Erhard M. Winkler

Crystallization Pressure of Salts in Stone and Concrete

Pressure by crystallizing salts against pore walls has caused disruption of rock, stone, and concrete, both in urban and desert environments. The Van9t Hoff-type equation for osmosis, as modified by Correns, is the basis for theoretical calculations of salt pressures. Under ideal conditions, halite can exert up to 650 atm pressure when it is oversaturated by a factor of 2. Air pollution, saline ground waters, and street salting in winter readily supply enough salt to inflict severe damage.

Salt Burst by Hydration Pressures in Architectural Stone in Urban Atmosphere

Salt bursts can cause extensive spalling in stone structures. Water-soluble salts and their hydrates Na2CO3 H2O (Na2CO3-7H2O, Na2CO3 10H2O), Na2SO4 (Na2SO4 10H2O), and MgSO4-H2O (MgSO4 7H2O) are trapped in pores of stone and concrete by both infiltration upward from the ground and by the reaction of carbonate and silicate rocks with sulfuric and carbonic acids from polluted urban atmospheres. The crystallization of the salts and their recrystalhzation from a lower to a higher hydrate within the range of mineral stability may develop stresses of high magnitude. Theoretical bursting pressures were calculated thcrmo-dynamically from vapor pressures of the hydrates and from humidities of the air for several air temperatures.

Important agents of weathering for building and monumental stone

Abstract The atmosphere has little corrosive effect on stone without the presence of water. Washout of aggressive ingredients from the atmosphere by rainwater, however, increases corrosion and solution of stone. Dissolved CO 2 , SO 2 , SO 3 , and Cl are the most effective corrodents. The urban atmosphere supplied much more CO 2 and sulfates through the combustion of fossil fuels than the atmosphere of rural areas; this accelerates stone decay in urban areas very much. Water with few ions in solution may be as corrosive as CO 2 and SO 4 charged water, as it is active to reach the equilibrium condition with the stone in contact. Silicate rocks can resist exposure to rainwater successfully for a long time. Exposure of facing stone and concrete aggregate to soft or acid running water or lake waters causes damage primarily to carbonate rocks. Resistant silicate rocks should be selected. Primitive animals and plants may inhabit bare stone surfaces paving the way to more extensive destruction through the production of organic acids along the root system. All rocks are subject to attack by low animals. Boring sponges, sea shells, and sea urchins may develop a dense network of pock marks near the waterline in different rock types; occasionally also in concrete.

Natural dust and acid rain

Atmospheric dust originates from three sources: terrestrial airborne matter, volcanic, and cosmic. Terrestrial natural dust makes up the main bulk reflecting the soil composition to 150 mi away. Soil erosion from flood plains, plowed fields and construction sites is the main source. Quartz, feldspar, the carbonates calcite and dolomite, and clay minerals are the components in decreasing order of frequency. Natural dust in the atmosphere interacts with rainwater converting the carbonates to benign gypsum (CaSO4.2H20). Naturally leached soils produce less calcite than unweathered sediments on flood plains and construction sites, and in granitic and crystalline rocks less than in limestone areas.Heavy industrialization associated with high emission of C02 and S02 on the one hand, and excess production of dust on the other appears to counteract mans interference with natural ecosystems in the opposite direction.

