ASTM C 595: Blended Hydraulic Cement Types, Suffixes, and Tests

ASTM C595 is the U.S. specification that governs blended hydraulic cements, meaning cements where portland cement clinker is combined at the manufacturing plant with slag, pozzolan, limestone, or a mix of those materials. The current active edition is ASTM C595/C595M-25, published by ASTM International.¹ It sorts those cements into four types, caps their chemical composition, requires a set of physical and strength tests, and dictates how each product must be labeled. Specifiers, buyers, and inspectors use it to decide which cement is right for a project and to confirm that what arrives on site is what was ordered.

The Four Cement Types Under ASTM C595

C595 organizes blended cements by what is blended with the clinker and how much.

Type IS: Portland-Blast-Furnace Slag Cement

Type IS incorporates slag, a glassy byproduct of iron smelting, at up to 95 percent by mass of the blended cement. When slag content reaches or exceeds 70 percent, hydrated lime is also permitted as an ingredient.² The standard treats IS(<70) and IS(≥70) separately because higher slag content changes both the chemical limits and the cement’s behavior, particularly its heat generation during curing. High-slag blends are common on massive pours like dam foundations and thick mat slabs for that reason.

Type IP: Portland-Pozzolan Cement

Type IP blends portland cement with a pozzolan, which can include fly ash, natural volcanic ash, silica fume, or calcined clay. Pozzolan content can reach 40 percent by mass.² Pozzolanic reactions develop strength more slowly than portland cement alone, but the long-term strength and density of the hardened concrete tend to be higher. Type IP is a common choice for structures exposed to moisture or aggressive chemical environments.

Type IL: Portland-Limestone Cement

Type IL requires limestone content above 5 percent but no more than 15 percent by mass, and the limestone must have a calcium carbonate content of at least 70 percent.² Because a portion of clinker is replaced with ground limestone, the carbon footprint is roughly 8 percent lower than traditional portland cement.³

Type IT: Ternary Blended Cement

Type IT combines portland cement clinker with two or more supplementary materials, such as slag plus pozzolan, or slag plus limestone. The total supplementary material content stays below 70 percent, with pozzolan capped at 40 percent and limestone at 15 percent. When slag content reaches or exceeds 70 percent, the limestone maximum remains 15 percent and hydrated lime is allowed.² Any slag, pozzolan, or limestone already present in the portland cement used to make the blend counts toward these limits.

Reading the Suffixes on a C595 Designation

Beyond the type, C595 uses parenthetical suffixes to communicate composition and special properties. The percentage of each supplementary material appears in parentheses right after the type, so Type IT(S25)(P10) tells the buyer the cement contains 25 percent slag and 10 percent pozzolan.³

Letter suffixes flag performance characteristics:

(A): Air-entraining cement, which puts tiny air bubbles in the mix to resist freeze-thaw damage.⁴
(MS): Moderate sulfate resistance, for concrete in soil or water with moderate sulfate concentrations.⁴
(HS): High sulfate resistance, for severe sulfate exposure.
(MH): Moderate heat of hydration, useful for large pours where excessive heat buildup could cause thermal cracking.

These suffixes stack. Type IT(S25)(P10)(MH)A reads as a ternary blend with 25 percent slag and 10 percent pozzolan, moderate heat of hydration, and air entrainment. Getting the suffix right in a specification matters because the wrong sulfate class, or an omitted (A) in a freeze-thaw environment, can lead to deterioration that may not show up for years.

Chemical Requirements

Every blended cement must satisfy the standard’s chemical caps before it can be sold under a C595 designation. The limits depend on whether the cement sits in the lower-slag group (Type IS with less than 70 percent slag, and Types IP, IL, and most Type IT blends) or the high-slag group (Type IS at 70 percent or more slag, and the corresponding Type IT variants).

For the lower-slag group, the main chemical caps are:

Magnesium oxide (MgO): 6.0 percent maximum. Excess magnesia can cause long-term expansion and surface cracking.
Sulfur trioxide (SO₃): 3.0 percent maximum. Excess sulfate can trigger delayed ettringite formation and internal cracking months or years after placement.
Sulfide (reported as S): 2.0 percent maximum.
Loss on ignition: 3.0 percent maximum.
Insoluble residue: 1.0 percent maximum.

