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Bentonite Slurry Testing Procedure

A reliable bentonite slurry test begins before the first instrument is used. The sample must represent the slurry at the stated location and operating stage, the test equipment must be clean and checked, and the result must be compared with the specification that applies at that moment. Freshly mixed slurry, circulated slurry, slurry recovered from an excavation and slurry tested immediately before concreting are not interchangeable samples.

This procedure covers the routine field sequence for sampling, density, Marsh funnel flow time, sand content and pH. It also explains when fluid-loss or rotational-rheology testing may be needed. It is a practical quality-control workflow, not a substitute for the project specification, the approved mix design, the equipment manufacturer’s instructions or the test standard named in the contract.

Quick Answer

Collect a representative bentonite slurry sample from the required location and depth, record its source, time, stage and temperature, then gently homogenize it without introducing air. Inspect the sample and run the most time-sensitive tests first. Measure density with a clean, water-checked mud balance; measure funnel flow time with a clean Marsh funnel and the specified receiving volume; determine sand content with the specified sieve and graduated tube; and measure pH with suitable paper or a calibrated meter. If the project requires it, add fluid-loss, filter-cake or rotational-rheology tests. Record the method, units, equipment ID and result, compare the result only with the limit for that sampling stage, and retest a new representative sample when a reading is suspect.

What This Procedure Tests—and What It Does Not

Test	What the result indicates	Important limitation
Density or slurry weight	The mass of a known slurry volume and, indirectly, changes in solids loading or contamination.	A normal density does not prove that viscosity, sand content or filtration behavior is acceptable.
Marsh funnel flow time	A rapid field index of flow behavior under one fixed funnel condition.	It is not a complete rheological profile and should not be reported as true dynamic viscosity.
Sand content	The volume percentage of particles retained by the specified test screen.	The retained material may include coarse non-sand solids; the result does not describe all fine solids.
pH	The acidity or alkalinity of the aqueous slurry and a useful warning of water or process changes.	pH alone cannot identify the contaminant or prove that the clay is fully hydrated.
Fluid loss and filter cake	The tendency of liquid to pass through a filter medium under the specified pressure and time.	The result depends on the exact apparatus, pressure, duration, sample condition and filter medium.
Rotational rheology	Shear-dependent behavior such as apparent viscosity, plastic viscosity, yield response and gel development under the chosen method.	Calculated values are method-dependent and must not be mixed with Marsh funnel seconds.

For a broader explanation of the material and its application context, see bentonite slurry and bentonite clay slurry. This page has a narrower responsibility: producing repeatable test results and a defensible field record.

Prepare the Test Plan Before Sampling

Do not start with a generic acceptance table taken from another job. Slurry limits vary with the construction method, geology, support requirement, slurry type, project stage and governing specification. The approved inspection and test plan should state where samples are taken, how often testing is required, which methods apply, the required units, the limits for each stage and who can accept corrective action.

Control item	Define before work starts	Why it matters
Slurry identity	Product, batch, make-up water source, formulation and whether the slurry is fresh, active or recovered.	Prevents results from different fluids being combined under one record.
Sampling point	Mixing tank, circulation line, storage tank, excavation surface, specified depth or base of excavation.	Slurry properties can vary by depth and location.
Sampling stage	After mixing and hydration, before introduction, during excavation, after treatment, before reuse or before concreting.	Each stage may have a different acceptance limit.
Test method	Named standard, project method and instrument instructions, including sample volume and reporting unit.	Similar-looking methods may produce results that are not directly comparable.
Frequency	Minimum routine frequency plus event-triggered testing after treatment, contamination, stoppage or soil change.	A single convenient sample cannot characterize an entire operation.
Action limits	Accept, investigate, treat, recirculate, desand, replace, retest or escalate.	Operators need an agreed response before an out-of-limit result occurs.

