These products fill pore space rapidly, contributing to early strength gain and providing the framework on which slower C-S-H gel forms over 28–90 days.

🔗 Mechanism 3: f-CaO expansion mitigation

Free lime (f-CaO) in steel slag hydrates to portlandite (Ca(OH)₂), causing a volumetric expansion of approximately 97% - which cracks and disrupts binder microstructure if not controlled. The hydroxyl groups of alkanolamines complex free Ca²⁺ released from f-CaO hydration, moderating the local calcium concentration spike that drives rapid portlandite crystallization. This "chemical aging" effect reduces the swelling tendency, making alkanolamine-activated slag more dimensionally stable than unactivated slag in stabilized soil applications.

NBEA research findings

• At 1–3% dosage on slag weight, NBEA accelerates 7-day compressive strength of slag-stabilized soft clay by 35–60% compared to slag alone
• The primary amine group shows higher reactivity with slag surface Si–O bonds than tertiary amines, promoting faster initial dissolution
• NBEA + gypsum (3%) synergy produces 28-day UCS gains of 40–75% over the reference mix in soft soil stabilization trials
• Effective in steel slag with f-CaO content up to 8% - higher f-CaO requires pre-treatment
• Published in: Construction and Building Materials, Journal of Hazardous Materials, Applied Clay Science

DMEA research findings

• At 0.5–2% dosage, DMEA selectively accelerates C₃A and C₄AF hydration in steel slag, contributing disproportionately to 28-day and 90-day strength
• DMEA-activated slag shows superior performance in high-slag (40–60% slag replacement) systems vs Portland cement-dominated mixes
• Lower addition levels needed than NBEA due to higher molar concentration per kg (MW 89 vs 103 for NBEA)
• DMEA + slag systems show reduced leaching of Pb, Cd, Zn, and Cu from contaminated soil compared to lime-only stabilization
• Published in: Cement and Concrete Composites, Journal of Cleaner Production, Waste Management

3–7 days

Early strength phase

Ettringite formation from activated C₃A + gypsum provides initial stiffening. Strength gain rate is 60–80% of Portland cement at equal binder content.

28 days

Pozzolanic phase

C-S-H gel formation from slag dissolution accelerates significantly. With DMEA + gypsum, 28-day UCS reaches 85–100% of the Portland cement reference at equal binder content.

90–180 days

Continued gain phase

Unlike Portland cement, activated slag binders continue to gain strength at 90–180 days. Long-term (1-year) UCS of alkanolamine-activated slag often exceeds the Portland cement reference by 10–25%.

1️⃣ pH elevation → metal precipitation

The alkaline pore solution generated by slag hydration (pH 11–12.5) causes most heavy metals to precipitate as insoluble hydroxides. Lead (Pb²⁺), cadmium (Cd²⁺), zinc (Zn²⁺), nickel (Ni²⁺), and copper (Cu²⁺) all have solubility minima in the pH 9–12 range. Once precipitated, these hydroxides are physically encapsulated within the hardening C-S-H gel matrix, preventing re-dissolution even if local pH later drops. The alkanolamine contributes to pH stability by buffering the pore solution against carbonation-driven pH reduction.

2️⃣ C-S-H gel sorption and structural incorporation

Calcium silicate hydrate gel (C-S-H) - the primary binding phase - has a large surface area (100–700 m²/g) and a layered crystal structure with high ion exchange capacity. Heavy metal cations (particularly Pb²⁺, Cd²⁺, and Zn²⁺) are incorporated into the C-S-H interlayer by substituting for Ca²⁺ in the crystal lattice. This structural incorporation is far more durable than surface adsorption - metals incorporated into C-S-H show minimal leaching even under extended TCLP (Toxicity Characteristic Leaching Procedure) or EN 12457 batch leaching testing.

3️⃣ Alkanolamine chelation - additional sequestration layer

The hydroxyl and amine groups of residual alkanolamine in the stabilized matrix can form coordination complexes with heavy metal ions, providing an additional sequestration mechanism beyond pH-induced precipitation and C-S-H incorporation. Research data on DMEA-stabilized soil shows that Pb leachate concentrations in TCLP tests are 40–65% lower than Portland cement-only references at equal binder dosage - a difference that is attributed in part to this chelation effect operating alongside the other immobilization pathways.

📋 Mix design procedure

1. Characterize the steel slag: XRF for f-CaO, MgO; XRD for phase composition; Blaine fineness
2. Characterize the soil: Atterberg limits, particle size distribution, organic content, pH, contamination profile (if applicable)
3. Design trial mixes at 3 binder contents × 3 alkanolamine doses × 2 gypsum levels = 18 mix combinations minimum
4. Cure at 20 °C and 95% RH; test UCS at 7, 28, and 90 days
5. If S/S application: also run TCLP or EN 12457 leaching tests on 28-day specimens
6. Select optimal mix based on UCS, leachate, and cost criteria

⚠️ Key constraints to check

• f-CaO content: if >8%, pre-treat slag with steam aging or limit slag content to avoid expansion
• Organic content of soil: if >5%, organic matter interferes with cementitious reactions - add lime pre-treatment step
• Sulfate-sensitive environment: if groundwater sulfate is high, use sulfate-resistant slag blend to avoid ettringite-related expansion
• Alkanolamine dosage ceiling: above 3% on slag weight, strength gains plateau and workability decreases - do not over-dose

−75%

Embodied CO₂

vs Portland cement at equal binder content (slag = ~50 kg CO₂/t; PC = ~800 kg CO₂/t)

0 kg

Primary raw material

Steel slag is an industrial waste product - using it as a binder displaces primary material extraction entirely

EN 14227

Regulatory pathway

EU standard for hydraulic road binders accepts slag-based materials; national waste framework regulations typically permit S/S treatment for brownfield remediation

🚜 In-situ stabilization (deep mixing)

For soft soil improvement using deep mixing equipment (single-axis or multi-axis mixing tools), the alkanolamine is pre-mixed with the slag slurry at the batch plant before injection. Slurry water/binder ratio is typically 0.6–0.8. The alkanolamine improves slurry fluidity and workability, reducing injection pressure and improving penetration into soft clay layers. Minimum column diameter: 500 mm; typical installation depth: 5–20 m.

