Hardness of Brick, Stone and Mortar

It is a basic rule of building conservation that mortars should not be harder than the bricks or stones they meant to hold together. A softer mortar has flexibility and also can act sacrificially, protecting stones or bricks from premature decay or degradation.

The concept of matching building materials or using certain building materials together originates from antiquity. The Romans have been aware of the very different properties of building materials and through centuries of observation they have drawn important conclusions about how long certain materials last or how well they work together.

In Book Two of "De Architectura"⁶Vitruvius (1999), Ten Books on Architecture De Architectura. Rowland, I D. and Howe, T N, (editors), Cambridge: Cambridge University Press. - the only major work on Roman architecture that survived from antiquity, which was the go-to textbook on architecture from ancient Roman times to the Renaissance - Vitruvius describes how the Romans gradually gained greater expertise in building construction by using the diverse materials in their surroundings including pozzolans - volcanic sands and ashes.

Throughout a period of about 400 years (200 BC - 200 AD) the Romans have made significant advancements in mortar technology, gradually transiting from very weak friable mortars to very durable hard mortars that were able to survive the millennia.

Before 200 BC the Romans have primarily used weak, friable mortars. By about 200 BC, builders began experimenting with lime and pozzolanic aggregates, gradually improving the strength and durability of pozzolanic mortars, deliberately selecting specific aggregates of red pozzolans (Pozzolane Rosse). After centuries of experimentation, Roman builders have acquired the technical expertise to also use hard volcanic rocks to their best advantage, creating innovative pieces of architecture from stone and Roman concrete.

Here is the historic evolution of Roman mortars⁷Jackson, Marie & Deocampo, Daniel & Marra, Fabrizio & Scheetz, Barry. (2010). Mid-Pleistocene volcanic ash in ancient Roman concretes. Geoarchaeology. 25. 36 - 74. 10.1002/gea.20295. throughout the centuries:

• 210 to 80 BC: very friable mortars, of predominantly grey colour with large amounts of earthy materials. Monuments of this era (e.g. the Temple of Castor and Pollux) were built from several types of tuffs and friable pozzolanic mortars. Soft porous tuffs can absorb 15-23% of their weight in water, losing up to about 50% of their dry compressive strength, having little durability. As a result many buildings from this period have mainly survived as ruins.
• 80 to 40 BC: less friable mortars than preceding period, ashy grey but darker in colour with less earthy materials.
• 40 to 20 BC: good quality mortars almost entirely free of friability using some red pozzolans and finely crushed tuffs. Red pozzolans introduced during the time of Julius Caesar.
• 20 BC to 40 AD: superior quality non-friable mortars of red or red-brown colour, although the mortars lacked the rock-like hardness of a century later. The quality of stones used in construction has also improved, using more durable moderate compressive strength stones, usually between 50-100 MPa. This period marks the beginning of Pax Romana (The Roman Peace), a roughly 200-year period of golden age of relative peace, stability, prosperity and expansion. 
• 40 to 90 AD: firm mortars of dark gey or reddish grey colour. The Colosseum has been built during this period. After the Great Fire of Rome of 64 AD which destroyed 70% of Rome, Nero introduced new building regulations to prevent yet another destruction of Rome by fires. During reconstruction a lot less wood has been used and travertine - a type of limestone liable to thermal expansion, cracking and disintegration when exposed to fire - has been replaced with other stones impervious to fire.
• 90 to 180 AD: very durable, rock-hard mortars free from earthy materials. During this period the Romans have made significant advancements in mortar technology. They became very skilful in using a wide range of pozzolans and other materials, the mortars being adapted to various building parts and structural demands. Mortar sands were partially replaced with clean white and red pozzolans. Vaults contained 20-30% light-coloured pumices.

What has history taught us? Here are some conclusions we can draw from history:

• Hardness is relative. In relative terms, a 5N (Newton) mortar is still 4 times softer than a 20N clay brick. This combination of materials satisfies the basic principle of building conservation that mortars must be softer then surrounding stones or bricks they meant to hold together.
• Matching mortars to applications: the Romans have matched the mortars to building materials, taking into account the part of the building and structural needs, using a wide variety of mortars for various needs.
• From historical perspective: hardness in itself is not a problem, as proven by several historical examples.  Roman buildings have survived for millennia.
  - Roman mortars: the harder Roman mortars provided adequate mechanical consolidation to ancient buildings, this playing an important role in their longevity.
  - Natural cements: 19^th century natural cements (also known as Parker's cement) - produced from limestone (marl) with high clay content (25-30%) fired at low temperatures without any chemical additives - have been used on the facades of many Victorian buildings, being around for more than 150 years. Natural cements had a completely different chemical composition from modern Portland cements and despite their hardness (over 40 N/mm²) they had none of the problems associated with Portland cements, causing no damages to historic buildings.
• From modern perspective: ancient Roman mortars had very good breathability and a traditional chemistry performed exceptionally well throughout history. Hardness of plasters/mortars became an issue during recent years due to the widespread use of Portland cements on historic buildings, causing significant damages. Modern cements are significantly harder than other mortars and most building materials - however there are other significant differences between Portland cement, NHL and lime mortars that must also be taken into consideration: breathability and chemical composition. In our practical observation the much reduced breathability and very different chemical composition of Portland cements - in particular due to the presence of salts in their chemistry - are far more responsible for damages to historic building than mere hardness.

References

• 1
  Jackson, Marie & Kosso, Cynthia & Marra, Fabrizio & Hay, Richard. (2006). Geological Basis of Vitruvius' Empirical Observations of Material Characteristics of Rock Utilized in Roman Masonry. Proc. Second Int. Congress of Construction History. 2. 1685-702.
• 2
  Fernandes, Francisco & Lourenco, Paulo. (2007). Evaluation of the Compressive Strength of Ancient Clay Bricks Using Microdrilling. Journal of Materials in Civil Engineering - J MATER CIVIL ENG. 19. 10.1061
• 3
  Abed Soleymani, Mohammad Amir Najafgholipour, Ali Johari (2022). An experimental study on the mechanical properties of solid clay brick masonry with traditional mortars, Journal of Building Engineering, Volume 58
• 4
  Goodman, R E, (1989). Introduction to Rock Mechanics, New York: John Wiley
• 5
  Suciu, Ovidiu & Cruciat, Radu & Ghindea, Cristian. (2014). Experimental Case Studies on Clay Fired Bricks Compressive Strength. Key Engineering Materials. 601. 10.4028
• 6
  Vitruvius (1999), Ten Books on Architecture De Architectura. Rowland, I D. and Howe, T N, (editors), Cambridge: Cambridge University Press.
• 7
  Jackson, Marie & Deocampo, Daniel & Marra, Fabrizio & Scheetz, Barry. (2010). Mid-Pleistocene volcanic ash in ancient Roman concretes. Geoarchaeology. 25. 36 - 74. 10.1002/gea.20295.