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Mortar Options: Pozzolanic Hydraulic Lime (PHL) & Natural Hydraulic Lime (NHL)

We rely on pozzolanic hydraulic lime (PHL) and natural hydraulic lime (NHL) for most of our historic restoration work. Both are hydraulic limes with either added pozzolans (PHL) or naturally occurring impurities (NHL).  This means that the mortar will set in two stages: the first hydraulic set will occur when water is added so that the mortar gains limited strength quickly during its initial set. The second begins when the mortar is exposed to the atmosphere and carbonates, which allows it to slowly accumulate strength over time.

PHL mortar is manufactured domestically and made by adding a controlled quantity of hydraulic material, usually metakaolin, fly ash, or slag to hydrated lime. NHL is not available from any domestic producer, and all NHL available in the United States is imported from Europe. NHL is mined from calcareous rock that has been infiltrated mainly by silica with only traces of other minerals.


Hydraulic limes are categorized as feebly hydraulic (2.0), moderately hydraulic (3.5), and eminently hydraulic (5.0). These classifications originated from the European Norm EN-459, which requires that the NHL 2.0 have a minimum strength of 2MPa; the 3.5 a minimum strength of 3.5mPA; and the NHL 5.0 a minimum strength of 5.0MPa. The acceptable compressive strength parameters according to EN 459 are:


2.0  —  Feebly Hydraulic (322-1015 PSI)

3.5  —  Moderately Hydraulic (507-1450 PSI)

5.0  —  Eminently Hydraulic (725 – 2175 PSI)


EN 459 also provides a standard for a minimum amount of free lime, which is important for workability, autogenous healing of microcracks, and elasticity. It will also contribute to the final strength. Free lime provides some tolerance for initial structural movements and both sufficient strength and plasticity to accept slight load shifts. The higher the free lime the more accommodating the mortar:


Product Minimum Free Lime (%) under EN-459
NHL 2.0 >35%
NHL 3.5 >25%
NHL 5.0 >15%


Hydraulic limes also facilitate vapor transmission, as seen in this graphic. St. Astier, the producer of the NHL that we supply, has also provided these relative vapor transmission rates of representative mortar types:

  • Type N (1:1:6) 0.23
  • Type O (1:2:9) 0.25
  • Hydrated lime mortars: 0.46-0.51
  • Lime putty mortars 0.57
  • Saint Astier NHL Mortars: 0.55 – 0.72*

Figures are expressed in grams of air x m2 x hour x mmHg.
*made with well graded #6 – #200 (3mm-0.075) sands to ensure good void structure

ASTM References

In the United States, NHL and PHL mortars fall under ASTM C-141 “Standard Specifications for Hydrated Hydraulic Lime for Structural Purposes.” PHL mortars also fall under C1707 “Standard Specification for

Pozzolanic Hydraulic Lime for Structural Purposes.” Compressive strength testing under ASTM C-141 is mandated at 28 days, but since hydraulic limes continue to gain strength for at least one year this should not be considered the maximum strength, and the specifier should be conservative and go with the lowest strength mortar that will suit the project.

PHL Mortar Options

PHL 3.5 | Data Sheet

50 lb bag – 45/skid

The average 28-day strength is 500 PSI, which is optimal in historic and sustainable applications where weaker materials are desired.

PHL 5.0  | Data Sheet

50 lb bag – 45/skid

The average 28-day strength is 750 PSI, which is optimal in historic and sustainable applications where weaker materials are desired.

NHL Mortar Options

As a supplier we have evaluated several lines of NHL and we have opted to carry St. Astier Natural Hydraulic Lime. It is a well-regarded and recognized brand in Europe that has been produced since 1851, and the product is supported by extensive test data:

  • Strength documentation for 1:2; 1:2.5, and 1:3 lime to aggregate ratios at 7 days, 28 days, 6 months, 12 months, and 24 months.
  • Whiteness index of 72, which provides a consistent neutral base for both pigment and aggregate
  • Test data documenting its compatibility with seawater due to very low aluminate content

St. Astier NHL 2.0: Detailed Information Sheet | MSDS Sheet

St. Astier NHL 3.5: Detailed Information Sheet | MSDS Sheet

St. Astier NHL 5.0: Detailed Information Sheet | MSDS Sheet