Two fiber surface treatment routes were compared on the same natural fiber and at the same dosage. One was an alkali route, the other an alkali-plus-paraffin route. The alkali route gave the best 56-day strengths, at 37 MPa compressive, 3.4 MPa tensile, and 5.3 MPa flexural. That is roughly 19, 26, and 20 percent above the unreinforced control. The paraffin route won on durability instead, which is the more interesting half of the result.
Why the fiber-matrix interface fails first in natural fiber composites
Natural fiber is mostly cellulose, and cellulose is covered in hydroxyl groups. In a composite that is a mixed blessing. Those groups are the only handles the matrix has for bonding, and they are also what pulls water into the fiber.
The failure sequence is predictable. Water gets in, the fiber swells, the interface opens, and load transfer stops. In a high-pH cementitious environment a second mechanism sits on top. Alkaline hydrolysis attacks the cellulose over time, and mineral deposits grow into the fiber wall until it turns brittle. Both mechanisms start at the interface, which is why interfacial adhesion is usually the first property engineered and the first one to fail.
This is the same reason silane coupling agents became standard in this field. A coupling agent bridges a polar fiber to a less polar matrix, and it does that with chemistry rather than with roughness alone. The two routes compared here sit at different points on that spectrum. One activates the fiber surface, the other passivates it.
The two fiber surface treatments compared, and where a coupling agent fits
| Route | What it does to the fiber | What it improves | What it gives up |
|---|---|---|---|
| Untreated | Nothing | Baseline | Weak interface, high water uptake |
| Alkali (1% NaOH, 2 h) | Dissolves hemicellulose, lignin, pectin and waxes | Surface roughness, mechanical interlocking, interfacial adhesion | Higher water uptake by the fiber |
| Alkali plus paraffin | Adds a hydrophobic film over the activated surface | Moisture barrier, dimensional stability | Some mechanical anchorage |
| Silane coupling agent | Grafts an organofunctional tail onto cellulose | Chemical bridge across the interface | Cost and process control |
The alkali route strips off the weak amorphous layer and leaves exposed cellulose microfibrils behind. You get a rougher surface, more available hydroxyls, and better mechanical keying. The paraffin route adds a continuous hydrophobic film, reported at roughly 8 to 10 percent mass uptake and about 5 to 10 micrometers thick. It seals pores and capillaries without filling the fibrillar structure.
The crystallinity index of the fiber rose from about 62.5 percent to about 75 percent after alkali treatment, measured by the Segal method, and the cellulose I peak positions did not move. That is what you expect when amorphous material dissolves away and the crystalline fraction is what remains.
Workability and setting: the two treatments pull in opposite directions
| Mix | Slump | Initial setting | Final setting |
|---|---|---|---|
| Untreated fiber | 65 mm | 75 min | 135 min |
| Alkali-treated fiber | 78 mm | 120 min | 190 min |
| Alkali plus paraffin | 85 mm | 165 min | 255 min |
Workability improved with treatment, which is worth noting because adding fiber usually does the opposite. Removing the surface impurities that soak up mixing water leaves more free water behind, and the paraffin film stops the fiber taking up water at all.
Setting time moved in the opposite direction, and by a wide margin. Against the untreated mix, the alkali route added 60 percent to initial setting and 41 percent to final setting. The alkali-plus-paraffin route added 120 percent and 89 percent.
That is a large effect, and it should be planned for rather than discovered on site. A mix that sets 2 hours later than the reference changes finishing schedules, formwork turnover, and cold-weather planning.
Strength at 56 days: alkali treatment won, paraffin came second
| Mix | Compressive 28 d | Compressive 56 d | Flexural 56 d | Tensile 56 d |
|---|---|---|---|---|
| Control, no fiber | 28 MPa | 31 MPa | 4.4 MPa | 2.7 MPa |
| Untreated fiber | 31 MPa | 34 MPa | 4.8 MPa | 3.3 MPa |
| Alkali-treated fiber | 34 MPa | 37 MPa | 5.3 MPa | 3.4 MPa |
| Alkali plus paraffin | 32 MPa | 35 MPa | 5.0 MPa | Not given as a figure |
Every fiber mix beat the control at every age, so the fiber addition itself clearly worked. The ranking among treatments held steady from early ages onward. Alkali-treated came first, alkali-plus-paraffin second, untreated third.
