Sunday, November 25, 2012

EN 459-1:2010 European Standard for Building Lime

Monsieur Louis Vicat
Europe has a continuous, documented history of building with lime dating from the Roman Republic, a period of well over 2,000 years. The above referenced standard is the UK implementation of the European Committee for Standardization (CEN) Cement and Building Lime Technical Committee 51. The European Committee for Standardization is an international non-profit providing a similar function to the American Society for Testing and Materials. The standard is a comprehensive document for defining and distinguishing manufactured limes used in construction. Parts 2 and 3 of the standard refer to Testing Methods and Conformity Evaluation respectively.

Whereas lime enjoyed only a brief history of widespread use being largely displaced by Portland cement in the US, in Europe the tradition is maintained and is inclusive of a wide range of limes. This post is a follow up on a previous article on Natural Hydraulic Limes. Hopefully it will serve to dispel some of the mystery and confusion surrounding the classification of NHL’s and highlight the balanced rationale based on experience, science and practical use reflected in the European standard.

Louis Vicat

In our previous article we briefly discussed the long history of hydraulic limes from Roman times, through the Renaissance and culminating in a scientific understanding in the early 19th century. Frenchman Louis Vicat began his career as an engineer conducting research into limes used for hydraulic works. Although hydraulic lime works had been already been underway since the mid-18th century Vicat was the first to consolidate the research and publish a comprehensive paper in 1818. Ten years later he would revise and expand upon his original work, publishing Résumé des Connaissances Positives Actuelles sur les Qualités, le Choix et la Convenance Reciproque des Matériaux Propres et la Fabrication des Mortiers et Ciments Calcaires, mercifully abbreviated to Mortars and Cements in Captain John Thomas Smith’s 1837 English translation.

Vicat’s testing procedures and classification index was to be the standard until recent times. The principles they were based on still are. His determination for classification was primarily twofold. The first was the chemical composition and percentage of argillaceous (clayey) infiltration in the given limestone under test. Second, was the reactivity and hydraulicity of the quicklime produced from said limestone. A summary of typical characteristics with approximate ranges for which Vicat himself acknowledged and documented exceptions to strict classification:

Rich limes
Containing less than 6% of inert impurities
Very reactive with a swelling during slaking exceeding 2 times in volume
No set with water

Lean or poor limes
Containing less than 30% of inert impurities
Significantly less reactive with minimal swelling during slaking
No set with water

Feebly hydraulic limes
Containing less than 12% of active* impurities
Reactive with minimal swelling during slaking
Set in 15 to 20 days

Moderately hydraulic limes
Containing less than 18% of active* impurities
Significantly less reactive with minimal swelling during slaking
Set in 6 to 8 days

Eminently hydraulic limes
Containing less than 25% of active* impurities
Almost unreactive with little to no swelling during slaking
Set in 2 to 4 days
*Vicat does explain that by active he is referring to silica not alumina

Vicat’s developed a precise method so he could consistently define when his tested limes achieved a set. However, he also furnishes his readers with a useful explanation that a “set” approximately corresponded to the hardness reached when the mean or average pressure exerted by the arm would resist an impression by the fingertip. I appreciated reading his book that he always provides both scientific, laboratory methods and results as well as practical tests that would be useful in the field for prospectors or workmen.

EN 459-1:2010

Despite our focus in this post on NHL’s, I will say the EU standard provides useful information for a broader range of building limes. For example, there are concise definitions for quicklime and hydrated limes, high calcium and dolmitic limes with impurity percentile categorizations roughly corresponding to Vicat’s rich, lean and poor classifications. Also, there are additional classifications for “Formulated” and “Hydraulic” limes that have additions of pozzolans, fillers, cements, fly ash etc.

Natural Hydraulic Limes fall under three classifications: NHL 2, NHL 3.5 and NHL 5. Not unlike Vicat the classification is based primarily on two factors: chemistry and set. However, the calculations are arrived at differently and I would argue more useful for construction.

Available hydrated lime, Ca(OH)2 ≥ 35%
Compressive strength at 28 days, ≥ 2 to ≤ 7 MPa*
*A megaPascal (MPa) or Newton (N/mm2) is a metric unit of pressure roughly corresponding to 145 psi

NHL 3.5
Available hydrated lime, Ca(OH)2 ≥ 25%
Compressive strength at 28 days, ≥ 3,5 to ≤ 10 MPa

Available hydrated lime, Ca(OH)2 ≥ 15%
Compressive strength at 28 days, ≥ 5 to ≤ 15 MPa

Courtesy of Lafarge Natural Hydraulic Limes

How do the NHL classifications compare with Vicat’s? The chemical requirements are a bit different. Vicat’s tests were based on setting underwater whereas the NHL testing is determined by compressive strength. So we can say they don’t compare exactly. 

