703

Some effects of masonry biocides on intact and decayed stone Rachael D. Wakefieldf and Melanie S. Jonestt

t School of Applied Sciences, The Robert Gordon University, Aberdeen, Scotland. AB2S I HG. ft Faculty of Design, The Robert Gordon University, Aberdeen, Scotland. AB9 2QB.

Abstract

Masonry biocides used in the control of biological growths are rarely formulated specifically for use on building materials and there is now growing concern for their overall effects on stone. A novel methodology was developed which used dried biocide residues after they had been in contact with various sandstone types. A common technique, energy dispersive X-ray analysis, was then utilised to assess the potential of the biocides to cause dissolution of stone components. Of the three biocides used in the investigation, a quaternary ammonium compound with inorganic borate had the least effect on dissolution of stone components. A biocide containing dichlorophenol appeared to cause some dissolution of silicate minerals, the effect likely to be related to its high pH. The third biocide, an alkyl amine, brought about the dissolution of Al, Si, K and Fe. Clay minerals appeared to be the most vulnerable to the effect of the alkyl amine biocide, the mode of action being related to the formation of clay-amine complexes, leading to clay dispersal. Stone minerals in the biologically decayed sandstone were less vulnerable to the action of the biocides. This could be related to the presence of various microbial products contained in the stone, though the action of masonry biocides on decayed stone requires more study to elucidate a precise mechanistic action. This work has shown that the application of alkyl amine biocidal formulas to clay containing substrates should be reviewed.

Introduction.

Buildings, monuments and other culturally important structures exposed to the environment in most climates are subject to colonisation by biological growths. Heavy colonisation can be aesthetically displeasing, obscure surface carvings or paintings, create an impression of poor maintenance and in some cases, promotes decay. The appearance of stone surface colonisers may often prompt the application of chemical solutions containing biocidal compounds to discourage their growth (Grant, 1982; BRE, 1992; Tiano, et al., 1993 ). The treatment of stone with masonry biocides is an area where much research is needed. Of particular importance is method development and standard tests; predictive tests for biocide efficacy which can be extrapolated to the field; in situ monitoring methods for the objective evaluation of biocide efficacy on stone and in testing new and existing substances for their immediate and long-term effects on the substrate.

In recent years there has been increasing concern regarding the potential of masonry biocides to cause stone decay. Such chemicals are typically formulated using active ingredients which have largely been developed for use in other areas where biological control is necessary, such as agriculture, medicine and the offshore industries. Very few formulations have been designed specifically for application on stone, or indeed other valuable historic material. Consequently, the effects of such chemicals on stone has

largely been ignored at manufacturing level.

Common ingredients used in masonry biocides, in particular masonry algaecides, include the quaternary ammonium compounds (QAC's) and the alkyl amines, used alone or in combination with zinc or borate salts. The highly effective combination of QAC's with organotins have now been withdrawn from use in the UK in situations other than the control of marine fouling. Other formulations include phenolics such as dichlorophenol, copper-containing compounds and sodium-containing compounds such as sodium pentachlorophenoxide and sodium hypochlorite.

704

The choice of an appropriate biocide to use in the control of algae and other micro-organisms on external building surfaces is usually an arbitrary process, since not enough is known regarding the activity of particular compounds on different substrates in different situations. Materials of cultural value are usually subject to various treatments before an effective one may be found (Agarossi et al., 1989). A number of requirements for biocidal compounds used in these situations have been suggested, such as a proved efficacy against a wide spectrum of organisms at a minimal dose, no interaction with the substrate and low toxicity to operators (Krumbein and Gross, 1992; Lisi, et al., 1992).

Detennination of the efficacy of biocides in killing target organisms at a minimal dose must be carried out in tests which utilise stone as a substrate since efficacies are usually not the same on different substrates. This is especially true in laboratory tests using growth media. On solid surfaces, the presence of organic debris or intentional applications, such as paint and plaster layers or polymers applied as consolidant surface films, all affect the availability of some biocides to the target organisms (Petersen, et al., 1993; Wainwright, 1988). Stone mineralogy also affects biocide activity, ionic bonding on exchange sites of clays may be expected, with interactions between stone minerals and biocides being similar to those known to occur between pesticides and soil components. Quaternary ammonium compounds have been shown to bind strongly to filter sand and other silicates, and are therefore likely to behave similarly with stone minerals (Isquith, et al., 1972; Mackrell and Walker, 1978; Walters, et al., 1973; Fitzgerald, 1959). Grant and Bravery (1981) suggested that the decreasing efficacies of QAC's observed on sandstone, limestone and mortar respectively, could be a function of the cement content of the materials, with the adsorptive sites of the siliceous components being masked by the carbonaceous materials in the limestone and mortar.

