A sand can enter a mixer and pass through a pump without producing the required coating. Choosing its fractions needs to connect three questions: what material is being prepared, how is it being conveyed, and what result must remain on the substrate after spraying? The simple description “fine sand” does not answer that sequence of questions.
In Algeria, the CTC separates render formulations, substrates and application methods in its defect-identification sheets.1 This local framework gives a reason for qualifying the complete process. It does not establish a volume of silica-sand demand or the existence of a particular installation.
The following guide proposes a comparison method for mortar containing sand. It distinguishes machine-applied rendering from dry-process spraying, then organises material, circuit and panel trials. The numerical examples are educational: they are neither a commercial recipe nor Minerallum test results.
1. Identify the process before choosing the fraction
“Sprayed mortar” can describe several arrangements of work. Mortar that has already been mixed with water can be pumped to a nozzle where air provides the spraying action. A dry mixture can also be conveyed pneumatically, with water introduced at the nozzle. Where water enters the system therefore changes the material travelling through it.
EFNARC describes these processes separately for sprayed concrete.3 This distinction helps define the stages; its concrete provisions should not automatically become requirements for a thin render.
In your trial specification, draw the actual route: storage, batching, any mixing stage, the pump or conveying machine, hoses, nozzle and substrate. Identify accessible sampling points. The selected sand needs to be compatible with this route and the formulation, rather than with an abstract machine.
2. Distinguish a dry premix from dry-process spraying
A bagged dry premixed mortar is not necessarily sprayed by the dry process. It can receive water in a mixing machine and then be pumped as a wet material. The product’s commercial presentation is therefore insufficient to classify its application process.
For example, the Putzmeister catalogue presents the MP 25 as a machine processing dry premixed mortars with water supplied in the mixing zone.2 That description concerns this equipment, rather than every rendering machine.
For qualification, describe what leaves the mixer and what enters the hose. Also state whether preparation takes place on site or in a factory. Two products delivered in bags can need different conveying trials when their water-mixing arrangements and equipment differ.
3. Read the machine limit without treating it as a recipe
A maximum particle size advertised for equipment is a compatibility constraint, rather than a recommended grading curve. The cited catalogue lists, for example, 4 mm for the MP 25 and 6 mm for certain other models.2 These values do not make every mixture within those sizes suitable.
Record the model, configuration, accessories and applicable instructions. The circuit actually assembled needs to be assessed, particularly after a hose or nozzle change. A limit taken from a different model does not validate your installation.
Ask the material supplier for a controlled distribution rather than a promise of pumpability. Keeping these questions separate allows the aggregate to be checked at receipt, followed by qualification of its use in a defined formulation and circuit.
4. Build a distribution that can be interpreted
A fraction designation does not, on its own, describe the distribution within that range. Two sands with the same nominal size range can contain different proportions of fines, intermediate grains and grains close to the upper limit.
The Mineral Products Association bulletin explains size designation using d/D and the separate identification of fines.4 It provides useful terminology; it does not set a mandatory curve for an Algerian site.
In the proposed enquiry, specify the sieves, preparation method and dry basis. Present cumulative percentages passing and, where helpful for analysis, the masses between sieves. Do not directly compare a measurement on wet sand with a distribution prepared differently without explaining the difference in preparation.
Also check whether each number refers to a cumulative percentage passing or to an individual fraction between sieves. In a cumulative presentation, the percentage passing cannot increase as the sieve opening becomes smaller. Resolve an inconsistency in presentation before calculating the blend, even when the reported numbers appear close to the target distribution.
5. Compare fraction blends by mass balance
A blend of two fractions can be calculated before a trial. Imagine fraction A with 80% passing a particular sieve and fraction B with 20% passing it. At 60% A and 40% B by dry mass, the predicted percentage passing is 0.60 × 80 + 0.40 × 20 = 56%.
Repeat the calculation for every sieve being used. An acceptable result at one sieve does not ensure an acceptable curve elsewhere. The blend percentages need to remain the same in every line of the calculation.
Then check the blend actually prepared. The calculation assumes representative distributions and homogeneous mixing. Fractions separating between batching and use can produce a sample that differs from the theoretical curve.
6. Treat fines as an identifiable variable
Comparing fines requires more than one overall number. State the size threshold measured and any prior treatment. Also retain useful information about their nature: a size difference does not automatically describe a mineralogical difference or contamination.
For the proposed programme, prepare a reference and a variant whose distributions are known. Maintain the other materials and mixing-water arrangement when investigating this variation. If water or an admixture needs adjustment, record that change as a second variable.
The objective is not systematically to obtain the finest sand. It is to define a reproducible distribution that permits qualified application and a finished product meeting the project requirements. A change favourable to conveying still needs to be evaluated against the completed panel.
7. Keep water and moisture in the calculation
Wet-sand mass and dry-sand mass are not interchangeable. In an example where 500 kg of dry sand carries a water content of 4% relative to dry mass, the wet material weighs 520 kg and contains 20 kg of water.
