Plaster and mortar durability

Strength, hardness and plaster or mortar diffusion depend on type and properties of applied sources, component mixture ratio and hardening conditions. Some historical plasters are better in properties after several centuries than building stone in structure. Example of high quality mortar is firm, consistent lime mortar (3 m2 of pre-Romanesque age), which was discovered in Mikulčice (Southern Moravia in Czech Republic).

Formation of firm lime mortar or plaster depends on drying of lime suspension, which is in close relation to shrinkage of mortar. Calcium silicates are formed by dissolution of quartz in alkali environment. Last process is carbonation.

The third above-mentioned process – carbonation – is the most significant for mortar strength. Lime matrix in mixture with water (together with soluble calcium alkali silicates) fill pores among particles of filling material (sandy grains). Reaction with atmospheric carbon dioxide forms calcium carbonate, which is identical with micrite by the texture. Rate of carbonation is the highest in relative humidity from 50% to 60%. Process does not occur in dry conditions and in higher relative humidity is almost stopped for difficult penetration of CO2 through the pore system filled by water (Hošek, Muk 1990). Original high alkalinity of lime mortar (pH 12,5 to 13,5) is gradually decreased to approximately pH 8. It may be said that strength of lime mortar depends on partial pressure of CO2 (its usual content in atmosphere is 0,03%) on amount, kind and stage of lime slaking (quick lime accelerates process of solidification but it blocks penetration of CO2 into mortar) on porosity of mortar (determined by hydraulic coefficient, ratio of matrix and filling material, size of sandy particles and particle size distribution curve) by moisture and temperature of surrounding environment.

Šujanová (1981) determined rate of carbonation of standard lime mortar in usual atmospheric conditions as follows: 59% of calcium hydroxide changed to calcium carbonate during 2 days, after 5 weeks it was 75% and after 2 years 86%. We have to find explanation stage of slaking of lime matrix (all studied mortar samples contain relicts of quick lime).
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