Blog

Polycarboxylate Superplasticizer in Machine made Sand Concrete

The growing scarcity of natural river sand has led to the widespread adoption of machine‑made sand (manufactured sand) in concrete production. However, the wet‑washing process used to remove fines from machine‑made sand often involves adding anionic polyacrylamide (PAM) flocculants to accelerate sedimentation. Residual flocculant on the sand particles causes serious issues: it competes with polycarboxylate superplasticizers (PCEs) for adsorption on cement surfaces, reduces fluidity, and lowers concrete strength.

To overcome this challenge, researchers have developed a sulfonate‑modified polycarboxylate superplasticizer that actively resists the negative effects of PAM flocculants. The results show a 19% increase in 28‑day compressive strength compared to a commercial PCE at the same dosage.

This article summarizes their work and explains why sulfonate‑modified PCEs are a promising solution for machine‑made sand concrete.

The Problem: Flocculant Residue in Washed Machine Made Sand

Machine made sand is produced by crushing rock, followed by washing to remove excess fines (particles <75 µm). To speed up settling of suspended fines in the wash water, anionic polyacrylamide (PAM) – a high‑molecular‑weight flocculant – is commonly added. Unfortunately, residual PAM remains on the sand particles even after rinsing. Typical residual PAM levels range from 0.2% to 0.8% by mass of sand.

When this sand is used in concrete, the anionic PAM strongly adsorbs onto cement particles, creating a dense polymer film. This leads to:

Competitive adsorption with PCE – the flocculant occupies the surface sites where the superplasticizer should anchor.
Reduced fluidity – the flocculant’s bridging effect increases interparticle friction.
Lower compressive strength – poor dispersion results in an inhomogeneous microstructure and weak interfaces.

Therefore, a PCE that can tolerate or counteract the presence of anionic flocculants is urgently needed.

Design Principle: Introducing Sulfonate Groups

Conventional polycarboxylate superplasticizer rely mainly on carboxylate groups (–COO⁻) for adsorption. While carboxylates are effective in clean systems, they are also strongly attracted to the same cationic sites (Ca²⁺, Al³⁺) that bind anionic PAM. Thus, PAM can outcompete carboxylate‑rich PCEs.

The new approach uses sulfonate groups (–SO₃⁻), which:

Carry a stronger negative charge than carboxylates.
Create electrostatic repulsion against the anionic PAM molecules.
Still adsorbs well onto positively charged cement surfaces (via Ca²⁺ bridging), but with less interference from the flocculant.

By copolymerizing a sulfonate‑containing monomer – sodium vinylsulfonate – into the PCE backbone, the resulting polymer can “push away” adsorbed flocculant and maintain its dispersing ability.

Synthesis of Sulfonate‑Modified Polycarboxylate Superplasticizer in Machine made Sand Concrete

Raw Materials

Component	Function	Amount (example)
Allyl polyoxyethylene‑polyoxypropylene ether (polyether F6)	Macromonomer (side chain)	20 g
Acrylic acid (AA)	Carboxyl‑containing monomer	50 g
Methyl acrylate (MA)	Ester monomer (slump retention)	27 g
Sodium vinylsulfonate	Sulfonate monomer	1–5 g (variable)
Ammonium persulfate ((NH₄)₂S₂O₈)	Initiator	0.5 g
Deionized water	Solvent	150 g

Polymerization Procedure

Dissolve sodium vinylsulfonate in water, then add the polyether macromonomer, acrylic acid, and methyl acrylate.
Heat to 50 °C, then add the ammonium persulfate solution (1–2 h).
Raise the temperature to 60–80 °C and continue the reaction for 4–6 h.
Cool and adjust pH to ~7 with 10% ammonia solution.
The final product has a solid content of about 40%.

The polymerization proceeds via free‑radical copolymerization of vinyl monomers, yielding a comb‑shaped polymer with –SO₃⁻, –COO⁻, and ester groups.

Characterization: FT‑IR Confirms Sulfonate Incorporation

Fourier transform infrared (FT‑IR) spectra of the purified polymer (Figure 2 in the original paper) show characteristic peaks:

3600–3060 cm⁻¹ – O–H stretching (from carboxyl and hydroxyl groups)
2860–2930 cm⁻¹ – C–H stretching (–CH₂–)
1729 cm⁻¹ – C=O stretching (carboxyl and ester)
1674 cm⁻¹ – C–O–O stretching (ester)
1440 cm⁻¹ and 628 cm⁻¹ – S–O stretching of sulfonate groups (absent in the control PCE without sodium vinylsulfonate)
1150–1050 cm⁻¹ – C–O–C ether (from the polyether side chain)

The presence of the sulfonate peaks confirms successful incorporation of the vinylsulfonate monomer into the PCE structure.

