The cement mortar strength molding test is a standardized laboratory procedure used to prepare prismatic mortar specimens for determining the mechanical strength of hydraulic cements. Performed in accordance with GB/T 17671 (identical to ISO 679) and EN 196-1, the test ensures that cement products meet specified flexural and compressive strength requirements at 3 and 28 days of age. This article provides a complete technical guide to the molding workflow, covering material proportioning, mixing, compaction, curing, and strength evaluation.
Technical Overview
The cement mortar strength molding test is the specimen preparation stage of cement strength determination, in which a cement–standard sand–water mortar is mixed, compacted into prismatic steel molds, cured under strictly controlled conditions, and subsequently tested for flexural and compressive strength. The molding operation directly governs the density, homogeneity, and dimensional accuracy of the specimens, and therefore the reliability of the final strength results. The governing standards include GB/T 17671 (the Chinese national standard, technically aligned with ISO 679), EN 196-1 in Europe, and ASTM C109/C109M in North America.
Under the ISO method, one batch of mortar consists of 450 g of cement, 1,350 g of ISO graded standard sand, and 225 g of water, corresponding to a sand-to-cement ratio of 3:1 by mass and a water-to-cement ratio of 0.50. The mortar is compacted in two layers in 40 mm × 40 mm × 160 mm triple prismatic molds using a jolting table, producing three specimens per batch. Because every parameter—material temperature, mixing time, jolt count, and curing regime—is fixed by the standard, the molding test provides a uniform, reproducible basis for comparing cements from different producers and production dates.
- Standard reference: GB/T 17671 / ISO 679, EN 196-1, ASTM C109/C109M
- Specimen type: 40 mm × 40 mm × 160 mm prisms, three per batch
- Mix proportion: 450 g cement, 1,350 g standard sand, 225 g water (W/C = 0.50)
- Compaction: two layers, 60 jolts per layer on a jolting table
- Test ages: typically 3 days and 28 days, with ±45 min and ±8 h tolerances respectively
Testing Samples
Cement
The sample shall be the cement to be evaluated, such as ordinary Portland cement, blended Portland cement, or other hydraulic binders within the scope of GB/T 17671. The cement should be protected from moisture and contamination, brought to approximately 20 °C before testing, and thoroughly blended so that the test portion is representative of the whole consignment. Where the cement contains settleable constituents, the full sample must be homogenized before a test portion is drawn.
ISO Standard Sand
Standard sand is a natural, rounded, siliceous sand with a grading of 0.08 mm to 2.0 mm, divided into coarse (1.0–2.0 mm), medium (0.5–1.0 mm), and fine (0.08–0.5 mm) fractions, supplied in pre-weighed 1,350 g bags. Its moisture content must be less than 0.2 % by mass. The use of standard sand is mandatory because it eliminates aggregate variability and allows the strength result to reflect the binding properties of the cement alone.
Mixing Water
Distilled water or clean potable water conforming to the standard is used for mixing, and clean water is used for curing. The water temperature at the time of mixing shall be 20 °C ± 1 °C to ensure reproducible hydration behavior. The exact mass of 225 g must be added to the mixer bowl before the cement, as prescribed by the standard procedure.
Testing Items
The cement mortar strength molding test and the subsequent strength determination cover the following examination items:
- Specimen molding quality: layer thickness, compaction, surface finish, and dimensional conformity of the prisms
- Mortar workability indicators observed during molding (cohesion, bleeding tendency) as supplementary information
- Flexural strength at 3 days and 28 days, determined by center-point loading on the whole prism
- Compressive strength at 3 days and 28 days, determined on the half prisms resulting from the flexural test using 40 mm × 40 mm auxiliary platens
- Conformity assessment of the measured strength values against the strength grade requirements of the applicable cement product standard
Each batch yields three prisms, from which three flexural results and six compressive results are obtained. The reported compressive strength is the mean of six valid results, with outlier handling performed as specified in the standard.
Testing Methods
Apparatus and Environmental Conditions
The essential equipment includes a planetary mortar mixer, triple 40 mm × 40 mm × 160 mm steel molds with base plates, a jolting (vibrating) table delivering 60 jolts in 60 seconds with a 15 mm drop height, a moist cabinet or room at 20 °C ± 1 °C and relative humidity of at least 90 %, a water curing tank at 20 °C ± 1 °C, a flexural testing machine with a loading rate of 50 N/s ± 10 N/s, and a compression testing machine with a loading rate of 2,400 N/s ± 200 N/s. The molding room shall be maintained at 20 °C ± 2 °C with relative humidity of at least 50 %, and all materials, molds, and apparatus shall be conditioned to room temperature before use.
Mix Proportions
For each batch, weigh 450 g ± 2 g of cement, 1,350 g ± 5 g of standard sand, and 225 g ± 1 g of water. One batch produces exactly one set of three prisms. The water-to-cement ratio of 0.50 is fixed for cements within the scope of the ISO method; any deviation required by special product standards must be explicitly stated in the test report.
