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Beginning in 2019 there will no longer be “low-alkali cement”! What? Has the portland cement manufacturing process changed such that low-alkali cement doesn’t exist anymore? The answer is NO. But in June of 2019 ASTM C150 Standard Specification for Portland Cement will eliminate the “low-alkali” optional requirement or designation in the standard. Why the change? Let’s first review some background.

What are alkalies?
Alkalies in portland cement are comprised of the ions sodium (Na+) and potassium (K+) and are inherently present in the raw materials used to produce portland cement. Alkalies are present in low amounts generally ranging from a few tenths of a percent up to about one percent. Alkalies are reported as the “Equivalent Alkali” content and is often denoted as “Na2Oeq”. Na2Oeq is calculated as the sum of Na2O + 0.685 K2O.

Why the Concern for Alkalies in Portland Cement?
The key concern is the alkali-silica reaction or ASR. Certain aggregate types have amorphous (glassy) and/or poorly crystalized forms of silica which can react with alkalies derived from the portland cement. This reaction forms an alkali-silica gel which can swell causing a deleterious expansion within either the aggregate particle, the cement paste, or both that can lead to cracking and subsequent degradation of the concrete. Four factors must be present for deleterious ASR to occur: reactive forms of silica, a sufficient quantity of alkali, a source of soluble calcium, and moisture. ASR was first identified and reported on as early as 1940 by T.E. Stanton as the cause of concrete deterioration in California. Subsequent research indicated that if the alkali content of the portland cement was below a certain level the deleterious ASR reaction did not occur. This was the basis for the adoption of the maximum alkali limit of 0.60% (expressed as Na2Oeq) to designate “low alkali” portland cement in ASTM C150 in 1961 as a means to avoid the potential for ASR. However a limit on alkalies of the portland cement alone did not address the alkali content in concrete which is a function of the cement content. As design strength requirements and corresponding cement contents continued to increase over the decades the concept of limiting the alkali content of just the portland cement itself did not seem to provide adequate assurance for the avoidance of ASR.

A New Approach
It became apparent that the total alkali content or “alkali loading” of the concrete was the more important factor related to ASR (i.e. portland cement alkali content times the Portland cement content). Continued research had demonstrated that an alkali loading on the order of 5 lbs./cu.yd. is an appropriate threshold for many reactive aggregates; below this level deleterious ASR is unlikely and above the level it is possible. In 2014 ASTM adopted ASTM C1778 Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete. ASTM C1778 provides a systematic approach to address concerns related to the selection of concrete making materials to avoid ASR using either a prescriptive approach or a performance approach. If the aggregates have been shown to be non-deleterious reactive by test C1260 or C1293 then no special precautions are needed. On the other hand, if the aggregates are deemed potentially reactive then some measures are required to avoid the potential for ASR.

The Prescriptive Approach
Under the prescriptive approach a “Level of Prevention” has to first be determined. This is based upon the potential reactivity of the aggregates, exposure conditions, and severity of consequences of ASR to the structure. Once the prevention level is established then either of two prescriptive options can be used.

Option 1 – Limit the alkali loading of the concrete. Calculate the alkali loading based upon the alkali content of the Portland cement alone; do not include alkalies contained in SCMs. The maximum permissible alkali loading will be in the range of 3 lbs./cubic yard to 5 lbs./cubic yard. If the alkali loading is less than the permissible limit then no further measures need to be taken.

Option 2 – Use an SCM. However, if the alkali loading is greater than the permissible limit then use an SCM at a minimum replacement level as prescribed to mitigate potential deleterious ASR. In extreme cases both a prescribed replacement with an SCM and limiting alkali loading may be required.

The Performance Approach
ASTM C1778 also allows for a performance based assessment to mitigate ASR. Under the performance criteria the SCM must be validated to mitigate as tested by either ASTM C1567 or C1293 using the aggregates proposed for use and at the SCM replacement level proposed for use. The need to avoid potentially deleterious ASR is now recognized in ACI 318 Building Code Requirements for Structural Concrete. Beginning with the 2019 edition of ACI 318 documentation related to the potential aggregate reactivity and proposed ASR mitigation will now be required. ASTM C1778 is cited in the ACI 318-19 Commentary as an appropriate method for addressing the materials evaluation and selection to avoid deleterious ASR.

The alkali content of portland cements will still be in the ranges typical for a given manufacturing source. There will still be portland cements of lower alkali content and portland cements of higher alkali content – just there will be no longer the designation of “Low-Alkali Cement”. For projects where an alkali content below a certain level (e.g. 0.60%) is desired this must be stipulated – just using the terminology “low-alkali cement” will no longer be accurate.

Courtesy of Robert E. Neal, FACI Technical Services Engineer with Lehigh Cement Co. 

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