Performance in Cement Paste Without Flocculant

Cement paste flow tests (water/cement = 0.29, PCE dosage = 0.15% solid on cement) were conducted with varying sulfonate content (sodium vinylsulfonate added as 1–5% of total monomers). The results (Table 1) show that as sulfonate content increases, paste flow increases.

Sulfonate addition (g)	Paste flow (mm)
1	225
2	248
3	253
4	258
5	261

The trend is attributed to the stronger anionic charge of sulfonates, which enhances the adsorption and dispersion of cement particles.

Resistance to Flocculant (Anti‑Flocculant Performance)

The critical test: cement paste containing 0.5% PAM flocculant (by mass of cement) was prepared, and paste flow was measured using PCEs with different sulfonate contents. The results (Table 2 and Figure 4) show:

Sulfonate addition (g)	Paste flow with 0.5% PAM (mm)
1	172
2	175
3	192
4	205
5	208

Without sulfonate (or very low sulfonate), the flow drops dramatically due to PAM competition.
With increasing sulfonate content, flow recovers – because the strongly charged –SO₃⁻ groups repel the anionic PAM and prevent its interference.
The optimum is around 4–5 g (≈3–4% of monomer mass); beyond that, improvement plateaus as PAM adsorption sites become saturated.

Even with higher PAM levels (0.2–0.8%), the 5% sulfonate PCE still maintains acceptable flow (Table 3), though flow decreases as PAM increases due to competitive adsorption.

Concrete Performance (C30 Mix with Machine Made Sand concrete)

The sulfonate‑type PCE (with 3 g sodium vinylsulfonate, ≈3% of monomers) was tested in a C30 concrete mix containing machine‑made sand (with residual flocculant). The mix proportions: cement 325 kg/m³, fly ash 70 kg/m³, water 150 kg/m³, machine‑made sand 700 kg/m³, gravel 950 kg/m³. PCE dosage varied from 0.3% to 0.7% (solid on cement). Compressive strengths at 3, 7, and 28 days were measured and compared with a commercial PCE.

Key results (Figures 5–7):

At 0.7% dosage, the sulfonate‑modified PCE achieved a 28‑day compressive strength of 39.8 MPa, compared to 33.4 MPa for the commercial PCE – an increase of 19%.
At all dosages (0.3–0.7%), the sulfonate PCE gave higher strengths at 3, 7, and 28 days than the commercial product.
The improvement is attributed to the sulfonate groups’ ability to counteract flocculant interference, allowing the PCE to properly disperse cement particles and produce a denser, more uniform microstructure.

Practical Recommendations

Based on the study, the following guidelines are suggested for using sulfonate‑modified PCE in machine‑made sand concrete:

Parameter	Recommendation
Sodium vinylsulfonate content (as % of total monomers)	≥3% (optimally 4–5%)
PCE dosage (solid on cement)	0.5–0.7% for C30 concrete
Compatibility test	Always test with actual machine‑made sand (PAM residue varies). Use cement paste flow with added PAM (0.5%) as a screening test.
Mix adjustment	Water‑to‑cement ratio may need slight adjustment to achieve target slump; the sulfonate PCE provides good retention.

Conclusion

A sulfonate‑modified polycarboxylate superplasticizer was successfully synthesized by copolymerizing sodium vinylsulfonate with acrylic acid, methyl acrylate, and a polyether macromonomer.
FT‑IR confirmed the incorporation of sulfonate groups (peaks at 1440 and 628 cm⁻¹).
The sulfonate groups carry a stronger negative charge than carboxylates, enabling them to electrostatically repel anionic PAM flocculant residues on machine‑made sand.
In cement paste containing 0.5% PAM, the sulfonate PCE gave significantly higher flow than a conventional PCE, and flow increased with sulfonate content (optimal at 4–5 g per 100 g monomers).
In C30 concrete made with machine‑made sand, the sulfonate PCE at 0.7% dosage produced a 28‑day compressive strength 19% higher than a commercial PCE (39.8 MPa vs. 33.4 MPa).
This technology provides an effective, practical solution to the flocculant‑induced performance loss in machine‑made sand concrete, supporting the wider use of manufactured sand in sustainable construction.