Cement Mortar Strength Testing Method: ISO 679 / GB/T 17671 Procedure for Flexural and Compressive S

Cement mortar strength testing is the principal method for evaluating the mechanical performance of cement, determining both flexural and compressive strength from standardized 40 mm × 40 mm × 160 mm mortar prisms. Based on ISO 679 and its Chinese equivalent GB/T 17671, the method underpins cement strength grade classification, factory quality control, and third-party conformity assessment worldwide. This article explains the complete testing workflow, from mortar proportioning and specimen preparation to curing regimes, loading procedures, calculation rules, and result evaluation.

Technical Overview

The strength of cement is not measured on neat paste alone; it is assessed through a standardized mortar that reproduces the binding behavior of cement in a reproducible and comparable matrix. The cement mortar strength test prepares prismatic specimens from a fixed mortar composition, cures them under strictly controlled temperature and humidity conditions, and loads them to failure to determine flexural strength and compressive strength at specified ages.

The internationally harmonized procedure is defined in ISO 679 (Cement — Test methods — Determination of strength), adopted in China as GB/T 17671 (Test method of cement mortar strength, ISO method) and implemented in Europe as EN 196-1. Results at standard ages — typically 3 days and 28 days — are used to classify cement into strength grades such as 32.5, 42.5, and 52.5, to verify conformity with product standards such as GB 175 and EN 197-1, and to support quality control, certification, and dispute resolution decisions.

Because the method is highly sensitive to temperature, timing, compaction energy, and loading rate, laboratories must maintain tight tolerances (for example, curing water at 20 ± 1 °C) and use calibrated equipment to keep inter-laboratory variation within acceptable precision limits.

Testing Samples

Cement

A representative cement sample is taken in accordance with recognized sampling practice, protected from moisture and contamination in sealed containers, and conditioned to laboratory temperature (20 ± 2 °C) before testing. The mixing room shall be maintained at 20 ± 2 °C with relative humidity of not less than 50%.

Standard Sand

ISO standard sand is a natural, rounded siliceous sand with a silica content of at least 98% and a moisture content not exceeding 0.2%. It is graded into three fractions — 0.08 mm to 0.5 mm, 0.5 mm to 1.0 mm, and 1.0 mm to 2.0 mm — in approximately equal proportions and is supplied pre-weighed in 1350 g bags, one bag per test batch. Using certified standard sand eliminates aggregate variability and ensures comparability between laboratories.

Water

Distilled or deionized water (or clean potable water) is used as mixing water, weighed to 225 g ± 1 g per batch. The water, cement, and sand shall be brought to laboratory temperature before mixing.

Mortar Proportions and Specimens

One test batch produces enough mortar for a gang of three prisms, yielding three flexural results and six compressive results:

• Cement: 450 g ± 2 g
• Standard sand: 1350 g ± 5 g
• Water: 225 g ± 1 g
• Water/cement ratio: 0.50; sand/cement ratio: 3:1 by mass
• Specimen geometry: prisms of 40 mm × 40 mm × 160 mm

Testing Items

The core deliverables of the test are the strength values at specified ages, which are then compared with the limiting values of the applicable product standard:

• Flexural strength (MPa), determined by three-point bending of full prisms at each specified age
• Compressive strength (MPa), determined on the half-prisms produced by the flexural test
• Strength grade conformity, including minimum early-age (typically 3 d) and 28-day compressive strength requirements for grades such as 42.5 and 52.5 and their rapid-hardening (R) variants under GB 175, EN 197-1, and equivalent standards
• Strength development evaluation between early and later ages where required by the product specification or the purchaser

Testing Methods

Apparatus

• Planetary mortar mixer with low-speed and high-speed stages and a standard mixing bowl and paddle
• Gang molds with three 40 mm × 40 mm × 160 mm cavities, watertight and fitted with base plates
• Jolting table (compaction apparatus) delivering 60 jolts in 60 s, or an approved vibrating table
• Flexural test device with two support rollers 100 mm apart and a centered loading roller
• Compression device with 40 mm × 40 mm auxiliary platens
• Testing machine capable of the specified constant loading rates
• Moist cabinet or curing box at 20 ± 1 °C and relative humidity not less than 90%, and water baths at 20 ± 1 °C
• Calibrated balances, timers, and calibrated temperature-measuring devices

Mortar Mixing Procedure

Place the water in the bowl, add the cement taking care to avoid material loss, and start the mixer at low speed. After 30 s, add the sand uniformly over the following 30 s while still mixing at low speed. Switch to high speed for 30 s, then stop the mixer for 90 s, during which any mortar adhering to the bowl wall and paddle is scraped into the mix. Finally, mix at high speed for 60 s. The complete program lasts approximately 4 minutes and must be followed without interruption.