Equipment and Consumables Checklist

• Depth-appropriate slurry sampler and clean, sealable sample containers.
• Mud balance with its base, cup, lid, rider, knife edge and bubble level.
• Marsh funnel, matching graduated receiving cup, stand, stopwatch and thermometer.
• Sand content kit with the specified sieve, graduated tube, funnel and wash bottle.
• pH paper with a suitable range or a calibrated pH meter with appropriate electrodes and buffers.
• Clean rinse water compatible with the method, lint-free wipes and a waste container.
• Optional filter press, filter media, pressure source and timer when fluid-loss testing is required.
• Optional rotational viscometer and conditioning equipment when a full rheology profile is required.
• Inspection sheet or digital record with unique sample and equipment identification.
• Appropriate personal protective equipment based on the slurry, additives, pressure equipment and site rules.

Use one unit system throughout the record. If the specification requires a second unit, show it as a clearly labeled conversion rather than reading one scale and comparing it with a limit written for another scale.

Step 1: Collect a Representative Bentonite Slurry Sample

1. Confirm the requested location and stage. Match the inspection plan rather than sampling from the easiest accessible point.
2. Identify the fluid. Record the slurry batch or tank, product, make-up water source, preparation time, treatment history and whether it is fresh, circulated or recovered.
3. Use the specified sampler. Where the procedure calls for a depth sample, collect it at that depth. A surface dip is not a valid substitute for a downhole sample.
4. Avoid stagnant edges and settled residue. Unless the method specifically targets a settled zone, sample from a representative moving or properly conditioned body of slurry.
5. Rinse the sampler when required. Prevent water, cleaning agent or residue from the previous sample from diluting or contaminating the new one.
6. Collect enough slurry for the complete sequence and repeat tests. Do not return tested or rinsed material to the original sample.
7. Label immediately. Include sample ID, location, depth, date, time, operator and operating stage.
8. Measure or record temperature promptly. Flow time and other properties can shift with temperature.
9. Homogenize gently before splitting. Redistribute settled solids without prolonged high-shear remixing, excessive shaking or aeration that changes the sample.
10. Test without unnecessary delay. If holding is unavoidable, record the container, holding time and conditioning before the test.

Sampling error often creates more variation than the instrument itself. When different depths or stages must be evaluated, keep them as separate samples with separate results; do not blend them into an average that hides stratification.

Recommended Field Test Sequence

Sequence	Action	Control point
1	Confirm sample identity, stage, location, depth, appearance and temperature.	Stop if the label or sampling point is uncertain.
2	Gently homogenize and split the sample into clean portions.	Avoid air entrainment and do not reuse a tested portion.
3	Run density and Marsh funnel flow time promptly.	Use clean, checked equipment and record the actual units.
4	Run pH on a fresh portion.	Prevent carryover from rinse water, buffers or previous samples.
5	Run sand content.	Rinse consistently and read the graduated tube after the retained solids settle.
6	Run fluid loss, filter-cake or rotational rheology if required.	Condition the sample exactly as the selected method specifies.
7	Review the results as a set and compare them with the stage-specific limits.	Do not accept one passing value as proof that the whole slurry is compliant.
8	Document treatment and retesting.	A corrected result must be linked to the original result and action taken.

The sequence may be changed when the governing method requires another order. The essential rule is consistency: use the same approved sequence for comparable samples and document any deviation.

Step 2: Measure Density with a Mud Balance

Density is measured by balancing a fixed-volume cup of slurry against a movable rider. It is quick and useful, but the small test volume makes representative sampling and complete cup filling critical.

1. Place the mud balance base on a stable, level surface away from vibration and wind.
2. Inspect the cup, lid, vent, knife edge, fulcrum, rider and bubble level. Clean and dry the contact surfaces.
3. Check the instrument with clean water at the frequency required by the quality plan and follow the manufacturer’s adjustment procedure if it does not balance correctly.
4. Gently homogenize the slurry portion and fill the cup completely. Avoid trapping air.
5. Fit the lid slowly so excess slurry and any trapped air can escape through the vent.
6. Clean and dry the outside of the cup and lid. Material left on the exterior adds false weight.
7. Set the beam on the fulcrum and move the rider until the bubble is centered.
8. Read the correct scale at the specified edge of the rider and record the value, unit, temperature, instrument ID and sample ID.
9. Empty and clean the cup immediately. Confirm that no coarse material remains under the lid or in the vent.

