Cement Mortar Strength Testing by the ISO Method (ISO 679 / GB/T 17671): Principles, Procedure and A

Cement mortar strength testing by the ISO method is the internationally recognized procedure for determining the flexural and compressive strength of cement, and it forms the technical basis for strength grading and conformity assessment of hydraulic binders worldwide. Based on ISO 679 and its Chinese equivalent GB/T 17671, the method evaluates standardized 40 mm × 40 mm × 160 mm mortar prisms prepared with ISO standard sand at a fixed water-cement ratio of 0.50. This article explains the principles, sample requirements, testing procedure, result evaluation, and typical applications of the method as performed by cement plants and accredited third-party laboratories.

Technical Overview

Cement mortar strength testing by the ISO method is the reference procedure for characterizing the mechanical strength of hydraulic cements. The international standard is ISO 679, "Cement — Test methods — Determination of strength", which is implemented in China as GB/T 17671, "Test method of strength of cement mortar (ISO method)", and in Europe as the technically aligned EN 196-1. Because the three standards share the same mortar composition, apparatus, curing regime and loading rates, strength grades such as 32.5, 42.5 and 52.5 can be compared consistently across different markets and certification systems.

The principle of the method is to prepare mortar prisms of 40 mm × 40 mm × 160 mm from a strictly defined mixture of cement, ISO standard sand and water. The prisms are cured under controlled temperature and humidity conditions and loaded to failure in flexure and then in compression. Since every variable is fixed by the standard, the measured strength reflects the quality of the cement itself rather than workmanship or mix design, which is why the method serves as the basis for product conformity, factory production control and trade inspection.

The key parameters of the method are:

• Mortar proportion: 450 g of cement, 1,350 g of ISO standard sand and 225 g of water (cement-to-sand mass ratio 1 : 3, water-cement ratio 0.50)
• Specimen geometry: prisms of 40 mm × 40 mm × 160 mm, compacted on a jolting apparatus
• Curing conditions: 20 °C ± 1 °C, with the first 20 h to 24 h in the mold at not less than 90 % relative humidity, followed by immersion in water until the test age
• Standard test ages: 3 days, 7 days and 28 days, each with strictly limited time tolerances
• Strength determination: flexural strength by center-point loading, followed by compressive strength on the broken half-prisms

Testing Samples

Cement Sample

The cement shall be sampled so that the laboratory portion is representative of the production batch or consignment, following the applicable sampling standard. Samples are delivered in sealed, airtight containers, protected from moisture and contamination, and are homogenized before testing. For each mortar batch, 450 g ± 2 g of cement is weighed to the nearest gram.

ISO Standard Sand

Only certified ISO standard sand may be used. It is a natural siliceous sand graded from 0.08 mm to 2.0 mm and supplied in three approximately equal fractions (0.08 mm to 0.5 mm, 0.5 mm to 1.0 mm and 1.0 mm to 2.0 mm). Each mortar batch requires 1,350 g ± 5 g of standard sand. Substituting ordinary construction sand or a non-certified sand invalidates the test because the grading and reactivity of the sand directly influence the measured strength.

Mixing Water

Clean potable water, or distilled and demineralized water, free from substances that could affect the setting and hardening of the cement, is used for mixing and for curing. Each mortar batch requires 225 g ± 1 g of water, corresponding exactly to a water-cement ratio of 0.50 by mass.

Testing Items

The principal test items determined by the ISO method are:

• Flexural strength at 3 days, 7 days (where specified) and 28 days, expressed in MPa
• Compressive strength at 3 days, 7 days (where specified) and 28 days, expressed in MPa
• Conformity evaluation of the measured strengths against the specified strength grade and early-strength requirements of product standards such as GB 175, EN 197-1 or equivalent national specifications

The 28-day compressive strength is the primary classification index for cement, while the 3-day results characterize early strength development, which is important for formwork removal schedules, winter construction planning and precast production cycles.

Testing Methods

Mixing Procedure

The mortar is mixed in a planetary-type mixer conforming to the standard. The sequence is: add the water, then the cement, and start the mixer at low speed for 30 s; add the sand steadily during the following 30 s at low speed; switch to high speed for 30 s; stop the mixer for 90 s, during which the bowl walls and blades are scraped during the first 15 s; finally mix at high speed for 60 s. The total mixing cycle is approximately 4 minutes and must not be altered.

Molding and Compaction

The mortar is immediately placed in a three-gang steel prism mold fixed to a jolting apparatus. The mold is filled in two layers of approximately equal depth, and each layer is compacted by 60 jolts of the table delivered within 60 s. After jolting, the surplus mortar is struck off and the surface is leveled with a straightedge. The molds are labeled and placed immediately in a moist cabinet or moist store at 20 °C ± 1 °C and not less than 90 % relative humidity.

