ASTM C595/C595M-21 separates blended hydraulic cement into Type IP, portland-pozzolan cement, and Type IS, portland blast-furnace slag cement [S1].
The distinction is the blended component: pozzolan for Type IP and blast-furnace slag for Type IS, with both designations evaluated under ASTM C595 performance requirements [S1][S3].
Type IP and Type IS perform different material functions
Type IP is portland-pozzolan cement, while Type IS is portland blast-furnace slag cement under ASTM C595/C595M-21 [S1]. The older ASTM C595-08a description establishes the same classification: Type IP denotes portland-pozzolan cement and Type IS denotes portland blast-furnace slag cement [S3].
The designation does not, by itself, identify one universal application. Portland cement, slag cement, pozzolan, fly ash, silica fume, calcined clay, limestone, and gypsum can participate in blended hydraulic cement manufacture, with intergrinding or blending used to combine the materials [S4]. Consequently, the letter identifies the defining blended component, while the complete specification and subdesignation control performance.
Compare composition, behavior, and specification fit
Type IP centers on pozzolan, while Type IS centers on blast-furnace slag; ASTM C595 classifies these as separate cement types rather than interchangeable labels [S1][S3]. The ASTM C595 blended hydraulic cement types include Type IP and Type IS [S4].
Type IS is the better selection when the project specification or mix design calls for a slag-bearing blended cement. Type IP is the better selection when the required cement family is defined by pozzolan content. Neither choice should be made from the two-letter label alone. The full ASTM C595 designation, chemical limits, physical performance requirements, applicable suffix designations, and project exposure conditions must be reviewed together [S1][S4].
One practical distinction is that Type IS is not a Type IP with a different percentage. It is a different compositional class: portland blast-furnace slag cement rather than portland-pozzolan cement [S1][S3]. Substituting one for the other requires the submittal to demonstrate compliance with the specified ASTM designation and the project mix design.
Select Type IP when the specification targets pozzolan
Type IP is the applicable ASTM C595 class when the specified blended hydraulic cement is portland-pozzolan cement [S1].
Type IP should be considered for concrete where the project requirements call for a pozzolan-bearing C595 cement and where the selected pozzolan is compatible with the exposure, placing conditions, and mixture proportions. The broader C595 system recognizes performance-oriented variants such as IP(HE), IP(MS), and IP(HS), as shown in the cement type matrix published by the American Cement Association [S4].
Type IP is not the automatic selection for every low-carbon or sustainable mixture. ASTM C1157 performance cement is a separate specification route, and Type IL uses limestone blended with Type I or Type I/II clinker, with limestone contents of 5–15% in the cited guidance [S8][S10]. If the contract requires IP, the submittal must remain within the Type IP framework rather than substituting IL without approval.
Select Type IS when the specification targets slag
Type IS is the applicable ASTM C595 class for portland blast-furnace slag cement [S1].
Type IS should be selected when the required cement is defined as slag-bearing and the approved concrete mixture is designed for that cement. The American Cement Association lists Type IS among the general ASTM C595 blended hydraulic cements and also identifies high-early-strength, moderate-sulfate-resistance, and high-sulfate-resistance variants [S4].
Type IS is not the same as a ternary blend simply because it contains slag. Ternary blended cements can combine more than one supplementary cementitious material, while Type IS remains the slag-centered C595 classification [S2][S5]. For procurement, require the exact designation and supporting compliance data rather than accepting a generic description such as “slag blend.”
Use-case recommendations for concrete specification
For a general-purpose C595 blend in which the specification calls for pozzolan, specify Type IP, which denotes portland-pozzolan cement [S4][S7]. For a specification calling for slag, specify Type IS, which denotes portland blast-furnace slag cement [S4][S7]. These are direct selections because the project requirement and ASTM classification align.
For applications requiring an additional property, retain the base family and add the applicable performance suffix. The published C595 matrix includes IP(HE), IS(<70)(HE), IP(MS), IS(<70)(MS), IP(HS), IS(<70)(HS), and related designations [S4]. The notation IS(<70) identifies the slag range category in that matrix, while the parenthesized property identifies the performance requirement.
General-purpose C595 cements are listed for pavements, floors, reinforced buildings, bridges, tanks, reservoirs, pipe, masonry units, and precast products, but those applications do not make IP and IS interchangeable [S4]. Local availability also varies, and the American Cement Association explicitly advises checking availability for specific cement types [S4].
Do not confuse C595 families or suffixes
ASTM C595 blended hydraulic cements include IL, IP, IS(<70), IT(S<70), IC, and their listed performance variants, while ASTM C1157 covers a different performance-cement framework [S4][S10]. Type IP therefore means portland-pozzolan cement, Type IS means portland blast-furnace slag cement, and Type IL is the limestone-blended family described as 5–15% limestone in the cited Type IL guidance [S8].
Air entrainment can also affect the specified form. The American Cement Association notes that any cement in the applicable columns can be used to make air-entraining cement, denoted with an “(A)” in its matrix [S4]. Air-entraining admixtures at the concrete plant do not automatically convert a Type IP or Type IS cement into a different ASTM C595 chemical type.
Comparison with adjacent Portland cement types can help where sulfate exposure drives selection. ASTM C150 Type V vs Type II for sulfate resistance provides that separate decision context, but it does not replace the C595 submittal check for Type IP or Type IS.
Verify the designation on the submittal
A compliant submittal should identify ASTM C595/C595M-21, the exact Type IP or Type IS designation, any parenthesized percentage or performance suffix, and applicable physical and chemical test results [S1][S4]. ASTM C595/C595M-21 is the edition tracked by ASTM with a standard historical update date of July 26, 2023 [S1].
For mixed project requirements, compare IP and IS against four criteria: defining blend component, blend percentage, performance suffix, and local supply. Confirm whether the specification names pozzolan, slag, heat of hydration, sulfate resistance, or early strength. Then require the submittal to match the full designation rather than relying on shorthand such as “blended cement” [S1][S4][S5].
The trackable procurement signal is the mill certification: it should state the ASTM C595 edition, Type IP or Type IS family, exact suffix, and test results. If the label is only “IS” or “IP” without the required qualification, resolve the ambiguity before approval rather than assuming a default blend percentage.