This calculation describes the total water associated with the sand under that definition. It is not sufficient to determine all the water available to the formulation: the protocol also needs to specify how absorption is taken into account.
For a comparison, retain the moisture definition and the method of correcting the batch quantities. Otherwise, a change presented as a grading change can also alter the dry quantities and mixture water. Check the sand conditions actually used, rather than moisture measured at a different stage.
8. Separate circulation from application on a panel
First organise a circulation trial under the manufacturer’s procedure, using a mixture admissible for the equipment. Record output stability, interruptions and accessible operating parameters. Any intervention on a pressurised circuit needs to follow its procedure for putting the system into a safe condition.
Next organise spraying onto a representative panel. Passing through the hose does not establish adhesion, consistent thickness or finish. The panel needs to address the product criteria and allow observation after hardening.
Keep these stages separate in the account of the trial. A variant can succeed in circulation but fail on the substrate, or require a different setting. The decision should describe that combination without attributing every result to the sand fraction alone.
9. Distinguish instantaneous output from useful production
Imagine 20 litres per minute during ten minutes of actual spraying. The conveyed volume is 200 litres. If preparation, a pause and cleaning bring the observation period to twenty minutes, the average output over that period is 10 litres per minute.
Both indicators are correct, but they answer different questions. Neither independently gives the conforming area produced. Thickness, losses and reworking still need evaluation through their own measurements.
To compare two variants, choose the same boundaries for measuring time. Describe whether priming, bag changes, cleaning or a technical stoppage are included. An improvement in active output can be cancelled by additional interruptions when the assessment considers the production that is actually useful.
10. Measure losses without calling everything rebound
Rebound describes material returning after impact, but the site can also lose material before impact or during other operations. Define the collected categories and their boundaries before comparing numbers.
EFNARC emphasises that rebound depends on spraying and that material in place can differ from the initial mixture.3 For this guide, the objective is to retain a clear balance without importing the dimensions of a sprayed-concrete test into a render trial.
Compare on a common mass or volume basis, with compatible moisture conditions. If categories are combined, call the result “collected losses” rather than assigning an identity that the measurement method does not establish.
11. Calculate a rate with the right denominator
Suppose 100 kg of material is used for a sequence, with 12 kg of collected losses on a comparable basis. Losses represent 12% of the input. If the balance identifies 88 kg retained, the ratio of losses to retained material is 12/88, or approximately 13.6%.
These two ratios are not interchangeable. State the denominator and included categories before announcing an improvement. A retained mass calculated by subtraction also assumes that all the other destinations of material are known.
Do not automatically present the 88 kg as conforming coating. Part of it can need reworking or removal. The material balance and product inspection complement one another: they should remain separate until the decision is made.
12. Control the substrate without redirecting the investigation
Sand remains the subject of qualification, but substrate condition needs to remain comparable. The CTC identifies cracks, adhesion defects and execution irregularities among render defects.1 Those observations do not prove a single cause in the aggregate.
For the proposed panel, specify substrate type and preparation. Retain the zones, thicknesses and application conditions. A variant sprayed onto a different surface can produce a result that cannot be attributed to the material change.
If a difference appears, describe its location: interface, mortar body or surface. Have the necessary analyses selected according to that observation. A photograph of detachment alone does not measure adhesion or establish where the defect originated.
13. Connect thickness to the volume actually required
For a geometrical area of 20 m² and a uniform thickness of 10 mm, the theoretical volume is 20 × 0.010 = 0.20 m³, or 200 litres. Changing to 12 mm gives 240 litres, and therefore 20% additional volume.
This calculation includes neither roughness nor losses nor actual thickness variation. It shows why comparing only the bags consumed can be misleading when the panels do not have the same geometry.
Measure or document the parameters appropriate to the project. Present geometrical volume, material committed and accepted area separately. Sand should not receive credit for apparent savings that simply result from a layer thinner than the one required.
14. Audit finishing and hardening
Set the visible criteria and planned checks before the trial: regularity, surface condition, thickness and applicable performance. Describe finishing operations rather than comparing panels that have received different treatments.
Plan observations at agreed ages and retain the conditions of storage or protection. An immediately observed difference is not the whole qualification; a later comparison without a history of the first days can also lose useful information.
The result needs to state which preparation was retained and for which application. A good render finish does not demonstrate suitability for structural repair. Different product families can require different tests and responsibilities.
15. Turn the trial into a receipt specification
The final specification connects the accepted distribution, checking method, mixture preparation and validated application scope. Define which changes require verification: fraction, formulation, circuit or spraying method.
Retain a relevant comparison reference and the panel results. A grading certificate allows a material to be checked; it does not replace mortar or process qualification. This distinction makes acceptance more precise and investigations faster.
Our silica-sand page supports preparation of your enquiry. A useful requirement states a measurable fraction and the context of its transformation. For a project in Algeria, selecting the sand thereby becomes clearer without confusing the material, machine settings and acceptance of the completed coating.