Controlling Swelling of Portland Brownstone
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1047-Y05-03
Controlling Swelling of Portland Brownstone Timothy Wangler1, and George W. Scherer2 1 Chemical Engineering, Princeton University, Eng. Quad. E-211, Princeton, NJ, 08544 2 Civil & Env. Eng., Princeton University, Eng. Quad. E-319, Princeton, NJ, 08544 ABSTRACT Many clay-bearing sedimentary stones such as Portland Brownstone will swell when exposed to water, and this can generate damaging stresses as differential strains evolve during a wetting cycle. Current swelling inhibitors, consisting of α,ω-diaminoalkanes, can reduce swelling in Portland Brownstone up to 50%. In this study, through X-ray diffraction and swelling strain experiments, we demonstrate that the α,ω-diaminoalkanes inhibit swelling by substituting for interlayer cations and partially hydrophobicizing the interlayer, then rehydrating on subsequent wetting cycles. We also introduce the copper (II) ethylenediamine complex as a potential treatment for swelling inhibition. INTRODUCTION Portland Brownstone, a sandstone widely used throughout the northeastern United States in historic buildings and monuments, shows damage that is related to the swelling of clay in the stone. These clays cause differential stresses and strains that lead to buckling and cracking, and the clays may also create small pores that make the stone susceptible to damage by salt crystallization or frost [1]. As the exterior surface of the stone absorbs water, the wet layer expands with a strain εs relative to the dry interior, which creates a compressive stress, σx, in the wet layer. When that layer is thin compared to the dry interior, the stress is [2] E ε σx = w S 1 − νw where Ew is the elastic modulus of the wet stone (which may be much less than that of the dry stone [3], and νs is Poisson’s ratio. If a treatment is applied that reduces εs, without increasing Ew, then the stress is reduced. Most of the damage observed is of the buckling type (as seen in Figure 1), where large surfaces buckle away from the surface during wetting, although exposed elements can crack upon drying. It has been demonstrated that treatment with surfactants can reduce swelling, and in this paper, we show the mechanism of strain suppression through tests on pure clays and whole stone.
Figure 1. Examples of buckling damage as seen on the Victoria Mansion (Portland, ME), a 19th century building constructed with Portland Brownstone. It is quite common to find swelling clays in sedimentary stones, and these clays can lead to many engineering problems in the fields of tunneling, oil well borehole stability, and foundation stability [4-6]. It was recognized as a problem in conservation by Delgado [7], among others. Wendler and Snethlage devised and tested an α,ω-diaminoalkane treatment that was found to be effective on some swelling German sandstones and on swelling stones on Easter Island [8-9]. This treatment consisted of a linear hydrophobic alkane chain with protonated amine groups at each end; the amines are believed to substitute for the alkali in the interlayer of the clay and act to bind o
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