The mechanism offered for the alkali result is surface roughening plus removal of hydrophilic amorphous material, and that is consistent with the microscopy. The paraffin route gave up a little strength, which the authors attribute to a smoother coating that offers less mechanical anchorage.
What the treatments did to the fiber itself
Scanning electron microscopy showed three distinct surfaces. Untreated fiber carried a weak, impurity-rich boundary layer. Alkali-treated fiber showed defibrillation, with the bundle separated into individual microfibrils and a rougher, more textured surface. Paraffin-coated fiber showed a smooth, occluded surface, with the natural grooves and pores visibly filled in.
That last observation is the caution attached to the durability win. Mechanical interlocking comes from topography, and a coating that fills that topography removes the interlocking even as it blocks water.
Durability is where paraffin overtakes alkali
| Property | Age | Untreated | Alkali-treated | Alkali plus paraffin |
|---|---|---|---|---|
| Sorptivity (mm/min to the power 0.5) | 28 d | 0.18 | 0.12 | 0.08 |
| Sorptivity (mm/min to the power 0.5) | 56 d | 0.14 | 0.09 | 0.05 |
| Chloride charge passed (coulombs) | 28 d | 920 | 780 | 760 |
| Chloride charge passed (coulombs) | 56 d | 600 | 540 | 500 |
Against the untreated mix at 28 days, the alkali route cut sorptivity by 33 percent and the paraffin route by 55 percent. By 56 days the paraffin mix sat 64 percent below the untreated mix. On chloride conductivity the paraffin route came in 17.4 percent below untreated at 28 days and 16.7 percent below at 56 days, while the alkali route managed 15.2 percent and 10 percent.
The sorptivity gap is larger than the chloride gap, and that is informative. Sorptivity measures capillary water movement, which the hydrophobic film blocks directly. Chloride transport depends on the whole pore network, so coating the fiber shifts it less.
Both treatments improved durability further between 28 and 56 days, which points to continued hydration densifying the matrix rather than to the fiber doing it alone.
The trade-off nobody gets to avoid
Put the two result sets side by side and the trade is clear. The route that activates the fiber surface gives the best strength and the worse durability. The route that passivates it gives the best durability and slightly less strength.
For a formulator this is the useful shape of the finding, because it transfers well beyond cement. Any fiber surface treatment that raises interfacial adhesion by exposing more polar groups will also raise water uptake. Any treatment that blocks water will also blunt mechanical anchorage. The question is never which treatment is better in the abstract. It is which failure mode the application actually faces.
Where the service environment is dry and the load case is demanding, activation wins. Where moisture cycling or aggressive ions are present, passivation wins. Where both matter, the sequential route the authors point to, meaning an alkali clean followed by a hydrophobic or silane coating, is the one that tries to hold both.
How to choose between the two routes
- Start from the failure mode, not from the strength table. Moisture-driven failure and load-driven failure want opposite surfaces.
- Budget for setting time. The longest-working mix here set 120 percent later on initial set than the untreated one.
- Measure durability early. Sorptivity at 28 days already separated the treatments, before the strength gap was fully developed.
- Do not read a smooth fracture surface as a stronger interface. It can just as easily mean a coating filled the anchorage sites.
- If both strength and moisture resistance are specified, test the sequential route rather than assuming one step covers both.
Limitations
The matrix in this work was cementitious rather than polymeric, so the absolute numbers do not transfer directly to a polymer composite. What does transfer is the shape of the trade-off between surface activation and surface passivation.
The reported setting-time ordering is internally inconsistent, and it is worth saying so plainly. The summary table gives untreated, alkali-treated, and alkali-plus-paraffin initial setting at 75, 120 and 165 minutes, growing in that order, while the narrative description of the same curves says the paraffin mix set first. The table values are used here because they agree with the abstract. The direction of the paraffin effect on setting should be treated as unresolved until the full data set is checked.
Compressive strength at 3 days is reported as about 1.7 MPa for the untreated mix and 1.8 MPa for the alkali mix against 15 MPa for the control, which reads as a typographical error rather than a measurement. Those values are not used here. Accelerated ageing used 30 wet-dry cycles, which is a screening protocol rather than a service-life prediction. Only one fiber dosage, one fiber length, and one mix design were tested, so no dosage response is available at all.