Demolding and Curing

The specimens are demolded between 20 h and 24 h after casting (specimens for a 24 h test age are demolded immediately before testing), marked with waterproof identification, and immersed in a water bath at 20 °C ± 1 °C. The prisms are stored so that water can circulate freely around them until they are removed for testing, wiped and tested within the permitted age tolerance:

• 24 h age: ± 15 min
• 48 h age: ± 30 min
• 72 h (3 day) age: ± 45 min
• 7 day age: ± 2 h
• 28 day age and beyond: ± 12 h

Flexural Strength Determination

Each prism is tested in flexure by center-point loading over a support span of 100 mm, with the loading rate controlled at 50 N/s ± 10 N/s. The flexural strength is calculated as Rf = 1.5 × Ff × L / b³, where Ff is the failure load, L is the distance between the supports and b is the side of the square cross-section. For standard 40 mm prisms, Rf = 0.00234 × Ff (MPa). Three prisms yield three flexural values; if one value deviates by more than ± 10 % from the mean of the three, it is discarded and the mean of the remaining two is reported.

Compressive Strength Determination

The six half-prisms produced by the flexural test are tested in compression between platens bearing on a 40 mm × 40 mm face, giving a loaded area of 1,600 mm². The loading rate is 2,400 N/s ± 200 N/s. The compressive strength is calculated as Rc = Fc / 1600 (MPa). The reported result is the mean of the six values; if one value deviates by more than ± 10 % from the mean, it is rejected and the mean of the remaining five is reported. If more than one value must be rejected, the test is invalid and shall be repeated with freshly prepared specimens.

Result Evaluation and Quality Assurance

Strength results are rounded to 0.1 MPa and compared with the limits of the applicable product standard. To guarantee comparability, the mixing machine, jolting apparatus, molds and testing machines must be verified at prescribed intervals, including the use of certified reference materials and calibrated force-measuring systems. Accredited third-party laboratories perform the test within an ISO/IEC 17025 management system and participate regularly in interlaboratory proficiency comparisons.

Applications

The ISO method is applied throughout the cement and construction materials sector, including:

• Factory production control in cement plants and conformity assessment of strength grades against GB 175, EN 197-1 and other product standards
• Third-party inspection and testing for import and export commodity verification, trade settlement and quality disputes
• Strength activity index testing of supplementary cementitious materials such as fly ash, ground granulated blast furnace slag and natural pozzolana
• Research and development of cement formulations, grinding aids, admixtures and blended cements
• Failure analysis and dispute resolution for construction projects where the quality of the supplied cement must be independently established
• Interlaboratory comparison and proficiency testing programs that underpin the mutual recognition of test data

FAQ

What is the ISO method for cement mortar strength testing?

It is the standardized procedure of ISO 679 for determining the flexural and compressive strength of cement mortars using 40 mm × 40 mm × 160 mm prisms made from a fixed 1 : 3 cement-to-sand mix at a water-cement ratio of 0.50, cured at 20 °C ± 1 °C and tested at specified ages. In China it is adopted as GB/T 17671.

How is the ISO method related to EN 196-1 and GB/T 17671?

EN 196-1 and GB/T 17671 are the European and Chinese implementations of the same internationally agreed method. Their mortar composition, specimen dimensions, compaction, curing and loading rates are technically aligned, so results obtained under any of these standards are directly comparable.

Why is the water-cement ratio fixed at 0.50 and the sand ratio at 1 : 3?

Fixing these parameters produces a workable "soft" mortar whose strength is governed by the hydraulic performance of the cement rather than by mix design choices. This ensures that strength values from different laboratories, dates and countries reflect genuine differences in cement quality and can be used for grading and certification.

How does the ISO method differ from ASTM C109?

ASTM C109 determines compressive strength using 50 mm (2 in) cube specimens with its own graded standard sand and, for Portland cements, a water-cement ratio of 0.485. Because the specimen geometry, sand and compaction differ, results from the ISO prism method and the ASTM cube method are not directly interchangeable, and conversions should only be made on the basis of established statistical relationships.

What happens if an individual strength value is an outlier?

For flexural strength, if one of the three values deviates by more than ± 10 % from their mean, that value is discarded and the mean of the remaining two is reported. For compressive strength, if one of the six values exceeds the ± 10 % deviation from the mean, the mean of the remaining five is reported; if two or more values must be rejected, the entire test is invalid and shall be repeated.

Why is the 28-day strength so important?

For most Portland cements, hydration approaches near-complete strength contribution by 28 days, so the 28-day compressive strength has been adopted worldwide as the classification index for cement strength grades. The 3-day and 7-day values complement it by documenting early strength development for construction scheduling.