Nevertheless, at least in regard to compressive strength, an average NHL 2 generally corresponds and tests within range of what Vicat had classified as Moderately hydraulic limes. NHL 3.5 overlaps between the stronger Moderately and weaker Eminently hydraulic limes. An average NHL 5 corresponds to the stronger Eminently hydraulic limes and stronger NHL 5’s exhibit compressive strengths corresponding to what Vicat might have classified as a Natural cement. Although there are other requirements under the NHL designation such as water demand and retention, bulk density, whiteness etc. this does provide an overview of how the two classifications bear some relationship to one another.

Practical Implications

Why the broad range of allowable compressive strengths for each NHL classification? I’ve yet to read any published documentation addressing this question; however, there appears a general consensus among those involved in manufacturing. Testing requirements for manufacturers as prescribed by EN 459-2:2010 are designed to achieve optimal compressive strengths under laboratory conditions. The mortar has a proportion of one part freshly baked NHL to 3 parts of specified sand by weight (approx. 1:1 by volume). Only enough water is added to the mix to vibrate and compress. 

Lafarge NHL 3.5
Typical field use NHL to sand ratios from 1:1.5 to 1:3 by volume, additional water (unpurified) for workability, lack of vibration/compression are just some of the factors that make it highly unlikely anything near a manufacturer’s published compressive strengths will be achieved in the field at 28 days. The various designated manufacturing requirements of NHL 2, 3.5 and 5 refer to minimum compressive strength requirements in MPa under lab conditions to ensure that mortars reach a practical compressive strength in the field. For sensitive restoration work it is best practice to perform tests on actual mortars under consideration for use in the field rather than rely exclusively on a published manufacturer’s compressive strength.

Average compressive strengths of the classification are as follows:
NHL 2 – 4.5 MPa
NHL 3.5 – 6.75 MPa
NHL 5 – 10 MPa

A significant requirement of the NHL classification is that almost no additions are allowed. The single exception is 0.1% of a grinding agent helpful in the manufacturing process. Two important components result from the baking and subsequent slaking of limestone utilized for NHL: hydrated lime, Ca(OH)2 and belite, a dicalcium silicate that forms in the baking process. The belite is the component responsible for the hydraulicity of the NHL. During the baking some of the belite agglomerates forming small pebbles. Manufacturers often retain these in the screening process. Manufacturers are permitted to grind these and add them back into the NHL to increase the hydraulicity. This is not considered an addition as it is a component of the original limestone. This is how some manufacturers are able to produce multiple NHL designations from a single limestone source.

There is some controversy over whether it is acceptable practice for an engineer or craftsman to add hydrated or putty lime to lower the compressive strength of NHL mortars in the field. As shown above hydrated lime is already a main component of NHL so there is no fundamental incompatibility. Extensive testing of the effect of high calcium hydrated lime mortars in NHL mortars have been conducted in the UK and results published in Hydraulic Lime Mortar for Stone, Brick and Block Masonry. Estimates for reduction in compressive strength from the addition of lime putty are more difficult to predict as factors such as length of slaking and water content can vary results considerably. 

Historically, pozzolans such as microsilicas  have been added for the occasional need to increase compressive strength, accelerate the set or otherwise alter the properties of NHL mortars. It would be advisable to consult with an expert in the potential long term effects of any such additions.

Contributed by Patrick Webb

Saturday, November 24, 2012

An American Couple’s Perspective on French Wine and Plaster Traditions: Viticulture

Château de Chambert
Nature. Culture. Perhaps these seemingly disparate aesthetics were no better reconciled than by the French Renaissance tradition of the formal garden.

“In the Renaissance taste the garden was an extension of the main design. It was a middle term between architecture and Nature. The transition from house to landscape was logically effected by combining at this point formality of design with naturalness of material.” – Geoffrey Scott, The Architecture of Humanism

To this point we have considered Varietals and Terroir…learning about grapes and minerals…exploring soils, weather and geology…recognizing all of nature’s generous contributions. All that we have hitherto discussed is most fundamental; however, wine and plaster are uniquely products of culture. The balance of our five part series will consider the human touch.

Viticulture in Wine

Although located in what is considered the “old world” of wine production, Bordeaux is squarely in the forefront with regard to wine-making technology.  So in this segment we are going to discuss an aspect of the Bordeaux wine industry that receives nowhere near the attention it deserves. We are talking about viticulture. Viti is latin for vine therefore viticulture roughly translates to vine cultivation.  In this article, we will examine two methods of viticulture that are essential to making a great wine; vine manipulation and pest control.

Vine leaves contain chlorophyll cells that absorb sunlight enabling the plant to extract carbon dioxide from the air and convert it to sugar. The nutrients imparted by the sugar feeds the vine roots, grape clusters and leaves ensuring the entire plant receives exactly what it needs, when it is needed.

Allowing too much foliage shields the grapes from the sunlight they need for the last stage of their healthy development, so pruning is crucial to producing a quality wine. However caution must be exercised with cutting, because every cut is an entry point for pests to enter and attack the vine.  On the other hand, if too many leaves are pruned, the plant does not have the means to absorb sufficient sunlight to sustain the entire vine.