Bottom line
Two fiber surface treatment routes were compared at equal fiber dosage in a cementitious matrix. Alkali treatment gave the best strength at 56 days, at 37 MPa compressive and 5.3 MPa flexural, about 19 and 20 percent above the control. Alkali plus paraffin gave the best durability, cutting 28-day sorptivity by 55 percent against untreated fiber and reaching 0.05 mm per minute to the power 0.5 at 56 days. Strength and moisture resistance pull in opposite directions here, and the choice has to be made against the failure mode.
FAQ
What is a fiber surface treatment
A fiber surface treatment changes the outer layer of a reinforcing fiber before that fiber is mixed into a matrix. The common routes dissolve the weak amorphous layer, graft on a coupling agent, or lay down a hydrophobic film. The aim is usually better interfacial adhesion or lower water uptake, and sometimes both at once.
Why does the interface fail before the fiber does
Cellulose is covered in hydroxyl groups, so the fiber takes up water and swells as it does. Swelling opens the interface, load transfer stops, and in a high-pH environment alkaline hydrolysis then attacks the fiber wall itself. Both mechanisms start at the interface rather than in the bulk, so that is where a treatment pays off first.
What did alkali treatment do to the fiber
Alkali treatment dissolved hemicellulose, lignin, pectin and surface waxes, leaving exposed cellulose microfibrils. The crystallinity index rose from about 62.5 percent to about 75 percent, with the cellulose peak positions unchanged. The surface became rougher, which improves mechanical interlocking.
What did the paraffin coating do
The paraffin coating laid down a continuous hydrophobic film, reported at roughly 8 to 10 percent mass uptake and about 5 to 10 micrometers thick. It sealed pores and capillaries without filling the fibrillar structure. It slowed water uptake and blocked capillary transport, and it cost some mechanical anchorage.
Which route gave higher strength
Alkali treatment gave the higher strength, reaching 37 MPa in compression, 3.4 MPa in tension, and 5.3 MPa in flexure at 56 days. Those figures sit about 19, 26, and 20 percent above the unreinforced control mix. The alkali plus paraffin route came second, at about 35 MPa in compression and 5.0 MPa in flexure at the same age.
Which route gave better durability
The alkali plus paraffin route gave the better durability on both of the measures reported. Sorptivity fell to 0.08 at 28 days against 0.18 for untreated fiber, which is a 55 percent cut, and it reached 0.05 by 56 days. Charge passed in the chloride test fell to 760 coulombs at 28 days, against 920 coulombs for the untreated fiber mix.
How much did setting time change
Initial setting went from 75 minutes for untreated fiber to 120 minutes for alkali-treated fiber and 165 minutes for alkali plus paraffin. Final setting went from 135 minutes to 190 and then 255 minutes, and that size of change is worth budgeting for in finishing schedules, formwork stripping times, and cold-weather placement.
Is the setting-time direction reliable
Not fully, and that is worth knowing. The summary table and the narrative description of the same curves disagree on whether the paraffin mix set sooner or later. This summary uses the table values, because the table is what matches the abstract of the work. The direction of that effect should be confirmed against the full data set before anyone acts on it.
Does a smoother fiber mean a weaker bond
Not always, but it certainly can. Mechanical interlocking depends on surface topography, and microscopy here showed the paraffin coating visibly filling the natural grooves and pores. That smoother surface blocked water very well and gave up a little mechanical anchorage at exactly the same time, which is why the strength ranking and the durability ranking came out reversed.
Can both strength and moisture resistance be improved at once
Only by combining steps. The sequential route, meaning an alkali clean followed by a hydrophobic or silane coating, tries to keep the roughened surface for anchorage while still protecting it from water. That combination was recommended in the discussion rather than tested head to head here, so it remains a hypothesis until someone runs the comparison directly.
How far do these numbers transfer to polymer matrices
The absolute values do not transfer across, because the matrix used here was cementitious rather than polymeric. What does transfer is the shape of the trade-off, meaning that raising interfacial adhesion by exposing more polar groups also raises water uptake in the same step. That pattern tends to hold across matrix types, which is why it is worth carrying into polymer work.