Hydraulicity & properties

of Saint-Astier® NHL

General information
Hydraulicity is the property of a binder to harden in contact with water.

Hydraulicity is produced by burning a limestone containing silica, alumina and iron oxides which above certain temperatures combine, totally or partially, with the Calcium Oxide. The resulting silicates, aluminates and ferrites give hydraulic properties to the product. Today, as in the past, natural building limes are obtained by burning and slaking limestone and the more or less hydraulic character of the finished product is directly related to the percentage of calcium silicates, aluminates and ferrites formed during burning. The composition of the Earth crust shows the predominance of silica and its presence is almost inevitable in all limestone deposits.

The building limes of the past, if the soluble (combined) silica content is analysed, will almost certainly show some hydraulic property, even if very feeble. The analysis of historical mortars today rarely takes this factor into account and, as sometimes the amount of combined silica in a mortar is minute, a number of findings will not identify the hydraulic component in the mortar.

For example: an amount of 4% of combined silica in a binder represents, in a typical mortar with a 17.5% binder content, only about 0.7 % of the total mass of the mortar but this mortar will still be feebly hydraulic. See example below based on an NHL 2 with a binder/sand ratio of 1:2.5

The existence of pure Calcium Carbonate deposits is not common. High Calcium limes are mainly exploited for industrial use (i.e. the steel industry), where it is essential to have an almost pure material. Even in metamorphic type calcareous stone such as marble, silica is found. The little amount of silica required to combine with the CaO during burning makes the production of hydraulic properties almost inevitable when the raw material is a calcareous stone.

This mortar will almost certainly be classified as non-hydraulic by most analysts. If a “match” is required, it might be erroneously made by adopting a non-hydraulic high calcium lime instead of a feebly hydraulic lime.

The quality of hydraulic limes derives from the mineralogical composition of the raw material as well as the skill and production controls of the manufacturer. See Mineralogy and chemistry of raw materials & products.

The absence of sulphates in St. Astier limestone and the low traces of alkali such as Potassium and Sodium cannot produce limes conducive to sulphate attack or alkali-silica reactions. The low amount of alumina will produce only very low levels of tricalcium aluminates – a necessary criterion in avoiding sulphate attack. Annex 1 clearly shows the potentially damaging components in binders responsible for many long-term deteriorations and failures in mortars.

The result of an efficient burning and controlled slaking is that Saint-Astier® products have a high percentage of free lime residual, greatly superior to the minimum limits required by the Standards for hydraulic lime.

Minimum Free (available) Lime	Table Header	EN/BS 459	Saint-Astier® NHL
requirements Ca(OH)2 %	NHL 5	3	15-20
	NHL 3.5	9	24-26
	NHL 2	15	over 50

A number of classifications have been put forward. The main ones are listed below, together with their shortcomings:

Classification related to setting time
This is based on the principle that limes with a setting time of over 1 day are not hydraulic.
The relevant tests are conducted on lime paste and therefore cannot be acceptable as hydraulic limes are used in mortars (lime + aggregates). The setting time in mortars depends not only on the hydraulic properties of the lime but also on the volumetric ratio of the mortar mix and other factors such as water content.

The cementation index
This index supposes that there is no unburned residue and that combined silica (SiO₂) is present as C₃S. Although this is correct for cement it is not the case with hydraulic limes where C₂S is the main hydraulic component and there is always unburned residue.
If hydraulic lime mortars contained a high level of C₃S, it would not be possible to rework them and St. Astier limes can be reworked.

Classification based on colour
Dismissed by their own authors.

Vicat classification
In the early 19th century L. Vicat established that Limestone containing Silica, Alumina and Iron Oxides produced Hydraulic Limes. He attributed the presence of these elements to “clay” impurities in the limestone and proceeded to classify according to the amount of “clay” content in a calcareous stone. He based his Hydraulicity Index on the following formula:

I = (SiO₂total+Al₂O₃+Fe₂O₃) / (CaO total)

Vicat omitted to note that not all SiO₂ is soluble (some of it is insoluble quartz) and therefore available to combine with the CaO. Furthermore, he supposes that all the CaCO₃ in the stone is converted to CaO during burning with no residue, which is also incorrect. Vicat’s formula is perfectly applicable to cement where the high burning temperature ensures that the components are combined in their near totality with the CaO. However it cannot be applied to hydraulic lime today. For example, according to the Vicat Hydraulicity Index, cement would have an Index (I) of 0.42 with a compressive strength of approximately 55 N/mm² @ 28 days whilst NHL 3.5 would have an Index of 0.37 with a compressive strength of 50 N/mm².