As some of the chemical ingredients which make up masonry biocides interact with stone minerals, it is to be questioned whether they can also bring about changes in the mineralogy of the material which may affect durability of the material. Studies into the effects of biocides on stone have shown for example, that porosity and penneability increase or decrease depending on stone type and biocide (Nugari, et al., l 993a). Colour change with biocide application has to date, been one of the main concerns of biocide application to stone, this effect mainly being related to the fonnation of iron oxide layers around crystals (Nugari, et al., l 993b; Lisi, et al., 1992). The formation of sodium salts within the stone has also been a recognised concern (Richardson, 1995).

The following investigation introduces a test which employs energy dispersive X-ray analysis (EDX) of dried biocide residues, following contact with stone, to detennine the effects of the biocides on the stones selected. The stone used in the investigations are Scottish sandstones which have been used in the construction of many historically significant buildings in the country. Samples of biologically decayed stone were also obtained for the sandstones. The latter, sampled from a field site currently the focus of an investigation into biological stone decay, was chosen in order to determine the response of the biologically decayed stone to different biocides. We hope that this aspect of the work provides additional interest, since in some cases masonry biocides may be considered to halt an ongoing decay process.

Methodology

Masonry biocides.

Three different masonry biocides were selected from those readily available on the UK market for use in the control of a~gae on exposed st?nework. The biocides chosen represent the main generic types comm~nly f?un~ m masonry al~aec1des; a QAC with a borate salt additive, an alkyl amine and a phenolic denvattve. The properties associated with these chemicals are detailed in Table 1 and their

705

structures shown in Figure 1. The trade names, or manufacturers details have not been referred to in this text.

Table 1. Table of masonry biocides used in biocide effects investigations

Biocide type Chemical make up Dilution factor pH PHENOL dichlorophenol 1: 10 10.3 ALKYL AMINE dodecylamine lactate 1:20 6.1

dodecylamine salicylate QAC + INORGANIC alkyl ammonium chloride none 7.2 BORON disodium octaborate

ethylene glycol

a:o- CH3 -I Cl 0 CH2-f-C°"2n+1

CH3

+ 2-b: 2Na Bg013

a: Benzalkonium chloride; b: Disodium octaborate

o-

c 1~ Cl

~ Dichlorophenol

a: Dodecylamine salicylate b: Dodecylamine lactate

Figure 1 Structures of QACB (benzalkonium chloride and disodium octaborate ), DCPH (dichlorophenol) and AMINE (dodecylamine salicylate and dodecylamine lactate) biocides used in

the control of algae on masonry.

Choice of Stone

Two Scottish sandstones were used in this investigation; Locharbriggs and Hermitage stone. Locharbriggs is a red, medium to coarse grained Scottish sandstone of Permian age. It is widely used in Scotland as a building stone. Freshly quarried stone samples were used for this experimental work. Hermitage stone was taken from Hermitage Castle which is situated on remote moorland in the Castleton Parish of Roxburghshire in the Scottish Borders. The 13th Century castle is currently the subject of an EPSRC funded research project looking at biodeterioration of sandstone by algae and application of stone conservation methods, and biocide effects on biodeteriorated stone are of

particular interest.

706

Summary of stone characteristics

Hennitage stone decays in a characteristic way producing a spall which varies in size from 5-10 mm diameter. The decay is due to the presence of an orange coloured algae of the genus Trentepohlia. The fresh and decayed stone samples used in this experimental work were yellow in colour, medium­grained, with ochre/rust coloured spots of Fe-oxyhydroxide ( 1-5 mm diameter). X-ray diffraction data shows the main components of the stone to be: quartz (Si02); potassium feldspar (KAISi30g); occasional traces of clays, illite (KL15Al4[Si7_65Al1_1.50 2o)(OH)4) and kaolinite (Al4(Si401o)(OH)s); traces of calcite (CaC03) and rutile (Ti02). Scanning electron microscopy shows that the cementing agent is idiomorpic siliceous cement crystals which closely abut one another. The cement was formed by pressure solution processes during diagenesis (Wakefield, et al, 1996). Light microscopy shows the presence of Fe oxides in the pore spaces of the sandstone. Porosity is estimated at 20%. The origin of this sandstone is as a river sediment deposited during the Carboniferous Limestone

period.

