Cement Activity Index Testing: Standards, Methods, and Quality Evaluation

Cement activity index testing is a laboratory procedure that quantifies the strength contribution of supplementary cementitious materials—such as fly ash and ground granulated blast furnace slag—by comparing the compressive strength of blended mortars against a pure reference cement mortar. Conducted in accredited third-party laboratories in accordance with GB/T 1596, GB/T 18046, ASTM C311, ASTM C989, and EN 196-1, this test is essential for classifying mineral admixtures, qualifying concrete raw materials, and supporting low-carbon concrete production.

Technical Overview

The cement activity index, also known as the strength activity index (SAI) or slag activity index, is expressed as the percentage ratio of the compressive strength of a test mortar containing a supplementary cementitious material (SCM) to the compressive strength of a control mortar made entirely with reference cement, both tested at the same age under identical conditions. A higher activity index indicates a greater capacity of the mineral admixture to react pozzolanically or hydraulically and contribute to strength development in concrete.

This test plays a central role in the classification and acceptance of mineral admixtures. For example, fly ash grades under GB/T 1596 and ground granulated blast furnace slag (GGBS) grades S75, S95, and S105 under GB/T 18046 are defined largely by their activity index values. Similarly, ASTM C618 classifies fly ash and natural pozzolans using the strength activity index determined by ASTM C311, while ASTM C989 grades slag into Grades 80, 100, and 120 based on the slag activity index. The test method itself relies on the standardized mortar strength procedures of ISO 679, EN 196-1, and GB/T 17671 for prism specimens, or ASTM C109 for cube specimens.

By providing a direct, performance-based measurement of reactivity, activity index testing enables concrete producers, admixture suppliers, and construction authorities to verify that cementitious materials will deliver the expected strength, durability, and sustainability benefits in blended cement and high-performance concrete systems.

Testing Samples

Fly Ash (Class F and Class C)

Fly ash collected from coal-fired power plant flue gas is one of the most frequently tested SCMs. Class F fly ash, derived from anthracite or bituminous coal, is evaluated for pozzolanic reactivity, while Class C fly ash from lignite or subbituminous coal exhibits both pozzolanic and cementitious behavior. Samples should be representative, homogenized, and sealed in moisture-proof containers, typically requiring 10–20 kg for a complete test series including parallel control mortars.

Ground Granulated Blast Furnace Slag (GGBS)

GGBS is a hydraulic SCM produced by rapidly quenching molten blast furnace slag. Its activity index determines the commercial grade (S75, S95, S105 under GB/T 18046; Grades 80, 100, 120 under ASTM C989). Because GGBS reactivity is highly sensitive to fineness and glass content, samples must be protected from moisture and contamination prior to testing.

Natural Pozzolans and Calcined Materials

Materials such as metakaolin, calcined clay, natural pozzolana, and limestone fines can also be evaluated using activity index methods adapted from the fly ash procedures. These materials are increasingly important in low-clinker, low-carbon cement systems, and activity index testing provides a standardized benchmark for comparing their strength contribution.

Reference Cement and Standard Sand

The test requires a certified reference cement with known chemical and physical properties, together with graded standard sand compliant with ISO 679 or EN 196-1 requirements. Using consistent reference materials is critical, because the activity index is a comparative ratio; any variation in the control mortar directly affects the reported result.

Testing Items

A comprehensive activity index evaluation typically covers the following parameters:

• Compressive strength of control mortar at 7 days and 28 days (flexural strength also recorded)
• Compressive strength of test mortar containing the SCM at 7 days and 28 days
• Activity index calculation at each test age, reported as a percentage of the control strength
• Water requirement ratio or flow comparison between test and control mortars
• Supporting characterization: density, specific surface area (Blaine), fineness or retained material on a 45-micron sieve, and moisture content
• Optional extended-age activity index at 90 days for slowly reacting pozzolans per EN 450-1

Testing Methods

Specimen Preparation

Mortar prisms of 40 mm × 40 mm × 160 mm are prepared in accordance with ISO 679, EN 196-1, or GB/T 17671. The control mortar consists of reference cement, standard sand, and water at fixed proportions of one part binder to three parts sand by mass with a water-to-binder ratio of 0.50. For the test mortar, a defined portion of the reference cement is replaced by mass with the SCM under evaluation:

• GB/T 1596 fly ash: 30% replacement by mass of cement
• GB/T 18046 slag: 50% replacement by mass of cement
• ASTM C311 fly ash and natural pozzolans: 20% replacement by mass, with 50 mm cube specimens per ASTM C109 and water adjusted to achieve the specified flow range
• ASTM C989 slag: 50% replacement by mass of cement
• EN 450-1 fly ash: 25% replacement by mass per the EN 196-1 method

Mixing, Casting, and Curing

Mixing is performed in a planetary mortar mixer following the standardized sequence of slow and fast mixing stages. The fresh mortar is cast into steel or plastic prism molds, compacted on a jolting apparatus or vibrating table, and leveled. Specimens are kept in the molds at 20 ± 1 °C with relative humidity of at least 90% for 20 to 24 hours, then demolded and submerged in a water curing bath at 20 ± 1 °C until the designated test age. Strict temperature control during curing is essential, because SCM reactivity is particularly sensitive to thermal conditions.

Strength Testing and Calculation

At each test age, prisms are first loaded to failure in flexure, and the resulting half-prisms are then tested in compression using standard loading rates and verified bearing blocks. The compressive strength is calculated from the failing load divided by the loaded area of 40 mm × 40 mm, with the result taken as the mean of six valid values for each mortar type. The activity index is then determined as follows:

Activity Index (%) = (mean compressive strength of SCM test mortar ÷ mean compressive strength of control mortar) × 100

Acceptance Criteria

Typical specification limits include:

• GB/T 1596 fly ash: 28-day strength activity index of at least 70%
• GB/T 18046 slag: 28-day activity index of at least 75%, 95%, or 105% for grades S75, S95, and S105 respectively, with corresponding 7-day minimums of 55%, 70%, and 95%
• ASTM C311 and ASTM C618: strength activity index of at least 75% of the control at either 7 or 28 days
• ASTM C989 slag: Grade 80 requires at least 75% at 7 days and 95% at 28 days; Grade 100 requires 95% and 115%; Grade 120 requires 115% and 135%
• EN 450-1 fly ash: 28-day activity index of at least 75%, with an optional 90-day value of at least 85%

Applications

Activity index testing supports a wide range of engineering, commercial, and sustainability objectives:

• Classification and certification of fly ash, GGBS, and other mineral admixtures prior to market release
• Factory production control at SCM grinding and processing plants, including fineness optimization
• Incoming material verification by ready-mixed concrete and precast concrete producers
• Qualification of blended cements and ternary binder systems for major infrastructure projects
• Technical support for low-carbon concrete design, green building certification, and carbon footprint reduction programs
• Research and development of calcined clay, metakaolin, and other next-generation supplementary cementitious materials
• Arbitration and dispute resolution when supplier and purchaser strength results differ

FAQ

What is the cement activity index?

The cement activity index is the percentage ratio of the compressive strength of a mortar in which part of the cement is replaced by a supplementary cementitious material to the compressive strength of a pure reference cement mortar at the same age. It directly measures how much strength the mineral admixture contributes through pozzolanic or hydraulic reactions.

How long does the test take?

Because the primary evaluation age is 28 days, the full testing cycle—including specimen preparation, curing, and reporting—typically requires 30 to 35 working days from sample acceptance. Extended 90-day evaluations for slowly reacting pozzolans under EN 450-1 require correspondingly longer schedules.

What sample quantity is required?

A representative sample of 10–20 kg of the supplementary cementitious material is generally sufficient. The laboratory separately supplies or procures certified reference cement and standard sand, as the accuracy of the ratio depends on a consistent, well-characterized control mortar.

Why is the 7-day index lower than the 28-day index for most SCMs?

Pozzolanic reactions are slower than Portland cement hydration because they depend on the availability of calcium hydroxide released during early hydration. As a result, SCMs typically contribute less strength at 7 days and progressively more at 28 days and beyond, which is why standards specify minimum values at multiple ages.

What factors can affect the test result?

Key influences include SCM fineness and glass content, moisture condition of the sample, water-to-binder ratio, mixing and compaction quality, curing temperature stability, and the uniformity of the reference cement. Any deviation from the standardized 20 ± 1 °C curing regime can significantly bias the activity ratio.

Which standards govern activity index testing?

The principal standards are GB/T 1596 and GB/T 17671 for fly ash, GB/T 18046 for slag, ASTM C311 with ASTM C109 for fly ash and natural pozzolans, ASTM C989 for slag, EN 450-1 for fly ash, and EN 196-1 or ISO 679 for the underlying mortar strength method. Accredited third-party laboratories select the applicable standard based on the material type and the destination market of the product.