Cement Mortar Strength Test Procedure: Step-by-Step Guide to ISO 679 and GB/T 17671 Methods

The cement mortar strength test is the fundamental procedure used worldwide to determine the compressive and flexural strength of cement, forming the basis for strength grade classification and quality conformity assessment. This article provides a complete, step-by-step technical guide to the cement mortar strength test procedure in accordance with ISO 679, EN 196-1, and GB/T 17671, covering apparatus, mix proportion, mixing, molding, curing, testing, and result calculation. It is intended for laboratory technicians, quality engineers, and construction professionals involved in cement testing and quality control.

Technical Overview

The cement mortar strength test measures the mechanical performance of hardened cement mortar by preparing standardized prismatic specimens and loading them to failure at specified ages. The internationally recognized method is specified in ISO 679 (Cement — Test Methods — Determination of Strength), which is technically aligned with the European standard EN 196-1 and the Chinese national standard GB/T 17671 (Test Method of Cement Mortar Strength, ISO Method). In North America, ASTM C109 is commonly used, which evaluates compressive strength on 50 mm cube specimens.

Under the ISO method, a mortar consisting of cement, ISO standard sand, and water at a fixed water-to-cement ratio of 0.50 is compacted into 40 mm × 40 mm × 160 mm prisms. The prisms are cured under strictly controlled temperature and humidity conditions and tested in flexure and compression at ages such as 3 days and 28 days. The resulting strength values are used to verify compliance with cement product standards, assign strength grades (for example, 42.5 or 52.5), and support quality control, research, and arbitration testing in accredited third-party laboratories.

Testing Samples

Cement Sample

The cement sample shall be representative of the consignment or production batch and taken in accordance with the relevant sampling procedures. It should be stored in airtight containers to prevent moisture uptake and carbonation, and brought to laboratory temperature (20 ± 2 °C) before testing.

Standard Sand

ISO standard sand is a natural, graded siliceous sand with particle sizes between 0.08 mm and 2.00 mm, distributed across coarse, medium, and fine fractions in defined proportions. Certified standard sand must be used because the grading and mineralogical purity of the sand directly influence the measured strength values.

Mixing Water

Distilled water, deionized water, or clean potable water is used for the mortar mix. The mass of water is fixed by the specified water-to-cement ratio of 0.50.

Environmental Conditions

• Laboratory air temperature: 20 ± 2 °C, relative humidity not less than 50%.
• Temperature of cement, sand, water, and apparatus at the time of mixing: 20 ± 2 °C.
• Temperature of the moist cabinet or room for initial curing: 20 ± 1 °C, relative humidity not less than 90%.
• Temperature of the curing water: 20 ± 1 °C.

Testing Items

The cement mortar strength test comprises two principal determinations:

• Flexural strength (bending strength): determined by center-point loading of a full 40 mm × 40 mm × 160 mm prism, expressed in megapascals (MPa).
• Compressive strength: determined on the two half prisms remaining after the flexural test, loaded through 40 mm × 40 mm auxiliary platens, expressed in MPa.

Both determinations are typically performed at 3 days and 28 days of age for ordinary Portland cement, and at additional or alternative ages (24 h, 48 h, 72 h, 7 days) depending on the cement type and product standard requirements.

Testing Methods

Apparatus and Materials

• Planetary mortar mixer with a stainless steel bowl and beater, capable of low and high rotational speeds as specified in ISO 679.
• Three-gang steel prism molds with internal dimensions of 40 mm × 40 mm × 160 mm per compartment.
• Jolting table (compaction apparatus) delivering 60 jolts in 60 seconds with a drop height of 15 mm.
• Flexural test device with center loading roller and two supporting rollers spaced 100 mm apart.
• Compression test device with auxiliary platens of 40 mm × 40 mm, and a compression testing machine of suitable capacity and verified accuracy.
• Moist cabinet or humid room, and water curing tanks with temperature control at 20 ± 1 °C.
• Balances accurate to ± 1 g for cement and sand, and ± 1 g or better for water.

Mix Proportion

One standard batch of mortar consists of:

• Cement: 450 g ± 2 g
• ISO standard sand: 1350 g ± 5 g
• Water: 225 g ± 1 g

This corresponds to a sand-to-cement ratio of 3:1 by mass and a water-to-cement ratio of 0.50, yielding enough mortar to fill one three-gang mold with a small surplus.

Step-by-Step Mixing Procedure

• Place the water in the mixer bowl, then add the cement, and immediately start the mixer at low speed for 30 seconds.
• During the next 30 seconds, add the entire quantity of standard sand steadily and evenly into the bowl while mixing continues at low speed.
• Switch to high speed and mix for 30 seconds.
• Stop the mixer for 90 seconds. During the first 15 to 30 seconds of this pause, scrape down any mortar adhering to the bowl wall and beater into the rest of the mix.
• Restart at high speed and mix for a final 60 seconds.
• The total mixing operation, from adding the cement to the final stop, takes 240 seconds.

Molding and Compaction Steps

• Assemble clean, lightly oiled molds on the jolting table, ensuring the molds are rigidly clamped.
• Immediately after mixing, place approximately half of the mortar (about 700 g) into each mold compartment and spread it evenly with a tamper so that the layer fully covers the bottom.
• Compact the first layer by jolting 60 times in 60 seconds.
• Fill the molds with the remaining mortar, spread it evenly, allow it to penetrate slightly, and compact the second layer with a further 60 jolts in 60 seconds.
• Strike off the excess mortar with a dampened straightedge or metal ruler and smooth the surface so that it is flush with the mold edges.
• Mark each specimen with an identification number or code, cover the molds to prevent water evaporation, and place them in the moist cabinet or humid room.

