Characterizing porosity in historic bricks is important for interpreting and comparing brick materials, evaluating the degree of deterioration, predicting behavior in future weathering conditions, studying the effectiveness of protective measures, and analyzing the potential effects of cleaning treatments. High-resolution micro-CT coupled with 3D image analysis is a promising new approach for studying porosity and pore systems in bricks. The research presented here has the primary goal of using micro-CT scanning and 3D image analysis to identify significant parameters for characterizing brick porosity. Its purpose is to expand the toolkit available to conservation scientists for investigating porosity variables, given how crucial porosity is for interpreting the history and preservation state of historic bricks.
Porosity refers to the volume fraction of a material that is empty space, or voids; the amount and characteristics of that pore fraction in bricks impact the mechanical performance of brick as a building material. Characterizing a brick’s porosity reveals information about its raw materials and production technology, use properties, and deterioration susceptibility and mechanisms. While many analytical methods have been used to study porosity, no one method has been found adequate, in part because the void space can span an enormous range in scale [1]. Experiments using micro-computed tomography (micro-CT) to study porosity in ceramics, bricks, or other architectural materials have been conducted for more than 15 years. Most often these experiments compared micro-CT to one or more other techniques for measuring porosity [2,3,4,5,6,7,8,9,10,11,12]. A frequent conclusion has been that porosity values can vary considerably depending on the technique used because each technique works in a somewhat different way and is measuring somewhat different characteristics, types of pores, and pore size ranges. Each technique has advantages and disadvantages.
Previous published applications of micro-CT analysis to brick porosity studies
As an example of the many published comparisons of micro-CT to other porosity analysis techniques, Coletti et al. [13] analyzed the porosity of one sample each from four industrial bricks of differing compositions and firing temperatures by using micro-CT with 3D image analysis. They compared the results to three other methods for characterizing porosity, focusing on total porosity and pore size ranges. They concluded that a porosity characterization benefits from using a variety of methods as each has limitations and no one method can completely characterize the entire pore system and its range of pore sizes. Bugani et al. [14] found that it was difficult to compare results between mercury intrusion porosimetry (MIP) and micro-CT for the pore size range that could be characterized by micro-CT (~ 10 µm and above), which they attributed to the fact the two techniques are based on different physical principles. Importantly, while MIP provides quantitative data, it cannot provide visual data regarding the location or distribution of pores.
Rather than performing yet another multi-technique comparison to micro-CT, this work seeks to highlight where micro-CT coupled with 3D image analysis has strengths that may be useful in characterizing the pore fraction of historic bricks. Some of the limitations of this approach are also highlighted, to emphasize where and when it needs to be augmented by other methods.
Importance of characterizing porosity in bricks
The study of porosity in bricks is important because porosity reflects the choices that were made in raw materials and technology, and because porosity is also strongly connected to a variety of physical and mechanical properties that influence the resilience of a brick structure [15]. Porosity refers to the volume of empty space within a material in the form of voids, or pores, along with cracks that have formed. A pore system consists of a group of pores, many with connections (pore throats) to other pores and/or open to the surface [16].
Porosity in brick results primarily from two technological choices. First, there is the choice of raw materials. If there is a significant fraction of non-clay particles present (whether naturally occurring with the raw material or added to improve drying and firing properties by providing more avenues for release of water) porosity can increase since clay tends to shrink away from those particles during drying and firing; this is especially true for large particles [17,18,19,20,21]. Second, production techniques can heavily influence the pore system. Fabrication methods and firing temperatures and atmospheres can influence the size, shape, and orientation of pores [22, 23]. During firing, additional porosity can be created if carbonates are present to dissociate and organics to burn out or char, and as other particles undergo thermal expansion. If firing temperatures are high enough, porosity will then decline as sintering and vitrification of the clay matrix occurs; the ratio of isolated closed pores to open connected pores will also increase [7, 23]. Round, secondary pores can be produced by trapped gases as the clay matrix and silica minerals begin to melt, off-gas, and vitrify. If temperatures become too high, the round pores can become bloated and expand in number, indicating overfiring [3, 24]. As the firing temperature rises, sealed (unconnected) porosity increases; pore size distribution also varies with firing temperature. Cracks can form during cooling if there are varied thermal expansion coefficients of different fabric constituents [22], which is more noticeable in heterogeneous handmade objects with a high percentage of varied additives. These cracks provide another avenue for pressure-induced water entry.
The connected pores provide a passageway for water to enter and move through the brick by capillary action or by pressure; the flow capacity and how freeze-thaw cycles affect bricks are also controlled by the size, shape, and number of pore throats per pore [25]. Very large pores (> a few mm) are sometimes termed “cavities” and may not contribute to capillary action but can provide ingress for water through pressure (such as wind-driven rain). Small pores (< 5 µm) allow water to enter but not escape easily, causing them to be especially affected by freeze-thaw cycles and enlarge over time. For pores larger than this, water can both enter and escape more freely [26]. Most analyses of pore systems have focused on flow capacity; however, isolated pores (unconnected to other pores or to the surface, or “closed pores”) are also important in the study of bricks because they can contribute to other characteristics such as density, mechanical strength, durability, and thermal conductivity [16, 25, 27, 28].
The resulting, often complex, pore system provides an avenue for ingress of water in liquid or vapor form. This is one of the main mechanisms of deterioration through freeze-thaw cyclic damage, salt crystallization damage, transporting of acid and particulate pollution and their reaction products, and disaggregation of the matrix in weakened zones [29]. It has been proposed that it is highly likely that in the future, with more intense and frequent precipitation or flooding events predicted by climate change models, brick structures in some regions will show higher tendencies for moisture-related damage [30].
