Calcareous Aggregate

Technical Advice TAN 5

Traditional lime mortars carbonate ‘within’ as well as on the surface.

Two groups of aggregate have a positive chemical effect on mortar performance:

  • Calcareous or limestone aggregates
  • Pozzolanic materials

Calcareous Aggregates

This is calcium carbonate present as limestone aggregate but also as un-burnt material in the lime. The reaction series that is critical to the performance of historic lime mortars is carbonation, and not the hydration of calcium silicates and calcium aluminates.

To understand this, it is necessary to understand the chemistry of carbonation, and this is more complicated than generally thought.

Carbonation is generally understood in the oversimplified reaction of:

Ca(OH)2 + CO2 → CaCO3   ( lime + carbon dioxide  calcium carbonate )

As moisture is necessary, this reaction should be written:

x.H2O + CO2 + Ca(OH)2 → CaCO3 + y.H2O

In fact:

The first reaction is that of CO2 + H2O to form H2CO3 (carbonic acid). Reactivity increases in the presence of acidity in rainwater. It is this carbonic acid that reacts with lime on the surface of mortar to produce calcium carbonate.

Ca(OH)2 + H2CO3 → CaCO3 + 2H2O

The depth of this surface reaction depends on many factors including time, permeability of mortar, and relative humidity.

Later:

Carbonated lime on the surface of mortar reacts with carbonic acid to form calcium bicarbonate:

CaCO3 + H2CO3 → Ca(HCO3)2(aq).

Both the calcium bicarbonate and the carbonic acid permeate into the mortar where they both react with lime to produce calcium carbonate:

Ca(OH)2 + H2CO3 → CaCO3 + 2H2O

Ca(OH)2 + Ca(HCO3)2 → 2CaCO3 + 2H2O

In a Traditional Lime Mortar containing calcareous material, carbonation occurs within the mortar:

As long as sufficient CO2 is available to stabilise the bicarbonate in solution which is likely to occur in the long term, calcium carbonate present as limestone aggregate, or as unconverted, partially converted, or reconverted material, will in turn dissolve in the carbonic acid to produce calcium bicarbonate. This bicarbonate will in turn react with lime to form calcium carbonate.

CaCO3  + H2CO3 → Ca(HCO3)2

Limestone aggregate + Carbonic acid → Calcium bicarbonate

Ca(HCO3)2  +  Ca(OH)2 → 2CaCO3  + 2H2O

Calcium bicarbonate + Lime → Calcium carbonate + water