No More Low-Alkali Cement – WHY!

Robert E. Neal, Lehigh Cement Co.

Beginning in 2019 there will no longer be “low-alkali cement”! What? Has the portland cement manufacturing processed changed such that low-alkali cement doesn’t exist anymore – the answer is NO. But in June of 2019 ASTM C 150 Standard Specification for Portland Cement will eliminate the “low-alkali” optional requirement or designation in the standard. Why the change? Let’s 1st review some background.

What are alkalies? Alkalies in portland cement are comprised of the elements sodium (Na) and potassium (K). Alkalies are inherently present in the raw materials used to produce portland cement, and are present in the portland cement in low amounts generally ranging from a few tenths of a percent up to about one percent.   As a result alkalies cannot be eliminated in portland cement and the content can only be controlled to a minor degree in the manufacturing process – there is no magic alkali “control knob” at the cement plant to select a desired alkali content. However, the alkali content does remains relatively consistent for a given cement plant.

Calculation and reporting of alkalies: Since sodium and potassium have different molecular weights it is necessary to adjust for these differences when reporting the alkali content so that they are reported based on equal footing rather than simply providing the sum of the sodium and potassium. This is known as the “Equivalent Alkali” content and is often denoted as “Na2Oeq”. Na2Oeq is calculated as the sum of Na2O + 0.685 K2O.   On a mill test report the “alkalies” are reported as “Na2Oeq” or “Equivalent Alkali”.

Why the Concern for Alkalies in Portland Cement: The alkalies (or the hydroxides thereof) greatly contribute to the pH or alkalinity of a portland cement paste. The higher the content of alkalies the higher the pH of the “pore water” within the paste, which has a pH of 13 or higher. A high pH or highly alkaline environment is a beneficial property of concrete as it provides protection of steel reinforcement from corrosion.

However, certain aggregate types have amorphous (glassy) and/or poorly crystalized forms of silica. In a highly alkaline environment such forms of silica can become unstable and susceptible to react with alkalies present in the pore water to result in what is known as the alkali-silica reaction or ASR. This reaction forms an alkali-silica gel which has a strong affinity to take on water and swell causing a deleterious expansion within either the aggregate particle, the cement paste, or both. This expansion can lead to cracking and subsequent degradation of the concrete. Three factors must be present for deleterious ASR to occur: reactive forms of silica, a sufficient quantity of alkali, and moisture.

The phenomenon of 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.  The approach of limiting the alkali content of the portland cement seemed to work well for decades.

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). The alkali loading takes into account both the alkali content of the portland cement and the portland cement content in the concrete. 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 this level it is possible. With this in mind a new approach has evolved: limit the alkali loading in concrete as a function of the reactivity of the aggregate, the environmental exposure, and the criticality of the structure.

In support of this concept ASTM adopted ASTM C1778 Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete in 2014. Although not a specification, ASTM C1778 provides a systematic approach to address concerns related to the selection of concrete making materials to avoid ASR.  If the aggregates have been shown by test C1260 or C1293 to be non-deleterious reactive 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. ASTM C 1778 provides both a prescriptive or performance based approach.

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 structural element. Once the prevention level is established then either of two prescriptive options can be used.

Option 1 – Limiting the Alkali Loading of the Concrete: A maximum permissible “alkali loading” for the proposed concrete can be determined. The maximum permissible alkali loading will be in the range of 3 lbs./cubic yard to 5 lbs./cubic yard. To calculate the alkali loading simply multiply the equivalent alkali content of the portland cement times the cement content of the proposed mix. Note that only the alkalies contributed by the portland cement are used; if the mixture contains a supplementary cementitious material (SCM), alkalies contributed by the SCM are not included in the calculation of the alkali loading. Provided that the alkali loading of the proposed concrete mixture is less than the maximum permissible alkali loading determined then no further mitigation measures are needed. This approach is applicable to evaluate straight portland cement mixtures as well as mixtures containing an SCM.

Option 2 – Using an SCM: Should the calculated alkali loading of the proposed concrete mixtures exceed the permissible maximum limit then additional measure must be taken. This requires the use of an SCM as a partial replacement for a portion of the portland cement. Within the prescriptive requirements of ASTM C1778 there are recommended minimum replacement levels with an SCM to provide adequate ASR mitigation.

The performance approach: Alternately ASTM C1778 allows for a performance based assessment to mitigate ASR. The performance-based approach is not applicable to straight portland cement mixture as there is currently no accepted accelerate test method to evaluate the effects of straight cement on ASR. The performance-based approach applies only to concrete mixtures containing an SCM.  Under the performance criteria the SCM must be validated to mitigate ASR with the aggregates proposed for use and at the SCM replacement level proposed for use. The efficacy of the SCM to mitigate ASR must be demonstrated by test of either ASTM C1567 or ASTM C1293. Note that both of these tests are accelerated test methods and subject the specimens to a highly aggressive alkali environments emulating a “worst case scenario” and therefore may result in an overly conservative estimate of the SCM content needed to mitigate ASR.

Utilizing the procedures in ASTM C1778 at first may seem to be quite a complicated and daunting task. However, once the critical factors of aggregate reactivity, alkali loading of the concrete, and criticality of the structure are understood ASTM C1778 represents a rather logical approach.  Lehigh’s Technical Service Department has developed an App which greatly simplifies the process of evaluating the required level of prevention and the permissible alkali loading.

In summary, if the aggregates have been shown by test C1260 or C1293 to be non-deleterious reactive then no special precautions as needed.   On the other hand, if the aggregates are deemed potentially reactive there are two basic options to consider:

  • Assess the alkali loading of the proposed concrete mixture:
    1. If less than the permissible limit then no further measures need be taken.
    2. If greater than the permissible limit then use an SCM at a replacement level deemed suitable to mitigate potential deleterious ASR.
  • Alternately perform testing of an SCM/Aggregate combination by ASTM C1567 or ASTM C1293.

The means of avoiding potentially deleterious ASR has now been recognized in ACI 318 Building Code Requirements for Structural Concrete. Beginning with the 2019 edition of ACI 318 documentation related to potential aggregate reactivity and 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 be 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 applicable.

Robert E. Neal, FACI

Technical Services Engineer

Lehigh Cement Co.

May 10, 2019