Molding and Compaction

Immediately after mixing, introduce the mortar into the mold in two layers of approximately equal thickness. Compact each layer with 60 jolts in 60 s on the jolting table. After compaction, strike off the excess mortar level with the top of the mold using a straightedge, mark the specimens for identification, cover the molds to prevent water loss, and place them in the moist cabinet at 20 ± 1 °C and at least 90% relative humidity.

Curing Conditions

Demold the prisms between 20 h and 24 h after casting, handling them carefully to avoid damage, and immediately submerge them in clean water at 20 ± 1 °C until the time of testing. Specimens shall be tested within the following age tolerances:

• 24 h: ± 15 min
• 48 h: ± 30 min
• 72 h: ± 45 min
• 7 d: ± 2 h
• 28 d: ± 8 h

After removal from the curing water, the prisms are wiped clean and tested while still in a wet condition, normally within 15 minutes of removal.

Flexural Strength Determination

Place a full prism on the two support rollers 100 mm apart and apply the load through the centered loading roller at a constant rate of 50 N/s ± 10 N/s until fracture. The flexural strength is calculated as Rf = (1.5 × Ff × L) / b³, where Ff is the failure load, L is the distance between supports (100 mm), and b is the side of the square section (40 mm). Each prism yields one flexural result, giving three results per batch.

Compressive Strength Determination

Test the two half-prisms from each flexural failure in compression between the 40 mm × 40 mm auxiliary platens, centering each half-prism so that the load is applied to a 1600 mm² bearing face. Apply the load at a constant rate of 2400 N/s ± 200 N/s until failure. The compressive strength is calculated as Rc = Fc / 1600, where Fc is the maximum load in newtons. Six compressive results are obtained per batch.

Calculation and Evaluation of Results

Current editions of ISO 679 and GB/T 17671 specify the following evaluation rules, with all strengths reported to the nearest 0.1 MPa:

• Flexural strength: reported as the median of the three prism results (earlier editions used the arithmetic mean after discarding values deviating more than ±10% from the mean)
• Compressive strength: calculate the median of the six half-prism values; if one value deviates by more than 10% from the median, discard that value and report the mean of the remaining five
• If a batch produces abnormal fractures or insufficient valid results, the batch is rejected and the test is repeated with a fresh batch

Applications

The cement mortar strength test is the reference method for strength evaluation throughout the cement and construction industries. Typical applications include:

• Factory production control by cement manufacturers and conformity testing for strength grade classification under GB 175, EN 197-1, and equivalent product standards
• Independent verification by accredited third-party laboratories supporting product certification and market surveillance programs
• Construction quality supervision, including incoming inspection of cement delivered to projects and verification of supplier declarations of performance
• Research and development, comparing cement formulations, supplementary cementitious materials, chemical admixtures, and grinding aids
• Dispute resolution between producers and purchasers, where the standardized method provides a technically defensible reference result

FAQ

Why is cement strength tested on mortar rather than neat cement paste?

Neat paste shrinks severely, is difficult to handle, and does not represent how cement performs in concrete. A standardized 1:3 mortar with a 0.50 water/cement ratio provides a workable, reproducible matrix in which the binding capacity of the cement dominates the result, while the certified standard sand removes aggregate variability.

At what ages must the specimens be tested?

Product standards typically require 3-day and 28-day strengths for grade classification, with 7-day testing also common. For special or rapid-hardening cements, 24 h and 48 h ages may be specified. The strict age tolerances (for example, ± 8 h at 28 days) must be observed to ensure valid, comparable results.

How does the ISO method (GB/T 17671) differ from ASTM C109?

The ISO method uses 40 mm × 40 mm × 160 mm prisms, a 1:3 mortar at a 0.50 water/cement ratio, flexural testing followed by compression on half-prisms, and compaction by jolting. ASTM C109 uses 50 mm (2 in.) cubes, a 0.485 water/cement ratio, and a different graded standard sand. The two methods are correlated but their compressive strength values are not directly interchangeable.

How many specimens are needed for one test?

One mortar batch fills a gang mold of three prisms. The three prisms provide three flexural results, and the resulting six half-prisms provide six compressive results, which is the minimum complete data set for reporting.

Which factors most influence the accuracy of the results?

Curing temperature control (20 ± 1 °C), the quality and moisture of the standard sand, consistent compaction (60 jolts per layer), accurate loading rates (50 N/s in flexure, 2400 N/s in compression), correct demolding time, and testing at the specified age all materially affect the outcome. Calibrated apparatus and trained operators are therefore essential.

How is a cement strength grade assigned from the test results?

The 28-day compressive strength must meet or exceed the grade value (for example, 42.5 MPa for grade 42.5 cement), and both the early-age and 28-day values must satisfy the minimum requirements of the governing product standard. Conformity assessment follows the statistical acceptance rules of that standard, typically applied to running production data in factory control.