The high-slag group follows different limits: sulfur trioxide rises to 4.0 percent maximum, loss on ignition to 4.0 percent, and the magnesium oxide cap does not apply. Ternary blends containing limestone get a more generous loss-on-ignition limit of 10.0 percent to account for calcium carbonate decomposition. A manufacturer can exceed the sulfur trioxide cap if testing demonstrates that expansion at 14 days stays below 0.020 percent, and the supporting data must be shared with the purchaser.

Physical and Performance Tests

Beyond chemistry, C595 requires each cement to pass a battery of physical tests before it reaches the market.⁵

Fineness, measured by both sieving and air-permeability methods. The air-permeability test determines the surface area of the cement particles, which influences hydration speed and strength gain.
Autoclave expansion. A cement paste specimen is subjected to high-pressure steam to accelerate any latent expansion. The maximum allowable expansion is 0.80 percent. The test catches cements with excessive free lime or magnesia.
Compressive strength. Standardized mortar cubes are cast and tested at 3, 7, and 28 days to confirm the cement develops adequate load-bearing capacity on a predictable timeline.⁶
Time of setting. Penetration needles measure initial and final set, confirming workers have enough time to place and finish the concrete.
Air content of mortar, particularly relevant for air-entraining cements designated with the (A) suffix.

Failure on any of these tests means rejection of the batch.

Labeling, Composition Statements, and Mill Test Reports

Every bag, bulk shipment document, or shipping vessel of blended cement must display identifying information so the buyer can verify the product. At minimum, the label includes the cement type with its full suffix designation (such as Type IP(25)(MS)), the manufacturer’s name, and the net weight.¹

If a purchaser requests a written composition statement, the manufacturer must supply one, and the actual cement composition must fall within specified tolerances of that statement. The standard allows a plus-or-minus 3 percent tolerance for major oxide components like silicon dioxide, aluminum oxide, and calcium oxide.

Purchasers also routinely require mill test reports documenting that each batch meets the chemical and physical requirements. For engineers and contractors, the practical rule is simple: request the mill test report and compare its values against the limits in the standard. If the supplier cannot produce one, or the numbers fall outside specification, reject the material before it reaches the forms.

How C595 Relates to C150, C1157, and AASHTO M240

Three ASTM specifications cover most cement used in U.S. construction. ASTM C150 covers straight portland cement without supplementary materials blended in at the manufacturing stage. C595 picks up where C150 leaves off, governing cements where the manufacturer blends in slag, pozzolan, limestone, or a combination before the product leaves the plant.¹ Both are prescriptive standards, setting specific composition limits that the cement must meet.⁷

ASTM C1157 takes a different approach. Rather than restricting composition, it sets performance targets and lets manufacturers use whatever blend achieves those targets. Most state transportation agencies have been slow to adopt C1157, in part because no equivalent AASHTO specification exists and because long-term durability data for some novel blends remains limited.⁷

For public transportation and highway projects, AASHTO M240 is the AASHTO-equivalent specification to C595, covering substantially the same ground. State departments of transportation commonly reference M240 in their own material specifications. When a cement manufacturer blends the supplementary materials at the plant under C595 or M240, quality control over the final product tends to be tighter than when a concrete supplier adds those materials separately at the batch plant, though the trade-off is less flexibility to adjust on site.⁷

1
ASTM International. ASTM C595/C595M-25 Standard Specification for Blended Hydraulic Cements
2
Portland Cement Association. Environmental Product Declaration Blended Hydraulic Cement
3
Transportation Research Board. Portland-Limestone Cements: History, Performance, and Specifications
4
AASHTO. User Guide for the Portland and Blended Cement Technical Committee
5
ASTM International. ASTM C595 Standard Specification for Blended Hydraulic Cements
6
ASTM International. ASTM C595/C595M-21 Standard Specification for Blended Hydraulic Cements
7
ROSA P. TechBrief: Blended and Performance Cements