STP ShaliPlast AF L
Alkali-free liquid set accelerator for wet-mix shotcrete — flash setting in minutes without the caustic handling risks, strength loss or groundwater alkalis of conventional accelerators.
What is it?
An alkali-free, chloride-free liquid set accelerator from STP Limited, metered at the nozzle in wet-mix shotcrete to flash-set sprayed concrete within minutes of impact.
Where is it used?
Tunnel linings, mine roof and rib support, slope stabilisation, shaft and cavern linings, and high-grade structural shotcrete in metros, hydro projects and hill roads.
Why use it?
It sets sprayed concrete fast enough for overhead work and thick layers while avoiding the caustic burns, long-term strength penalty and alkali leaching that come with traditional aluminate and silicate accelerators.
How is it applied?
Dosed at 3-10% by weight of cementitious material through a calibrated liquid dispensing system feeding directly into the shotcrete nozzle, on a base mix designed at w/c below 0.45.
About STP ShaliPlast AF L
ShaliPlast AF L is STP Limited’s alkali-free set accelerator for the wet-mix shotcrete process. Injected at the nozzle just before the concrete leaves the gun, it turns a pumpable base mix into a material that grips rock and stays put — initial set in roughly 3-4 minutes and final set within 10-12 minutes under laboratory conditions at 25 °C. That speed is what lets a nozzle crew build thick layers overhead, secure freshly excavated rock quickly, and cycle back to the face sooner. The product conforms to ASTM C 1141 Type II, Class A, Grade 9.
The “AF” matters as much as the speed. Older aluminate and waterglass accelerators are strongly caustic, cut into 28-day strength, and wash alkalis into groundwater — a real problem in Himalayan hydro and metro tunnelling where environmental conditions are monitored. ShaliPlast AF L is alkali-free and chloride-free, so accelerated shotcrete retains its strength, reinforcement stays protected, and site handling is considerably safer. Space Arc Engineering supplies genuine STP material with application support across Delhi NCR and pan-India.
The technical insight
Accelerators of this class work by disturbing the sulphate balance of fresh cement paste the instant they meet it at the nozzle. The acidic suspension (pH 2.5-3.5, specific gravity 1.42 ± 0.02) triggers rapid formation of early hydration products that bridge cement grains and stiffen the paste within minutes — without flooding the system with sodium or potassium ions. That is why accelerated cores show little of the long-term strength loss typical of alkali-rich accelerators: the cement’s normal silicate hydration continues largely undisturbed underneath the fast-set skeleton. Dose is the control knob — higher dosage within the 3-10% band shortens set and allows thicker passes, which is why the TDS anchors around 6% by weight of cementitious for a 10 cm single-pass layer. Because the reaction rides on the cement’s own chemistry, every change of cement source or blend genuinely changes the result — hence the non-negotiable trial-mix requirement.