Frost damage to stone and concrete: geological considerations

Abstract Frost damage to stone and concrete is due to expansion during freezing of water. This process is presented in the p—v—t diagram of ice which is updated and lines of density added for the phases water and ice I. Water also expands 0.4% at 1 atm when heated from 4 to 30°C, and 0.6% at 1 atm, if undercooled from 4 to —20°C. A modified diagram with t on the abscissa and the density change in percent on the ordinate gives expansion curves for 1, 25, 50, 75, and 100 atm. Water is enough compressible that no expansion can take place from 4 to 30°C, if the capillaria walls can resist stresses to 100 atm which are insufficient to break up sound rock. The efficiency of water transportation to the zone of freezing in soils and rocks depends on the size and distribution of capillaria, and the presence of non-swelling clay minerals which act like water pumps along the channels without closing the channels. Water in micropores, however, is unfreezable and much less mobile than water in larger channels, if only a few molecules thick, oriented, and stressed. Engineering geological applications of the p-v-t ice diagram are as follows: (a) the frost stability of stone, if strength, climate, and capillaria size are known; (b) the stability of point-bearing piles in muddy frozen sediments; (c) the knowledge of the rock strength in estimates of minimum Pleistocene rock temperatures of felsenmeers. The freezing process of water is known to produce a spontaneous expansion from a density of about 1.000–0.9165. The expansion pressure (or crystallization pressure) may be of considerable importance as it aids in the mechanical disintegration of natural rock, structural stone and concrete. The interest in the process of freezing started at the beginning of the 19th century when the expansion as well as the contraction of ice at increased pressures was recognized. Only recently we have learned that the damaging frost action does not depend on the expansion of ice alone, but rather on a variety of processes which diagram of ice may find an application in engineering geology. The discussion also extends to the implications which the lithology may have in the frost safety of rocks and soils. It is the hope that the previous discussion will be helpful to both the geologist and the engineer.

The importance of air pollution in the corrosion of stone and metals

Abstract Evidence is presented that polluted air not only affects life but also stone, concrete and metals as well. CO 2 and the sulfates are the strongest corrodents; less strong are the nitrates and chloride. Evidence is presented here that the ionic increase almost doubled in the Great Lakes - St. Lawrence River drainage basin, whereby about 20–50% of the ions are believed to be contributed from polluted atmospheres surrounding the lakes. Improvement of automotive combustion will climinate toxic CO and smog-producing unburned hydrocarbons, but on the other hand will augment CO 2 and NO 2 to such a high level that the corrosion of stone and metals probably will increase rapidly unless a switch to new power sources is accomplished.

Decay of Stone

The accelerating rate of decay of cultural treasures of stone and concrete is becoming a familiar story. Fig. 72 a, 72 b, 72 c suggest the progress of decay in a sandstone sculpture, exposed to the elements since 1702 and photographed in 1908 Open image in new window Fig. 72 Stone decay in the industrial atmosphere of the Rhein-Ruhr; sculpture is of porous Baumberg sandstone (Upper Cretaceous) at Herten Castle near Recklinghausen, Westphalia, Germany, built in 1702. a Appearance in 1908, showing light to moderate damage, b appearance in 1969, showing almost complete destruction and ? estimate of the change in rate of decay since 1702. Photos and information supplied by Dr.Schmidt-Thomsen, Landesdenkmalamt, Westfalen-Lippe, Muenster, Germany and 1969. The weathering damage in the first 200 years was relatively mild compared with that suffered in the 60 years of the present century.

Iron in Minerals and the Formation of Rust in Stone

The mean iron content of the earth’s crust is 5%. Iron is locked in ferromagnesian silicates in rocks at the earth’s surface mostly as green or black ferrous-ferric iron. The black ferrous-ferric form is magnetite, the red ferric oxide, hematite, and the yellow-brass ferrous sulfides are commonly cubic pyrite and orthorhombic spearhead-shaped marcasite. Iron also appears as white to dark brown ferrous carbonate (siderite) and green iron silicate, glauconite, which adds a greenish color to sedimentary rocks (Sect. 4.4).

Color and Color Stability of Stone

The color of structural and monumental stone has challenged the architect for the most effective and harmonious appearance in architectural design since ancient times. The utilization of different color shades of stone has given new life to many existing structures. Stone colors are influenced by the color of the predominant mineral, but also by the adjacent minerals, grain size, and grain cement.

Physical Properties of Stone

Stone is a heterogeneous substance characterized by a wide range of mineral compositions, textures, and rock structures. Consequently, the physical and chemical properties and the resulting durability are quite variable. The suitability of a stone for a given building can be easily tested in the laboratory. Although some tests are expensive and consume considerable amounts of rock material, others are simple, inexpensive, and nondestructive (see App. A).