Technical Questions

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Could a sulfate-resisting cement and slag cement be combined and used for concrete exposed to sulfates attack?

Q. We are designing a project that will be exposed to sulfates in the soil. Should our specification prohibit mixtures that combine sulfate-resisting cement and slag cement?


A. We are not aware of any ACI document that cautions against combining Type V (sulfate-resisting) cement and slag for protection against sulfate attack. On the contrary, for very severe sulfate exposure, ACI 318 and ACI 201.2R include this combination as an option. Section 6.1.7 of ACI 201.2R also states that: “Studies have shown that some pozzolans and slag cements used, either in blended cement or added separately to the concrete in the mixer, increase the life expectancy of concrete considerably in sulfate exposure. Many slag cements and pozzolans significantly reduce the permeability of concrete. They also combine with the alkalies and calcium hydroxide (CH) released during the hydration of the cement, reducing the potential for gypsum formation.”

The documents state that the Type V cement must meet the requirements of ASTM C150, “Standard Specification for portland Cement,” while the slag must meet the requirements of ASTM C989, “Standard Specification for Slag Cement for Use in Concrete and Mortars.” Furthermore, for very severe sulfate exposure, the documents indicate that the specified blend of Type V cement and slag must demonstrate an expansion equal to or lower than 0.10% at 18 months when tested in accordance with ASTM C1012, “Standard Test Method for Length Change of Hydraulic- Cement Mortars Exposed to a Sulfate Solution.” In binary mixtures, ACI 201.2R indicates that a portland cement replacement with slag cement “can be expected in the range of 35 to 70% by mass of the total cementitious material.” The aluminum-oxide content of a slag used on a project should be no more than 2% higher than the content reported for the slag used in test mixtures (analyzed in accordance with ASTM C114, “Standard Test Methods for Chemical Analysis of Hydraulic Cement”). For more information on requirements for concrete exposed to sulfates in soil or water and detailed guidance on material selection, qualification, and proportioning, refer to ACI 318 and ACI 201.2R.


References: ACI 318-19; ACI 201.2R-16; ASTM C150; ASTM C114; ASTM C989; ASTM C1012

Topics in Concrete: Cementitious Material; Durability; Slag

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