The theory of soluble (combined) silica
This is by far the most reliable method of classifying hydraulicity.
The principle is simple but indisputable: the silica contained in a calcareous limestone is combinable or inert. The appropriate burning process determines the quantity of silica that will combine. This explains how from a uniform deposit, such as the Saint-Astier® quarry, it is possible to obtain different hydraulic characteristics from the same stone. Soluble silica combines with the CaO (ratio of approx. 1:3) during burning at 900^o-1,000^oC, forming CS (Calcium Silicates) which are responsible for hydraulicity. See Annex 2.

The amount of available silica in the stone is the determining factor. Limestone containing less than 4% will not produce hydraulic lime. From 4% and above, hydraulicity will be generated in direct proportion to the combined amount of available silica and CaO. See Annex 3.

Soluble silica and ancient mortar analysis
The soluble silica theory is of great value when studying ancient mortars in order to separate their different hydraulic behaviours. If we agree that the soluble silica combines with the CaO to produce reactive Calcium Silicates and then find the levels of soluble silica in ancient mortars, we can establish their degree of hydraulicity and match them if so required. Using this method would show that a surprising number of ancient mortars have hydraulic properties. This is because the building lime used by our ancestors rarely came from limestone that was free from silica, alumina and iron oxide. (These minerals are also present in clay, hence the popular definition of “clay contamination”). As previously stated, it would be enough for the soluble silica to be as low as 4% to generate hydraulic properties in the lime.

Mortar setting: pozzolanic additions to mortars not made with hydraulic limes
Due to the properties of today’s Air Limes (putty or hydrated), pozzolan addition is usually necessary to enable the builder to get on with his work. However its presence can be problematic: greater care must be taken with the water content, the variable setting properties and granulometric and colour requirements. In the end, using this aggregate can lead to unnecessary complications, higher costs and a disappointing result.

Saint-Astier® limes do not require pozzolans. If the main reason for using pozzolans is to create a hydraulic effect the correct grade of natural hydraulic lime will achieve the same or better result in a safe and reliable manner.

The composition of the raw material, Saint Astier®’s experience in the production process as well as its strict quality control procedures have brought to the user a range of Natural Hydraulic Limes suitable for all construction purposes.

Annex 4a shows some of the most important performance characteristics of Saint-Astier® NHL mortars compared with blended NHL/putty mixes and cementitious mixes (1:1:6 and 1:2:9)

Here are some of the reasons why Saint-Astier® NHL are widely accepted and appreciated:

1) Purity
No addition of any kind is made to the Saint-Astier® NHL products to enhance their performance.

2) No need for blending
The Saint-Astier® range permits the builder to select the most suitable product for the work at hand without having to add pozzolans, cement, plasticisers, water retainers, waterproofers etc.

Blending introduces considerable risks of errors, added costs and final short and long-term results which are uncertain and therefore hazardous.

3) Compatibility
The availability of a range of pure binders with different performance
characteristics ensures the compatibility of Saint-Astier® NHL mortars with existing mortars whatever their age.

4) Free lime content (available lime)
Responsible for workability and self-healing in NHL mortars.

5) Economy
Generally binders are bought by weight but mixed by volume, their bulk density therefore determines the volume used. The lower the density, the lighter the weight of product used when mixing by volume. The low bulk density of all Saint-Astier® NHL products is such that when compared with cement, lime putties and some other hydraulic limes, one can obtain a sizeably larger quantity of mortar with the same weight of material purchased.

Example: taking the density of NHL 2 at 550 gr/litre versus lime putty (1,350gr/litre), the density of putty is 145% greater than NHL 2, so at the same volume, NHL 2 will produce more mortar.

6) Versatility of use
NHL products can be used to make building, rendering and plastering mortars, grouts, injection, concrete and paint.

