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Type I/II

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For those who have been wondering what exactly that Type I/II cement they bought at Home Depot is.

Type I/II Cement

Portland cements are made up of four principal compounds: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). These four compounds constitute approximately 90% by weight of the cements, with the C2S and C3S being about 75%.   (When referring to the compounds in cement, the letters refer to oxides; C= CaO, S=SiO2, A=Al2O3, F=Fe2O3. For example, C3A is actually 3CaO-Al2O3.)

ASTM C 150, Specification for Portland Cement, includes five basic types of cement.  Although the cements are differentiated on the basis of their chemical composition and physical properties, composition requirements for each type are given as a range.  Thus, some cements meet the requirements of more than one type.   

Most cement currently produced is actually Type I/II, which meets the requirements of both Type I and Type II. To cut down on the costs of storing two cements, many ready-mix producers use a Type I/II cement for projects requiring either Type I or Type II cement. Type I/II cement meets all of the standard physical and chemical requirements for both Type I and Type II.  The chemical requirements of the two cements are actually very similar; the main difference is that Type II cement has a limit of 8% on tricalcium aluminate (C3A). The physical requirements for Types I and II are almost the same, except that Type II is allowed to gain strength more slowly.   Type I/II cement meets the Type I requirements for strength development and meets the Type II  limit on C3A.

The limit of 8% on C3A for Type II and Type I/II cements provides moderate sulfate resistance; if low heat of hydration is required, an optional ASTM C150 chemical requirement that limits the sum of the C3S and C3A to 58% of the cement must be specified. Alternatively, an optional physical requirement on heat of hydration may be specified.  A Type II or I/II cement will not necessarily have a low heat of hydration unless one of these optional requirements is specifically required.  

Gail Kelley