Porosity in a fired ceramic material such as brick can also be advantageous, so is often deliberate and controlled. Production factors that influence total porosity, pore size distribution, and the type of pores present and their distribution (open and closed, and overall pore morphology) can have a major effect on the physical properties important for bricks such as strength, water absorption, and frost resistance [31, 32]. However, deterioration over time can lead to significant changes in the original pore system, such as an increase in pore size and pore connectivity; pores and throats can become enlarged because of water flow or freeze/thaw cycles and combine with others, and new cracks can form as a result of use and environmental exposure. Understanding the total porosity percentage and the pore structure (size ranges, morphologies, and connectivity) is useful for characterizing the material and comparing it to other similar materials, evaluating the current degree of deterioration, predicting behavior in future weathering conditions, studying the effectiveness of protective treatments, and analyzing the potential effects of cleaning treatments [26, 27, 33, 34].
Micro-CT analysis of bricks
Conventional computed tomography (CT) has been used for decades [35, 36]. In this imaging technique, hundreds or even thousands of X-ray projection images are acquired at 360° of rotation angles around a specimen. X-ray radiation passing through the sample is absorbed, with the radiation weakened to varying degrees depending on the varying densities within the object. The 3D volume is reconstructed by a computer algorithm, producing maps of X-ray attenuation based on the composition/density of the material, with each pixel having an intensity value associated with the component/phase it represents. Depth information is much better than in 2D radiographs, making the specimen interpretation of 3D images obtained by CT more accurate. This method has been used since its inception in the 1970s as, for example, a medical diagnostic tool to image bone structures [36]. However, such medical CT systems have low spatial resolution, typically hundreds of microns, because of the size of the object being investigated (humans) and the large size of the X-ray source focal spot. Newer high-resolution micro-CT instruments have a much smaller focal spot, with much higher resolution for the internal study of materials (typically 50–100 microns or less in resolution with sample sizes below 5 cm) [37, 38]. Nano-CT can achieve as low as 325 nm spatial resolution but requires small sample sizes (generally 0.5–2 mm) [39].
Synchrotron micro-CT systems have been used for decades, with excellent spatial resolution [40]. However, the difficulty of accessing synchrotron facilities limited the range and quantity of research applications. In recent years, more widely accessible laboratory-based (non-synchrotron) systems have greatly improved with advances in detector technology, more accurate rotation stages, and new acquisition geometries. These laboratory-based systems, often affordable benchtop instruments, are now widely available and can achieve good spatial resolution and high-speed scanning [14], leading to micro-CT now being regularly used as a research tool in materials and composites science and in geology for the study of rock fractures, internal microstructure, porosity, and phase quantification, replacing or augmenting petrography [38, 41, 42]. This technique has been applied less often to the study of bricks or other ceramic materials, and only rarely with quantitative 3D image analysis. Yet, emerging work in allied disciplines and in industry shows that this technique can give a tremendous amount of information about ceramic properties and technologies.
Du Plessis et al. [43] noted that a past downside of using micro-CT in porosity studies was that it usually required time-consuming scans and custom software, but current laboratory-based micro-CT scanners have relatively fast scan times with immediate image reconstruction. Powerful and comprehensive 3D image analysis packages now exist for analyzing and interpreting those scans, eliminating the need for in-house programming. Some of the software packages commonly used are VGStudio, Avizo, Mimics, ImageJ/Fiji, and Dragonfly.
Issues of micro-CT sample size, representativeness, variability, and replication
Achievable voxel size is limited by the sample size, with smaller samples allowing for finer spatial resolution. This means there is a tradeoff between being able to adequately image smaller pores and the representativeness of samples from heterogeneous materials [10]. With handmade and historic bricks, even a cursory examination of the brick interior and of thin sections taken from different locations within the brick highlights the fact that these materials are often quite heterogeneous. A 1-mm sample can be analyzed on a nano-CT instrument with spatial resolution of 350 nm to examine some of the smallest pores and their locations but will not be representative of the brick as a whole or of larger pores within the pore network system. A sample of 2.5 cm will be much more representative of pore sizes, but quantitative data will be limited to larger pores (with our instrument, 50 µm or larger). Higher-resolution scans also greatly increase file size and increase computational demands on image processing. Porter and Wildenschid [44], for example, found that doubling resolution from 11.8 to 5.9 µm/voxel increased datafile sizes by a factor of 8, yet it provided few new insights regarding porosity (leading them to settle for 13 µm resolution for most of their work).
Du Plessis et al. [43] used micro-CT to study the porosity of a concrete sample. They selected a range of sample sizes and resolutions (from 200 µm spatial resolution with 100 mm fields of view so that very large pores could be characterized, down to a 5 mm field of view with 10 µm spatial resolution to characterize smaller pores). The same sample was scanned at six resolutions; for the finest resolutions, the sample was cut. The finer resolution/smaller fields of view allow for smaller detectable pores, but very large pores could not be fully characterized because they were too large for the field of view. They also compared slow and fast scans. Fast scans at lower resolution were found useful when the objects of interest were the largest pores. However, fast scans result in more noise, making detection of smaller pores more difficult.
Most papers reporting micro-CT scanning on cultural materials appear to analyze only one sample per brick or other object. Several studies that did include replication have shown that porosity values can vary greatly from spot to spot for rocks with high heterogeneity [9] and can also vary considerably across fired bricks [11]. Given the high heterogeneity of most bricks, especially handmade ones, a single sample is unlikely to be representative of the brick material.