Unexpected density should trigger a check for air, exterior contamination, an incompletely filled cup, settled solids, incorrect scale reading or an unrepresentative sample before the slurry is treated. For the difference between density, relative density and mineral-property measurements, continue to bentonite specific gravity and bentonite clay specific gravity.

Step 3: Measure Marsh Funnel Flow Time

The Marsh funnel provides an efflux time: the seconds required for a specified volume of slurry to leave a standard funnel. It is valuable for rapid trend control, but it is an index measurement influenced by solids, gel development, temperature and the condition of the funnel. Do not convert funnel seconds into a full rheology model without a separately validated method.

1. Confirm that the funnel, screen and outlet are clean, undamaged and free from dried slurry.
2. Verify the funnel with clean water under the condition stated in the selected method. Record or investigate a failed check before testing slurry.
3. Set the clean receiving cup on a stable surface and have the stopwatch ready.
4. Close the funnel outlet with a finger and hold or support the funnel vertically.
5. Pour a freshly homogenized sample through the screen until the liquid reaches the specified fill level.
6. Position the outlet over the receiving cup. Release the outlet and start the timer at the same instant.
7. Stop timing when the slurry reaches the specified receiving-volume mark. Many field procedures use a 946 mL or one-quart mark; use the volume named in the governing method.
8. Record the efflux time to the required precision together with the sample temperature, volume, funnel ID and any interrupted or irregular flow.
9. Clean the screen and outlet immediately and inspect for retained coarse fragments that may have affected filling.

A repeat reading should be taken from a newly homogenized portion, not by pouring the discharged slurry back through the funnel. If repeat results differ more than the project’s allowed tolerance, inspect the equipment, sampling and timing technique before averaging or reporting a value.

Step 4: Determine Sand Content by Volume

The field sand-content test separates particles retained on the specified fine screen and reports their settled volume as a percentage of the original slurry volume. A commonly specified kit uses a nominal 200-mesh screen, approximately 74–75 micrometers, but the actual screen and method must match the project requirement.

1. Inspect the graduated tube, screen and funnel. Confirm that the screen is intact, seated correctly and free from blocked openings.
2. Fill the graduated tube with slurry to the marked slurry line.
3. Add clean water to the marked water line, close the tube and shake until the slurry is fully dispersed in the dilution water.
4. Pour the mixture through the wetted screen. Use controlled rinsing rather than forcing solids through the mesh.
5. Rinse the tube and screen until fine slurry has passed and the retained coarse material is clean enough to read according to the method.
6. Fit the funnel to the screen assembly, invert it and wash all retained particles into the graduated tube.
7. Allow the retained solids to settle as required, then read the percentage by volume directly from the correct graduation.
8. Record the result, screen size, sample ID and any observation of unusual coarse fragments, fibers, cement, aggregate or other non-sand material.
9. Clean the screen from the reverse side with a method that does not stretch or damage it.

High retained-solids content can accompany increasing density, but the two tests are not interchangeable. Density reflects the mass of all components in the test volume, while the sand kit isolates a defined coarse fraction. Review both before deciding whether circulation, desanding, dilution, treatment or replacement is appropriate.

Step 5: Measure pH

pH is a useful process indicator because make-up water, salts, cementitious contamination and additives can alter hydration and rheology. It should be measured on a fresh, representative portion at a recorded temperature.

1. Select pH paper with adequate resolution for the specified range or prepare a calibrated meter suitable for slurry testing.
2. For a meter, calibrate with fresh buffers that bracket the expected pH and follow the electrode manufacturer’s cleaning and storage instructions.
3. Gently homogenize the sample. Avoid measuring rinse water or only the clear liquid from a settled sample unless the method specifically requires it.
4. Immerse the indicator or electrode as directed. Allow a meter reading to stabilize without scraping the bulb against settled solids.
5. Record pH, temperature, measurement type, instrument or strip lot and any instability in the reading.
6. Rinse the electrode between samples without carrying rinse water into the next test portion.