7) Mortar performance
Mortars made with Saint-Astier® NHL binders achieve:


Elasticity	A factor in building without construction joints. Important in diminishing shrinkage and cracking. Allows for minor movements.
Permeability	Good vapour exchange qualities allow for condensation dispersion. No rot. Great benefits to the living environment.
Resistance to salts	The absence of any potentially damaging addition (i.e. gypsum or cement) make sulphate attack, alkali-silica reactions impossible. Existing salts in the building fabric will pass through and eventually can be washed off. Excellent performance in marine environment
Suitable Compressive Strength	Unlike cement or cementitious mixes (1:1:6 etc..) the compressive strength will be achieved gradually, allowing for movement. The availability of a range will permit the making of mortars with the required strength without having to add or blend.
Resistance to weather	Early setting will provide quicker protection from adverse weather. See "Protecting Lime Mortar".
Self Healing	The available lime provides this quality. A timely light water mist over a minor shrinkage mark will help to heal it.
Resistance to Bacteria and Vegetable growth	The alkalinity of the binder does not favour their development
Insulation	The porosity of the mortar present good insulation values.
Sand colour	The whiteness of the NHL binders will reproduce the colour of the aggregate used
Reworking	All St. Astier mortars can be reworked (8 - 24 hours), reducing wastage and increasing work speed. This is due to the absence of cement, gypsum or pozzolans.
Recycling	Materials built with NHL mortars can be reused.
CO2 absorption	probably the most Eco friendly contribution of using limes. Damaging CO2 is re-absorbed during the carbonation of the free lime.

Annexes

Annex 1

		% content in
		OPC	NHL	Potential damaging effect
Tricalcium Aluminate	C3A	3 - 10+	<1	Reacts with Sulphates and water producing sulphate attack causing mortar deterioration and eventual failure. Reacts with sea salts. Affects bricks/stones.
Tetracalcium Aluminoferite	C4AF	8 - 10	NIL	Reacts with Gypsum causing expansion
Sulphates	SO3	2 - 7	0.4 -0.6	Contributes to sulphate attack
Alkalis	Na2O/K2o	1 - 3	>0.1	Reacts with the silicates in cement and sand producing gradual disintegration ALKALI-SILICA REACTION
Gypsum	CaSO4	2 - 9	NIL	Subject to expansion, efluorescence. Deteriorates in contact with sea salt

Note: in marine locations the air contains sea salt which reacts with C₃A in OPC, causing serious damage to cementitious renders.

Annex 2

Chemical Process in the production of pure and Natural Hydraulic Lime:
Limestone – Quicklime – Pure Natural Hydraulic Lime

Annex 3

EU Norm 459-1		NHL Classification
NHL 5	5-15
NHL 3.5	3.5-10	N/mm2 @ 28 days (mortars prepared with a binder:sand ratio of 1:1.3)
NHL 2	2-7

Hydraulicity: % of soluble silica and relative hydraulic properties

Annex 4a

Characteristics of Saint-Astier® NHL mortars versus those of cement mortars and OPC/CL lime mortars
(Putties/hydrated lime) Sand used: well graded sharp sand 3mm – 0.075

The water addition in the shrinkage test was regulated to obtain mortars with the same workability
(flow table test 190 +/- 5mm)

Notes
A – Mortars containing OPC start setting too quickly.
B – OPC mortars are not as flexible as lime mortars.
C – NHL mortars achieve a good compressive strength gradually, allowing for movement
D – Cementitious mixes will retain moisture
E – At complete carbonation

		Tests on pure NHL mortars				Tests on cement/ hydrated lime mixes
			NHL5	NHL3.5	NHL2	OPC/CL(hydrated)		Notes
Volumetric mix (binder/sand)			1 : 2.5	1 : 2.5	1 : 2.5	1:1:6	1:2:9