Locharbriggs sandstone is composed of mainly of quartz (Si02), though each grain displays an iron oxide rim. Approximately 10% of the rock is composed of: potassium feldspar (KAISi30g ) {microcline and orthoclase}; plagioclase feldspar (CaNa Si308) ; muscovite mica. (K2Al4 Si6Al2020 (OH,F)4); opaques; associated clays; smectite (1!2CaNa)07 (AIMgFe)4 [(SiAl)s020] (OHk nH20 and illite (K1. 1.5 Al4 [Si7_6.5 Al1_1.5 0 20] (OH)4). The grains are well sorted with a high spericity. The porosity is reasonably high at 24% and is intergranular rather than within grains, however the feldspar appears porous due to dissolution. There is some alignment of grains to form fine banding and also evidence of compaction and pressure solution with overgrowths on the quartz and some feldspar grains. The origin of this sandstone is probably aeolian, that is wind blown and being subsequently deposited in a semi arid environment.

Exposure of sandstone to masonry biocides

Masonry biocides were made up to the manufacturers recommendations using deionised water for dilutions where indicated. Biocides were added in 1.0 cm3 aliquots to acid-washed glass bijou bottles which contained 0.15 g of selected samples of prepared sandstone. Bottles containing 0.15 g stone and 1.0 cm3 of deionised water were simi larly prepared as controls.

Fresh and decayed samples of both sandstones were prepared by lightly crushing in a pestle and mortar, and the stone particles were repeatedly shaken and washed with deionised water to remove all fine suspended particulates. The resultant stone samples were dried in an oven at 80°C for 1 hour. Bottles containing the stone and biocide/water mixtures were sealed with parafilm and left at room temperature for 12 hours. At the end of each incubation period, the bottles were briefly shaken, the contents allowed to settle, and subsamples of the biocide residues decanted by autopipette and introduced into clean glass bottles. The decanted residues were pipetted in 20 µL aliquots onto the surface of 1 cm diameter standard scanning electron microscope stubs coated with pure carbon EDX tabs to provide a low contamination conductive surface. Each 20 µL aliquot was dried onto the stub surface at 80°C, before application of the next, until a total of 180 µL had been added. Control samples of water/biocide only were similarly prepared onto stubs.

Energy dispersive X-ray analysis of biocide residues

Stubs coated with dri~d biocide resi~ues were analysed by SEM/EDX. An element map of the surface of each stub was provided to determine chemical homogeneity of the sample, and suitably homogenous

707

areas (generally fairly even over the whole surface of the stub) were selected for qualitative EDX (magnification lOOOx, working distance 24 mm and kV at 15). Mineral elements present in biocide or water residues were identified and the spectra of elements obtained from biocides in contact with stone were compared with those obtained from the biocide controls. Following biocide exposure, the stone grains were reserved for SEM examination.

Results and discussion

Energy Dispersive X-ray analysis of biocide residues

The sandstones used in these investigations were not intact, but washed with water several times following crushing in the pestle and mortar. This treatment of the stone was carried out to ensure that all readily exchangeable ions and small particulates, including some of the clay fractions, were removed so as to minimise sources of contamination by mineral components not directly interacting with the biocidal applications. The removal of mineral components which would go readily into solution or suspension prior to biocide application, therefore ensured that ions appearing in solution, following the application of the biocide, were present as a result of stone interaction with the biocide.

Comparisons were made of the spectral analysis of dried deionised water only (Figure 2, i) and dried deionised water/stone residues following contact with stone (Figure 2, ii-iv). Specta were similar between the samples analysed, with K, Si and Al present in trace amounts in all samples. When the water/stone residues (Figure 2, ii-iv) are compared with biocide/stone residues shown in Figures 3, 4 and S, it is evident that the water-stone interactions were minimal for the stone types examined.

The EDX spectrum obtained from the biocide QACB only (quaternary ammonium compound and inorganic borate), and those of QACB/stone following 12 hours contact with the various stone samples, are shown in Figure 3, i-iv. Major elements present are Na and Cl, the origin of which is the biocide itself; the Na ions from the inorganic boron component as disodium octaborate, the Cl ions from the benzalkonium chloride component of the QAC fraction. With the chemical components being in their ionic forms (Figure 1) the formation of NaCl is expected, although whether the possible crystallisation of salt within the stone may lead to damage requires further investigation.