Do not correct an unexpected pH automatically. First verify the sample and instrument, then review the water source, recent additives, contamination risk and the paired density and flow-time results. Any chemical treatment should follow the approved slurry plan and be followed by adequate mixing, conditioning and retesting.

When to Add Fluid-Loss and Rotational-Rheology Tests

Routine field control often focuses on density, funnel flow time, sand content and pH. Additional testing becomes important when the specification controls filtration behavior, gel development, reuse, long circulation periods or a more demanding support-fluid condition.

Additional test	Use it when	Procedure control
Low-pressure fluid loss	The slurry must form a controlled filter cake or the contract sets a filtrate-volume limit.	Fix pressure, duration, filter medium, sample condition and reporting units; depressurize safely before opening.
Filter-cake observation	Cake thickness, texture or integrity is part of the approved evaluation.	Use the specified measurement and avoid unsupported visual labels such as “good” or “bad.”
Rotational viscometer	Funnel seconds are insufficient to describe shear response, yield behavior or gel development.	Control cup geometry, temperature, speed sequence, rest period and calculation model.
Gel-strength measurement	Static suspension and restart behavior must be evaluated.	Use the stated pre-shear and rest times; do not compare readings produced by different sequences.
Water hardness or ion testing	Make-up water changes or suspected salt contamination may affect clay hydration.	Test the water and slurry under the method named in the quality plan.

Water-based bentonite slurry and organophilic clay dispersed in an oil-based system are different materials. Their test programs, sample preparation and interpretation must not be mixed. For the latter, use a method written for the actual organic system; see organoclay density testing for the narrower density-testing context.

Compare Results with the Correct Operating Stage

There is no single universal pass range for every bentonite slurry. A fresh slurry may be judged after a defined hydration period, while a circulated or recovered slurry contains different solids and may be governed by separate reuse or pre-concreting criteria. The record should therefore show the applicable limit beside the result rather than relying on operator memory.

Sample stage	Typical decision purpose	Do not confuse it with
Make-up water	Confirm water condition before bentonite addition.	A finished-slurry pH or density result.
Freshly prepared and conditioned slurry	Verify the approved mix and readiness for use.	An immediate as-mixed sample that has not completed the specified hydration period.
Slurry entering the excavation	Confirm the fluid being supplied to the work.	A tank sample taken before recent treatment has circulated through the system.
Slurry in the excavation	Monitor change with depth, soil contact and operating time.	A surface sample used to represent the base without a depth sampler.
Recovered slurry	Determine treatment, desanding or reuse suitability.	Fresh-slurry acceptance limits.
Slurry before concreting or final operation	Confirm the property set required at the critical handover stage.	An earlier passing result taken before continued excavation or contamination.

A useful companion page is bentonite slurry testing, which explains how the individual results support overall slurry control. This procedure focuses on executing and documenting the tests correctly.

Read the Results as a System

Observed pattern	Possible direction to investigate	Verification before treatment
Density rises and sand content rises	Coarse soil loading, ineffective solids removal or an unrepresentative settled sample.	Repeat a depth-appropriate sample; inspect circulation and solids-control records.
Density rises but sand content remains low	Fine solids, added material, concentration change or a measurement error.	Check cup filling, water calibration, slurry history and additional solids-control indicators.
Funnel time rises while density is stable	Hydration, gel development, temperature change, chemistry change or treatment effect.	Repeat at matched temperature and review pH, water quality and conditioning history.
Funnel time falls and pH changes	Contamination, dilution or water-source change may have altered the clay structure.	Verify pH measurement and compare with a fresh controlled sample.
pH shifts but other field tests appear normal	An early chemistry change may exist before a larger rheology response becomes visible.	Confirm with a calibrated meter and review recent additions or cement contact.
Repeat results scatter widely	Stratification, poor homogenization, air, dirty equipment, inconsistent timing or damaged test components.	Use a new representative sample and a second checked instrument where practical.

These patterns are diagnostic prompts, not automatic treatment instructions. The approved technical owner should decide the response after considering the application, stage, geology, mix design and full set of results.