Set (beginning) / hours			2-4	4-6	6-9	1.0	1.0	A

Elasticity Moduli	28 days	MPa	11000	9000	9800	16200	15595	B
	6 mths	MPa	17050	13505	12030	22010	19300	B
	12 mths	MPa	17280	13620	12030	22210	19700
	24 mths	MPa	18020	13785	12000	22150	19650
Compressive strength	28.days	N/mm2	2	1.47	1.36	7.7	5.56	C
	6 mths	N/mm2	5.9	5.30	3.00	8.1	5.75	C
	12 mths	N/mm2	8.44	5.90	2.90	8.7	6.05
	24 mths	N/mm2	8.81	6.00	3.00	8.5	5.95
Vapour exchange (air gr/m2/h/mmHg)			0.55	0.65	0.71	0.23	0.25	D/E

Shrinkage 28 days mm.m.			0.13	0.44	0.6	0.63	0.42

Annex 4b

Characteristics of Saint-Astier® NHL mortars versus those of cement mortars and OPC/CL lime mortars
(Putties/hydrated lime) Sand used: well graded sharp sand 3mm – 0.075

The water addition in the shrinkage test was regulated to obtain mortars with the same workability
(flow table test 190 +/- 5mm)

Notes
A – Mortars containing OPC start setting too quickly.
B – OPC mortars are not as flexible as lime mortars.
C – NHL/Putty mortars are considerably weaker than NHL mortars.
D – Cementitious mixes will retain moisture
E – At complete carbonation.

		Tests NHL/Putty(CL)/Sand Mixes NHL 5/CL				Tests on cement/ hydrated lime mixes
						OPC/CL(hydrated)		Notes
Volumetric mix (binder/sand)			0.9/0.1:3	0.7/0.3:3	0.5/0.5:3	1:1:6	1:2:9

Set (beginning) / hours			3.5	5.25	9.5	1.0	1.0	A

Elasticity Moduli	28 days	MPa	11000	10020	8000	16200	15595	B
	6 mths	MPa	16000	14000	12030	22010	19300
	12 mths	MPa	16510	14320	12030	22210	19700
	24 mths	MPa	16500	13950	13220	22150	19650
	28.days	N/mm2	1.4	1.1	0.6	7.7	5.56
	6 mths	N/mm2	4.8	3.95	2.97	8.1	5.75
	12 mths	N/mm2	5.3	4.1	2.8	8.7	6.05
	24 mths	N/mm2	5.25	4.31	3.85	8.5	5.95
Vapour exchange (air gr/m2/h/mmHg)			0.60	0.59	0.63	0.23	0.25	D/E

Shrinkage 28 days mm.m.			0.25	0.60	0.84	0.63	0.42

Annex 4c

Characteristics of Saint-Astier® NHL mortars versus those of cement mortars and OPC/CL lime mortars
(Putties/hydrated lime) Sand used: well graded sharp sand 3mm – 0.075

The water addition in the shrinkage test was regulated to obtain mortars with the same workability
(flow table test 190 +/- 5mm)

Notes
A – Mortars containing OPC start setting too quickly.

B – OPC mortars are not as flexible as lime mortars.
C – NHL/Putty mortars are considerably weaker than NHL mortars.
D – Cementitious mixes will retain moisture
E – At complete carbonation

			Tests on pure NHL mortars NHL 3.5/CL			Tests on cement/ hydrated lime mixes
						OPC/CL(hydrated)		Notes
Volumetric mix (binder/sand)			0.9/0.1:3	0.7/0.3:3	0.5/0.5:3	1:1:6	1:2:9

Set (beginning) / hours			6.5	8.5	10.0	1.0	1.0	A

Elasticity Moduli	28 days	MPa	8400	8050	7510	16200	15595	B
	6 mths	MPa	13220	12600	11000	22010	19300	B
	12 mths	MPa	13410	12900	11050	22210	19700
	24 mths	MPa	14250	13010	10850	22150	19650
Compressive strength	28.days	N/mm2	1.3	1.1	0.75	7.7	5.56	C
	6 mths	N/mm2	3.9	3.63	2	8.1	5.75	C
	12 mths	N/mm2	4.8	4.45	3.75	8.7	6.05
	24 mths	N/mm2	4.75	4.55	2.65	8.5	5.95	Content
Vapour exchange (air gr/m2/h/mmHg)			0.69	0.71	0.68	0.23	0.25	D/E

Shrinkage 28 days mm.m.			0.35	0.67	0.89	0.63	0.42

Hydraulicity & properties

of Saint-Astier® NHL

Annexes

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