The sandstones are composed of silicate minerals such as quartz, feldspars and clays, the main components of which are Si and Al. Various cations are held at the exchange sites of smectitic and other clays, K, Mg, Ca and Fe are present at these sites. Feldspars also contain K, Na and/or Ca, and Fe is associated with stone components in general. The occurrence of these elements from stone minerals in solution is indicative of weathering or transformation processes. Such processes can be influenced in certain situations as for example, by the introduction of biocidal chemicals to the stone. The detection of elements such as Al and Si in the dried biocidelstone residues would suggest that some disruption of the stone minerals has been brought about by an interaction between the stone and the biocidal components. This appears to be the case with the remaining 2 biocide types tested, the DCPH (dichlorophenol) and the AMINE (alkyl amine) biocides; Figures 4 and S respectively. The elements Al and Si appear in significant amounts in biocidelstone residues of Locharbriggs and fresh Hermitage

sandstone.

The spectra of biocidelstone residues of DCPH applied to Locharbriggs and fresh Hermitage stone shows Si and Al and Ca to be present. After application of DCPH biocide to decayed Hermitage stone for 12 hours, only Na and Cl are present in the dried biocidelstone residue (Figure 4, iv; the scale of the spectrum from DCPH only is double that of spectra ii-iv). It would appear the DCPH action on dec~yed stone is not as influential as that on fresh stone. The biologically decayed stone from Hermitage

708

contains up to 15% dry weight of organic material, together with significant amounts of monohydrocalcite concentrated around the organisms, along with large amounts of extrapolysaccharides secreted by the organisms (Jones et al., 1996). The interactions between the DCPH biocide, the organisms themselves or their products and the stone minerals are not clear, and require further study. However, the high pH of the biocide solution (10.3) would suggest that many silicate minerals might be subject to Si dissolution, as observed in the fresh samples of Locharbriggs and Hennitage.

Figure 5, i-iv shows similarities in the pattern of elements found in dried biocidelstone residues exposed to the AMINE for 12 hours, for Locharbriggs and Hennitage fresh stone. In the spectra K, Fe, Ca and some Mg are present, as well as Al and Si. Identical patterns of mineral elements have also been observed in other sandstone types (Cat Castle and Hennitage white sandstone) for the same biocide (author's unpublished data). These data indicate that the AMINE biocide could have an indiscriminate action on certain minerals common to all the sandstone types investigated. Energy dispersive X-ray analysis of the biocide/decayed stone resiude showed that unlike Locharbriggs and fresh Hermitage

stone, the only element vulnerable to the action of the biocide was Si.

The mechanisms of action of the biocides on the stone minerals has not been fully elucidated in this exercise, however some deductions can be made. The most obvious factor to take into consideration when examining the dissolution of mineral elements is pH. The pH of a solution applied to stone can influence desorption rates of exchangeable ions from their adsorption sites on clays, as well as cause etching and pitting of minerals such as quartz and feldspar. The effect of acidic and alkaline cleaning agents of extreme pH on sandstone has been reported (Webster et al., 1992). The pH of the working solutions of the biocides investigated are shown in Table 1, the pl-1 of the deionised water used in controls was 6.6. In the case of DCPH, a pH of 10.3 could cause some dissolution of Si from silicate containing minerals such as feldspars, clays and quartz, since Si dissolution begins at pH 8 and above. Scanning electron microscopic examination of Locharbriggs stone grains after contact with DCPH did not reveal significant areas of etching of the quartz grains where exposed grains were found (Wakefield and Jones, 1995). This could mean that the source of the Si observed in EDX spectra could be feldspar or clay rather than quartz.

The AMfNE biocide has an effect on the minerals constituting the sandstones examined. Its action however, is unlikely to be a pH related mechanism since the pH of the working solution is 6.1 , not close to the lower pH limit for Si dissolution. Nor is it an extreme value at the acid end of the scale which may lead to dissolution of cations held on exchange sites, such as Fe, K, Mg or Ca, or the dissolution of any calcite minerals such as CaC03.

In previous preliminary studies during the development of the EDX technique for this investigation, SEM of Locharbriggs stone grains following the various biocidal treatments (Wakefield and Jones, 1995), sandstone treated with the AMINE biocide appeared to have lost the clay layers which were observed coating the surface of grains treated with water only. The clay layer was porous and honeycomb like in appearance and composed of illite with some smectite. On exposed grains, some pitting was evident on the feldspar and quartz crystal faces, although it was difficult to attribute this to the effects of the AMINE on the stone for the following reasons; 1) the clay could well have masked the pitting which could have been present prior to the clay removal by the AMINE; 2) the pitting could be a function of earlier erosion of the grains during deposition of the sediment or produced during the sediments diagenetic history.

For fresh Hermitage stone, it was difficult to determine by SEM if the small amounts of clay in this san.dston~ h~d actually been removed by any of the biocides tested. However the EDX analysis of the r~s1~ues md~cates that clay ~as been re~oved under these experimental conditions. Pitting of grains by b1oc1dal action was also difficult to discern because the mineral grains were already significantly

709

weathered prior to deposition by processes of attrition in the depositing media (river water). However, the quartz idiomorphic cement crystal faces did not show any pitting in the samples observed under SEM, implying negligible effect on quartz in fresh stone.