Where it is used
- Primary and initial shotcrete linings in rail, metro and highway tunnels
- Securing freshly excavated rock in tunnels and underground mines
- Slope stabilisation and cut-face protection on hill roads and portal slopes
- Shaft and cavern linings in hydroelectric and underground storage projects
- Backfilling of overbreak and voids behind tunnel linings
- Overhead and vertical guniting where sag control is critical
- High-grade structural shotcrete in general construction and repair works
Key advantages
- Alkali-free and non-toxic — far safer for nozzle crews than caustic aluminate or waterglass accelerators
- Fast set: initial in about 3-4 minutes, final within 10-12 minutes (lab conditions, 25 °C)
- No strength loss in the accelerated concrete, unlike conventional alkali accelerators
- Low rebound during spraying, cutting material wastage and cleanup
- Chloride-free (nil), so no corrosion risk to rebar, lattice girders or rock bolts
- No alkali wash-out into groundwater — helpful where environmental compliance is monitored
- Thick single-pass layers — around 10 cm at a 6% bwc dose
- Improved water-tightness of the finished sprayed layer
Limitations & considerations
- Set and hardening behaviour vary with cement brand, blend and dose — site trial mixes with the actual cement and SCMs are essential before production spraying
- Requires a calibrated liquid dosing pump at the nozzle; it is not a hand-dosed or mixer-added admixture
- The suspension is acidic (pH 2.5-3.5) — dosing equipment must suit it, and gloves, eye protection and ventilation are mandatory
- Base mix must be designed at w/c below 0.45; a wet, poorly controlled mix blunts the acceleration
- Shelf life is 6 months in unopened packing, so procurement on long tunnel packages needs planning
- Dose rates of 3-10% bwc are higher than alkaline accelerators — compare cost per cubic metre at equal performance, not per kg
Selection & best practice
Start with trial panels using the actual project cement, fly ash or slag, and aggregates — set times move with every change in cementitious source, so fix the working dose from trials, not from the datasheet mid-point. Prepare the substrate mechanically (bush hammer, sandblast or jackhammer) to a clean, rough profile of roughly 3 mm and vacuum or pressure-wash away all residue; no bond coat is needed. Set up the wet-mix rig near the placement area and feed ShaliPlast AF L through a liquid dispensing system directly at the nozzle tip — skilled nozzle operation and thorough in-stream mixing decide the outcome as much as the chemistry does. Condition the material to 20-30 °C before use, follow ACI hot- and cold-weather practices in adverse temperatures, and wash tools and lines with water before the material hardens. Store drums in a cool, dry, shaded place.
Specifications
| Form | Liquid suspension, colourless |
| Standard compliance | ASTM C 1141 Type II, Class A, Grade 9 |
| Setting time (lab, 25 °C) | Initial 3-4 min; final 10-12 min |
| pH | 2.5-3.5 |
| Specific gravity | 1.42 ± 0.02 |
| Chloride content | Nil |
| Dosage | 3-10% by weight of cementitious material (typically 6% bwc; w/c below 0.45 recommended) |
| Packing | 275 kg drum |
| Shelf life | 6 months in original unopened packing, stored cool and dry under shed |
Industry use cases
Metro & Rail Tunnelling
Initial shotcrete support in NATM-driven metro and rail tunnels, where alkali-free chemistry protects crews working at the face and keeps 28-day lining strength intact.
Hydropower
Head-race tunnels, surge shafts and powerhouse caverns in Himalayan projects, where nil alkali wash-out into groundwater supports environmental compliance.
Mining
Rapid roof and rib support in underground mines — fast final set lets crews re-enter and advance sooner after spraying.
Highways & Slope Protection
Shotcreted cut slopes, portal faces and landslide-prone stretches on hill highways, sprayed overhead and on steep faces with low rebound.
Repair & Structural Shotcrete
High-grade sprayed concrete for strengthening and section rebuilding where quick setting and water-tightness are specified.
Frequently asked questions
What is the dosage of ShaliPlast AF L?
How is it safer than conventional shotcrete accelerators?
Does it reduce the final strength of shotcrete?
Can I add it into the transit mixer or the pump hopper?
Which cements is it compatible with?
What pack size does it come in, and how long can I store it?
How we position STP ShaliPlast AF L
ShaliPlast AF L is the safety-and-durability pick within STP’s shotcrete range — specify it wherever alkali-free chemistry is mandated or permanent linings cannot afford strength loss. Where the specification permits conventional accelerators, STP ShaliPlast Shotcrete L (liquid) and Shotcrete P (powder) are the economical siblings. The wet-mix base itself needs a high-range water reducer to stay pumpable at w/c below 0.45 — pair it with STP ShaliPlast PCE 100 from the same family, or Fosroc Auramix 400 on projects already standardised on Fosroc admixtures. Space Arc supplies and supports all of these from stock, pan-India.
Fosroc Auramix 400 — Fosroc has no direct alkali-free nozzle accelerator in our range, but specifiers on Fosroc-standardised tunnel packages pair ShaliPlast AF L with Auramix 400 as the PCE water reducer that keeps the wet-mix base pumpable at w/c below 0.45.
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