Common Testing Errors and Corrective Checks

Error	How it distorts the result	Corrective check
Sampling only from the surface	May miss settled solids or property changes at depth.	Use the location and depth specified in the inspection plan.
Testing an unlabelled container	Breaks traceability and may apply the wrong acceptance limit.	Quarantine the result and collect a correctly identified sample.
Aerating the sample	Can lower apparent density and disrupt repeatability.	Homogenize gently and refill the mud balance without trapped air.
Dirty Marsh funnel outlet	Artificially increases flow time or causes irregular discharge.	Clean, inspect and perform the required water check.
Wrong receiving volume	Produces a time that cannot be compared with the stated method.	Use the matching graduated cup and record the volume.
Damaged or blinded sand screen	Changes the retained fraction and may give a false percentage.	Inspect against light and replace a damaged screen.
Reading the mud balance on the wrong scale	Creates a unit or transcription error.	Circle the required unit on the record before testing.
Exterior slurry on the balance cup	Adds weight and raises the indicated density.	Clean and dry the exterior before balancing.
Comparing tests at different temperatures	Can make funnel-time trends misleading.	Record temperature and compare under the project’s conditioning rule.
Averaging a failed and a passing retest	Hides the cause of a nonconforming or invalid result.	Keep both readings, document the investigation and report the valid result under the quality procedure.

Minimum Bentonite Slurry Test Record

• Project, location, element or bore identification.
• Unique sample ID, slurry batch or tank and product identification.
• Sampling point, depth, operating stage, date and time.
• Slurry preparation time, conditioning or hydration period and recent treatment.
• Make-up water source and any recorded water-quality result relevant to the method.
• Sample appearance and temperature.
• Test method or project procedure revision.
• Equipment ID and calibration or verification status.
• Density result with unit.
• Marsh funnel time with receiving volume and temperature.
• Sand content with screen designation and percentage by volume.
• pH result and measurement type.
• Fluid loss, filter-cake or rheology results where required.
• Applicable acceptance limit for that exact operating stage.
• Pass, investigate or fail decision and authorized reviewer.
• Corrective action, mixing or treatment details, retest sample ID and final disposition.

The record should preserve the original result. Never overwrite an out-of-limit value with a later passing retest. Link the two records so the treatment history and decision remain auditable.

How to Compare Bentonite Samples Fairly

A supplier comparison is meaningful only when the test preparation is controlled. Use the same make-up water, bentonite concentration, weighing basis, addition rate, mixer geometry, mixing energy, batch size, temperature, hydration time, sample handling and test sequence. If one slurry is tested immediately and another after full conditioning, the comparison measures procedure differences as well as material differences.

Variable to fix	What to record	Risk if uncontrolled
Water	Source, temperature, pH and relevant hardness or dissolved-ion data.	Different hydration and gel development.
Concentration	Dry mass, water mass or volume and calculation basis.	An apparent material advantage caused by a higher loading.
Powder addition	Rate, addition point and method used to avoid lumps.	Unequal wetting and persistent agglomerates.
Mixing	Mixer, impeller, speed, time, batch geometry and temperature rise.	Different dispersion energy and structure development.
Conditioning	Container, rest time, temperature and remixing before test.	Comparison at different hydration stages.
Test method	Instrument, sample volume, sequence, temperature and units.	Results that look comparable but were generated differently.
Replicates	Number of independent preparations and repeat readings.	A decision based on one accidental result.

Camp-Shinning provides product recommendation, technical consultation, formula optimization, remote technical support and sample testing. The company also supports TDS, SDS and COA documentation for relevant products, subject to confirmation for the selected material. Send the complete test method and raw observations rather than only a final pass/fail statement.

Frequently Asked Questions

Request a Controlled Bentonite Evaluation

Send Camp-Shinning the slurry application, water analysis, preparation method, equipment, conditioning time, named test standard, target property window and current raw results. The technical team can review the screening direction without assuming that a limit from another project applies to yours. If material is required for a controlled comparison, review sample testing options or request technical consultation.