Examination by SEM of Locharbriggs sandstone after treatment with the AMINE biocide showed that clay had been removed from grains. The Al, Si, K and Fe observed in the dried biocide/stone residues is likely to be from clay particles dispersed throughout the liquid, after being removed from the stone mineral grains. Indeed, some pinkish clouding of the biocide residues was evident during the experiment. The mechanisms underlying clay dispersion from the stone minerals could be explained by considering the nature of clay-amine complexes. Such complexes are known to form through the adsorption of alkyl amines onto clay exchange sites by cation exchange. Such studies as those carried out by Lagaly and Weiss ( 1969) of layer charges in clays, established the use of alkyl amines as charge indicators of various smectites, vermiculites and micas. In these and other studies, the orientation of the alkyl ammonium ions between the silicate sheets depends mainly on layer charge and the chain length of the cations. The nature of the interaction between clays and the alkyl amines are shown in Figure 6, where alkyl chains gradually assume positions perpendicular to the silicate layers of the clay, depending upon chain length.

The swelling of clays through the adsorption of alkyl amines could lead to the situation where ionic charges are disrupted to such an extent as to result in break down of the crystalline structures and dispersal of the clay into surrounding solution. It is unclear as yet as to how the lactate and phenol moieties on the alkyl amines of the biocide may influence this interaction, or which clay types are more vulnerable.

Figure 6 Diagram showing the orientation of alkyl amine chains in the interlayer space of clay. The angle of orientation is dependent upon the number of C atoms in the chain and the layer charge of

the silicate sheets. (Lagaly and Weiss, 1969).

Conclusion

More research on the effects of masonry biocides on stone is needed to determine not only how biocides may be made more effective in terms of biocidal longevity on stone, but h~w they may interact with stone minerals to bring about changes to stone integrity. A cause for concern 1s the extreme pH of some

710

biocidal solutions, as a pH of 8 or higher may be expected to cause some dissolution of Si from silicate minerals. Acidic pH may cause loss of cations on exchange sites of clays, or cause etching and dissolution of mineral grains themselves.

Of the three biocides used in this investigation, only two appeared to have any noticeable effect on sandstone, in terms of dissolution of stone components. The action of the DCPH may be related to its pH while the action of the AMINE is perhaps more complex; involving dispersal of clay from the stone, related to the formation of amine-clay complexes.

Energy dispersive X-ray analysis can give additional information on the effects of masonry biocides which are difficult to determine using SEM observations alone. Comparison of the Hermitage sandstone with the Locharbriggs stone for example was difficult because of the low incidence of clay in the former compared to the latter, and the fact that stone grains were naturally weathered prior to the experiment. Finally, decayed stone appeared to be less interactive to the action of both DCPH and the AMINE biocides compared to the fresh Hermitage stone, this could be related to the presence of micro­organisms and their products.

Ongoing work is being carried out into the effects of the biocides investigated on other sandstone types and X-ray diffraction is being used to determine is present in the biocide residues following contact with stone, and whether the AMINE biocide interacts with all clay types, or only expanding clays. Clearly, the continued use of such chemicals on substrates which contain clays should be reviewed. A better understanding of the way such chemicals interact with stone minerals may lead the way to the development of novel or improved biocides which have been designed specifically for use on stone.

Acknowledgements. The authors wish to thank Ian Touch, The Robert Gordon University for assistance with EDX, Dr A. Shirayev for his comments on the biocide structures, Professor M.J Wilson of the Macaualy Land Use Research Institute, Aberdeen for comments on clay interactions, to Historic Scotland for collaborative work on the Hermitage Castle field site and finally, to the EPSRC for funding this research (ref. GR/191500).

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iv)

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Figure 2 EDX spectra of dried residues of deionised water only ( i); water following

J 2 hours contact with Locharbriggs sandstone ( ii); water following 12 hours contact with

Hermitage fresh sandstone ( iii); and water following 12 hours contact with Hermitage

decayed sandstone (iv).

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contact with Locharbriggs sandstone (ii); QACB following 12 hours contact with Hermitage

fresh sandstone (iii); and QACB following 12 hours contact with Hermitage decayed sandstone (iv).

10.2 > ?714 cts

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fresh sandstone (iii); and DCPH following 12 hours contact with Hermitage decayed

sandstone ( iv).

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10.2 > 213 ct.s

715

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