🔄 Ex-situ stabilization (pugmill mixing)

Excavated soil is mixed with dry slag + alkanolamine solution (or pre-mixed liquid activator) in a pugmill or pug mixer. The activated mix is then returned to the excavation or placed in a designated treatment cell. This approach allows more precise quality control of the mix design and is preferred for S/S remediation of heterogeneous contaminated soil where the contaminant distribution is irregular.

⏱️ Working time and pot life

Alkanolamine-activated slag mixes have a working time (time to initial stiffening) of 2–6 hours at 20 °C, compared to 0.5–2 hours for Portland cement-dominated mixes. This extended working time is an operational advantage in large-area stabilization works. At temperatures above 30 °C, working time shortens to 1–3 hours - plan batching and placement accordingly.

💧 Water and moisture management

The hydraulic reaction of slag requires water - but excessive moisture dilutes the binder and reduces strength. Optimal treatment moisture is typically OMC (optimum moisture content) + 0–3%. If the natural soil moisture exceeds this, pre-drying or addition of dry quicklime (to consume free water and raise temperature) before slag addition is recommended. The alkanolamine activator is added as a dilute aqueous solution (5–15% concentration) to facilitate even distribution during mixing.

Q: Is alkanolamine-activated steel slag approved for use in regulated remediation projects?

In most jurisdictions, S/S technology using novel binder formulations is permitted provided the treated material meets the required performance criteria (UCS, leachate concentrations, durability) specified in the remediation design. The alkanolamine is a processing aid at low concentration - it is not the primary binder constituent. Regulatory acceptance typically requires: (1) treatability study demonstrating TCLP or equivalent leachate compliance; (2) UCS demonstration at 28 days; (3) materials characterization of the slag including REACH compliance documentation; (4) in some jurisdictions, classification of the slag as a product (not waste) under the applicable waste framework regulation. Consult your regional environmental regulator and confirm the waste/product classification of the steel slag at project inception.

Q: Does the alkanolamine itself leach from the treated soil and present an environmental risk?

At typical dosage (1–3% on slag weight, corresponding to 0.1–0.6% on dry soil weight), the alkanolamine concentration in pore water leachate is very low - typically below 1 mg/L in standard batch leaching tests. Both NBEA and DMEA are readily to inherently biodegradable (DMEA is readily biodegradable per OECD 301B), meaning any leached alkanolamine will be metabolized by soil microorganisms. The environmental assessment of alkanolamines by Health Canada (2013) and REACH registration data confirm no significant aquatic or terrestrial risk at these concentrations. The alkanolamine also becomes partially bound within the C-S-H gel structure over time, further reducing its mobility.

Q: Can this technology treat arsenic-contaminated soil?

Arsenic presents a different immobilization challenge from most heavy metals. At high pH (above 10), arsenate (As(V)) forms insoluble calcium arsenate precipitates - which the alkaline slag binder promotes. However, arsenite (As(III), the more mobile and toxic form) is less effectively immobilized at high pH and may require additional treatment. For arsenic-contaminated sites, adding ferrous sulfate (FeSO₄) to the mix is recommended - iron-arsenic co-precipitation produces insoluble iron arsenate at any pH. The combination of alkanolamine-activated slag (for mechanical strength and other heavy metal immobilization) plus ferrous sulfate (for arsenic-specific treatment) is an effective dual-mechanism approach documented in the literature.

Q: How does alkanolamine-activated slag perform in seismic zones or areas prone to wetting/drying cycles?

For seismic applications, the key criterion is that the stabilized material should not be brittle - it should deform without sudden failure. Alkanolamine-activated slag binders, when cured to UCS in the 500–800 kPa range, show higher strain-to-failure ratios than Portland cement-stabilized soils at equivalent UCS, making them suitable for seismic stabilization of liquefiable ground. For wetting/drying durability, slag-based binders generally perform comparably to Portland cement - the C-S-H gel is stable in the presence of water. The key vulnerability is sulfate attack if the groundwater sulfate content is high (above 1,500 mg/L SO₄²⁻) - sulfate-resistant slag blends or Portland slag cement (PSC) should be used in sulfate-rich environments.

Q: Where can I source steel slag for use in stabilization projects?

Steel slag is available directly from integrated steel mills (BOF slag, EAF slag) and secondary steelmaking facilities. In most countries, it is classified as a co-product rather than waste, meaning it can be sold commercially without waste management authorization. Key specification requirements for geotechnical use: Blaine fineness ≥280 m²/kg; f-CaO content ≤8%; MgO ≤10%; loss on ignition ≤3%. Chinese, Japanese, and European steelmakers are the largest suppliers of processed and quality-certified steel slag for construction applications. Sinolook Chemical supplies the alkanolamine activator (NBEA, DMEA) for use with steel slag from any supplier - contact us for recommended mix design protocols and sample quantities for your treatability study.