Curing Procedure

• Initial curing: keep the molds in the moist cabinet or humid room at 20 ± 1 °C and relative humidity not less than 90% for 20 to 24 hours after molding.
• Demolding: remove the prisms from the molds between 20 h and 24 h after molding. If the 24-hour strength is to be tested, demold the prisms immediately before testing.
• Water curing: after demolding, immerse the prisms in clean water at 20 ± 1 °C, positioned so that they do not touch each other, until the time of testing. The water should be changed at intervals to keep it clean.
• Testing ages and permitted tolerances: 24 h ± 15 min; 48 h ± 30 min; 72 h ± 45 min; 7 days ± 2 h; 28 days ± 8 h. Surfaces of prisms shall be wiped clean and free of surface water before testing.

Flexural Strength Test Steps

• Place the prism on the two supporting rollers of the flexural device, 100 mm apart, with a lateral face in contact with the rollers.
• Apply the load vertically through the center loading roller at a uniform rate of 50 N/s ± 10 N/s until fracture.
• Record the maximum load at failure.
• Calculate the flexural strength using the formula Rf = 1.5 × Ff × L / b³, where Ff is the failure load in newtons, L is the distance between supporting rollers (100 mm), and b is the side of the square section (40 mm).
• Each broken prism yields two half prisms, which are immediately used for the compressive strength test. Test three prisms in total for one batch.

Compressive Strength Test Steps

• Place each half prism between the 40 mm × 40 mm auxiliary platens, with the loading faces being the two lateral faces that were in contact with the mold walls during compaction, so that the load is applied perpendicular to the compacted layers.
• Center the half prism relative to the platens and the machine loading axis.
• Apply the load at a uniform rate of 2400 N/s ± 200 N/s until failure.
• Record the maximum load. Six half prisms are tested per batch.
• Calculate the compressive strength using the formula Rc = Fc / 1600, where Fc is the maximum load in newtons and 1600 mm² is the loaded area (40 mm × 40 mm).

Calculation and Expression of Results

• Flexural strength: report the arithmetic mean of the three prism results. If any single value deviates from the mean by more than ± 10%, discard that value and report the mean of the remaining two.
• Compressive strength: report the arithmetic mean of the six half-prism results. If one of the six values deviates from the mean by more than ± 10%, discard that value and report the mean of the remaining five. If two or more values exceed the ± 10% limit, retesting is required.
• Express all strength results in megapascals (MPa) to one decimal place.
• The test report should include specimen identification, cement type and source, date of molding and testing, test ages, individual and mean strength values, and any deviations from the standard procedure.

Key Precautions and Quality Control Points

• Maintain strict control of temperature and humidity throughout mixing, curing, and testing, since even small deviations alter hydration and strength development.
• Verify and calibrate the mixer, jolting table, molds, and testing machines at specified intervals in accordance with the standard requirements.
• Use calibrated timers for mixing steps and compaction jolts; timing errors are a common source of variability.
• Ensure loading rates are held within tolerance, as excessive rates artificially inflate strength values.
• Handle prisms carefully during demolding and transport to avoid microcracks that reduce measured strength.

Applications

The cement mortar strength test procedure is applied across the entire cement and construction value chain, including:

• Factory quality control and routine production testing at cement manufacturing plants.
• Conformity assessment and certification of cement against product standards and strength grade requirements, such as 32.5, 42.5, and 52.5 grades.
• Acceptance and verification testing of cement delivered to construction projects by contractors, supervisors, and accredited third-party laboratories.
• Arbitration and dispute resolution where independent strength data are required.
• Research and development of new cementitious materials, supplementary cementitious materials, and admixtures.
• Comparative studies between the ISO prism method and the ASTM C109 cube method for international projects.

FAQ

Why is the standard test age 28 days?

At 28 days, the hydration of Portland cement has progressed to a stage where strength development has substantially stabilized, and the measured value correlates well with long-term structural performance. Cement standards worldwide use 28-day mortar strength as the principal conformity criterion, while early ages such as 3 days or 7 days verify early strength development.

Why must ISO standard sand be used instead of local sand?

The grading, particle shape, and mineral composition of sand strongly affect mortar strength. ISO standard sand is produced and certified to a tightly controlled specification, which ensures that measured strength differences reflect the cement itself rather than aggregate variability, making results comparable between laboratories and countries.

What is the difference between the ISO prism method and ASTM C109?

The ISO method (ISO 679, EN 196-1, GB/T 17671) uses 40 mm × 40 mm × 160 mm prisms compacted on a jolting table at a 1:3 sand-to-cement ratio and 0.50 water-to-cement ratio, testing both flexural and compressive strength. ASTM C109 uses 50 mm cube specimens, a different sand-to-cement ratio of 1:2.75, a water-to-cement ratio of 0.485 for portland cements, and determines compressive strength only. Results from the two methods are not directly interchangeable.

How are abnormal results handled?

For flexural strength, if one of the three values deviates by more than ± 10% from the mean, it is discarded and the mean of the remaining two is reported. For compressive strength, if one of the six values deviates by more than ± 10% from the mean, it is discarded and the mean of the remaining five is reported. If more values fall outside the limit, the test must be repeated.

What factors most commonly cause inaccurate results?

Frequent causes include temperature deviations during curing, incorrect loading rate, inaccurate weighing of materials, worn or uncalibrated jolting tables and molds, delayed or premature demolding, and improper positioning of half prisms in the compression device. Rigorous adherence to the specified procedure and a regular calibration program are essential for reliable results.

Can this procedure be used for blended or specialty cements?

Yes. The ISO 679 procedure is the reference strength method for common cements, blended cements containing supplementary cementitious materials, and other hydraulic binders, although some product standards specify modified ages or additional requirements. Always confirm the applicable product standard before testing.