Mixing Procedure
The mixing sequence is strictly time-controlled and executed as follows:
- Place the water in the mixer bowl, add the cement, and start the mixer at low speed for 30 seconds
- Add the sand steadily and uniformly during the next 30 seconds while continuing at low speed
- Switch to high speed and mix for a further 30 seconds
- Stop the mixer for 90 seconds; during the first 15–30 seconds, scrape the mortar adhering to the bowl wall and blade into the mix
- Resume mixing at high speed for the final 60 seconds
The total cycle, including the rest period, lasts 240 seconds. Any interruption or deviation invalidates the batch, and mixing must restart with fresh materials.
Molding and Compaction
Immediately after mixing, place a first layer of mortar, of approximately 13–14 mm compacted thickness, into each mold compartment and compact it with 60 jolts of the jolting table. Then fill the molds with a second layer of mortar and compact it with a further 60 jolts. The jolting table must complete exactly 60 jolts in 60 seconds, and the counter shall be verified before each session. After compaction of the second layer, remove excess mortar with a wet trimming knife held nearly vertical and sawing slowly, level the surface, and identify each specimen with a durable marking. The interval between the end of mixing and the completion of the second compaction shall not exceed 15 minutes, and preferably should be shorter.
Demolding and Curing
Place the molds, with base plates and cover plates, in the moist cabinet at 20 °C ± 1 °C and relative humidity of at least 90 %. Demold the specimens 20 to 24 hours after molding, taking care not to damage them, and record the time. Immediately after demolding, immerse the prisms in the curing water tank at 20 °C ± 1 °C, positioned so that water circulates freely around all faces and with at least 5 mm of water above and below each specimen. The water shall be renewed every 15 days, and specimens shall not be allowed to dry before testing.
Strength Determination
Test the specimens at the specified ages—3 days ± 45 minutes and 28 days ± 8 hours are the standard control ages—within 15 minutes of removal from the curing tank. First determine flexural strength by center-point loading at 50 N/s ± 10 N/s; the strength is calculated from the maximum load and the section modulus of the 40 mm × 40 mm prism. Then test the two half prisms in compression at 2,400 N/s ± 200 N/s using 40 mm × 40 mm auxiliary platens applied to the sides that were in contact with the mold walls. Acceptance of results and rounding follow the rules of GB/T 17671 and ISO 679. Under the ASTM C109/C109M alternative, 50 mm cube specimens are compacted by tamping and tested in compression only, so results from the two systems are not interchangeable.
Applications
The cement mortar strength molding test is one of the most widely performed procedures in construction materials laboratories and serves multiple purposes across the industry:
- Factory quality control at cement plants, where every production lot is sampled and tested before release
- Acceptance testing and conformity verification of delivered cement at construction projects
- Certification and surveillance testing conducted by accredited third-party laboratories for product marking and regulatory compliance
- Verification of strength grade declarations (for example, 42.5, 52.5) against the requirements of the applicable product standards
- Research and development of new binders, supplementary cementitious materials, and admixtures, where standardized molding enables direct comparison of experimental mixes
- Arbitration and dispute resolution, since the method is legally recognized and internationally harmonized
Because the molding conditions are fixed by the standard, the test isolates the influence of cement quality from other variables, making it the reference basis for trade specifications, engineering contracts, and national supervision inspections.
FAQ
Q: Why must ISO standard sand be used instead of local natural sand?
A: Standard sand has a controlled grading, particle shape, and mineral composition, which eliminates aggregate variability. This ensures that the measured strength reflects only the properties of the cement, allowing valid comparison between laboratories worldwide.
Q: Why is the mortar compacted in two layers of 60 jolts each?
A: Two-layer filling with 60 jolts per layer guarantees uniform density throughout the 160 mm depth of the prism and removes entrapped air. Insufficient or excessive compaction changes the porosity of the specimen and directly distorts the strength result.
Q: When should specimens be demolded, and what happens if they are demolded late?
A: Specimens must be demolded between 20 and 24 hours after molding. Demolding too early can damage immature specimens, while demolding after 24 hours must be noted in the report because the effective curing history differs from the standard regime.
Q: What temperature and humidity must be maintained during molding and curing?
A: The molding room shall be at 20 °C ± 2 °C with relative humidity of at least 50 %; the moist cabinet at 20 °C ± 1 °C with relative humidity of at least 90 %; and the curing water tank at 20 °C ± 1 °C. Even small deviations in curing temperature measurably affect early-age strength.
Q: Are results from GB/T 17671 (ISO 679) and ASTM C109 directly comparable?
A: No. The ISO/GB method uses 40 mm × 40 mm × 160 mm prisms compacted on a jolting table with a W/C of 0.50, whereas ASTM C109 uses 50 mm cubes compacted by tamping, typically at a W/C of 0.485 with a flow requirement. The different specimen geometry and preparation regime produce systematically different strength values.
Q: How many specimens are needed for one valid result?
A: One batch of mortar yields three prisms. Testing provides three flexural values and six compressive values; the reported flexural strength is normally the median of three, and the compressive strength is the mean of six valid half-prism results, subject to the outlier rules of the standard.