Wine grapes emerge at the end of the growing season so the plant’s nutrients must further be shared with the new grape clusters. If there are too many clusters, the sugar and acid levels will likely be undeveloped and/or unbalanced resulting in a poor showing as a wine.  Too few clusters negatively affects potential profits from wine sales.

Pest control is another very important aspect of viticulture. In the 1870s a small, deadly phylloxera louse made it’s way to Europe and all but wiped out all wine production. Phylloxera destroys the grapes, rots the vines and often leaves its larvae in the root, eventually killing the vine completely.  Although Bordeaux and Europe at large have regained their wine producing capabilities, phylloxera and other lice, along with viruses, bacteria, fungi, mites and insects are still among the many threats to healthy vines.

In an effort to eliminate ongoing threats to their vineyards and livelihoods, many late 20th century wine growers often used chemical fertilizers and pesticides indiscriminately.  Thankfully much has changed since then with most of the region’s winegrowers using more environmentally conscious, natural pest control methods.  For example, Bordeaux wine growers are currently and constantly experimenting with root grafting in order to find the genetic combination that is naturally resistant to harmful bacteria and viruses.  Scientists and wine growers are also experimenting with sea algae as a natural deterrent to gray rot. 

There is no doubt that viticulture is both science and art.  Winemakers must have intimate knowledge of their vineyard’s terroir as well as which viticulture methods will work best within its parameters. It is with this intricate knowledge and dedication to quality that winemakers are able to extract the best wines from the best grapes.

Viticulture in Plaster

France is a geologically, minerally rich country. Correspondingly rich in culture, the French have been very successful in exercising their influence over a number of raw mineral materials to produce some of the finest plasters in the world. The plaster equivalent to Viticulture is baking. Let’s now take a closer look at how 3 minerals are prepared for our blended plaster, Terre de Séléné.

Clay is the primary mineral used for plaster in Terre de Séléné. It is an abundant mineral worldwide, the result of millions of years of erosion. In parts of France a relatively pure form is available just under the topsoil, just a few feet below ground. It is easy to excavate and is still traditionally dried by the sun. Later, with minimal effort, it is ground into a powder ready to be used for plaster. While there are a variety of clays in France, clay with a low shrink-swell capacity such as Kaolinite is desirable for Terre de Séléné.

Historically, the French were enamored with this type of clay for additional uses. The word “Kaolin” comes to us directly from French. They in turn inherited the term from China. In the early 18th century the French were obtaining an extremely pure form of clay useful for porcelain, “China”, from a deposit near a mountain the Chinese called Kao “high”, Ling “hill”.

Gypsum is the secondary mineral used in Terre de Séléné plaster. Gypsum is plentiful in France and particularly so in Paris. Gypsum plaster is almost synonymous with the expression “Plaster of Paris”. Paris in fact sits on a “massif” or deposit of mineral gypsum that is among the largest and finest in quality on earth. Naturally occurring gypsum is a type of salt that precipitates through cycles of evaporation from lime or other calcium compounds, typically in lagoons or inland seas.

Preparing gypsum plaster requires a little more effort and energy than clay. It is usually mined from underground deposits. Relatively soft as a stone, it is easily pulverized to a coarse sand ideal for baking. Most of the gypsum plaster useful for Terre de Séléné only needs to be baked at under 350° F for less than an hour. In general, considerable influence can be exercised in the baking process. Adjustments to the grind, temperature, length of baking and even barometric pressure can produce an amazing range of properties in gypsum plaster such as fast setting plasters good for casting or extremely dense, hard plasters appropriate for floors or countertops.

Limestone is the third mineral used for our plaster blend. In abundance in the South of France, limestone is a sedimentary stone, the result of millions of years of marine skeletons accumulating on ancient sea beds. The lime most useful for Terre de Séléné plaster is very pure, having little contamination from magnesium or silicates. By itself, limestone is very useful as a building material; however, to produce a plaster requires considerable fuel and labor.

Limestone is found underground but is plentiful and easier to extract from surface mines. Much harder than gypsum or clay, extraction is laborious. For baking limestone is broken into golf ball size pieces. Traditionally, it was baked for 24 hours in vertical kilns at an extremely high temperature of 1500° F. Modern production methods utilizing crushers and horizontal kilns have reduced the time considerably.

The resulting “quick” lime is highly caustic, potentially hazardous to handle. At this point of production enough water is introduced to cause a partial reaction that reduces reactivity and danger. The slaked lime, also known as dry hydrate, is now ready to be blended with the clay and gypsum plaster to make Terre de Séléné.

As you have read, the French traditions of Viticulture and plaster preparation are very sophisticated. The usefulness of our modern scientific, chemical understanding still lags behind the practical experience gained through centuries of empirical observation and practice. This is especially evident in our subsequent, fourth segment considering the art of the blend, Viniculture.

This article was coauthored by Angela and Patrick Webb