Concrete, Mortar, and Cement as Evidence

Portland cement is made by heating limestone and clay to around 1450 °C to produce clinker nodules, then grinding the clinker with a small amount of gypsum. The clinker is a mixture of four main phases: alite (C3S), belite (C2S), aluminate (C3A), and ferrite (C4AF). When cement is mixed with water, these phases hydrate over hours to days, producing C-S-H gel, portlandite, and ettringite. The proportions of clinker phases vary between cement plants and between production batches, and XRD quantification of unreacted clinker in hardened paste can in principle distinguish cements from different sources.

The aggregate added to make concrete is even more discriminating. Quarries draw on local bedrock, so the mineralogy of the coarse and fine fraction reflects the geological formation that was quarried. Flint gravel from a chalk-plain river terrace differs sharply from granite crush from a Precambrian highland or basalt grit from a volcanic region. Even within a single rock type, trace-element profiles measured by XRF or ICP-MS can distinguish aggregate from one quarry from another producing apparently similar material.

Thin-section petrography is the workhorse of concrete forensic analysis. The sample is impregnated with fluorescent epoxy (to fill pores and reveal the void structure), cut, and ground to 30 micrometres. Under plane-polarised light, the aggregate grains are identified by colour, cleavage, and form. Switching to crossed polars reveals birefringence: quartz shows grey interference colours, feldspar shows cream, calcite blazes bright white, and the C-S-H gel appears dark because it is nearly isotropic.

• Aggregate characterisation: identify rock types and mineral species in both coarse and fine fractions; count proportions; note grain shape (rounded river gravel vs. angular crush).
• Cement paste assessment: estimate the water-to-cement ratio from paste porosity and C-S-H texture; note portlandite content and carbonation depth.
• Void system: measure air-void size distribution (relevant to freeze-thaw durability and, in forensic context, to whether air-entraining admixtures were used).
• Secondary products: identify ettringite recrystallisation, alkali-silica reaction gel, or delayed calcite deposition, which each point to specific environmental histories.

X-ray diffraction of ground concrete or mortar identifies the crystalline phases present: quartz, feldspars, calcite, portlandite, and whatever clay minerals are in the aggregate. Rietveld refinement quantifies the proportions. In forensic comparisons, a sample from a crime scene and a reference sample from a suspect site are run side by side; differences in phase proportions or the presence of an unusual mineral (volcanic ash, blast-furnace slag, fly ash from a specific source) can support or exclude a common origin.

XRF provides major- and trace-element profiles. The major elements (Ca, Si, Al, Fe, Mg, K, Na) give the bulk chemistry of the mix. The trace elements are more discriminating: titanium and phosphorus concentrations in the aggregate, and sulfur and chloride levels in the cement, vary enough between sources that a multi-element fingerprint can separate materials that look identical under the microscope. Portable XRF allows field triage of material at a scene before destructive sampling.

When concrete is exposed to air, CO2 diffuses inward and reacts with portlandite to form calcite. The depth of this carbonation front advances roughly as the square root of time. Spraying a fresh-cut concrete surface with phenolphthalein indicator reveals the front: alkaline un-carbonated paste turns pink; the carbonated outer zone stays colourless. Measuring the colourless depth in millimetres, combined with knowledge of the cement type and the humidity conditions during exposure, gives a rough construction-age estimate.

The method is not a precise clock. Carbonation rate varies with cement content, water-to-cement ratio, CO2 concentration (higher near roads, lower in rural areas), and moisture. But in a case where the question is whether a concrete pour was made within the last five years or thirty years ago, carbonation depth combined with thin-section assessment of portlandite content often gives a defensible bracket.

Concrete fractures along irregular paths that cut through aggregate grains and around them, producing a surface whose geometry is determined by the specific grain arrangement at that location. When a fragment recovered from a suspect's vehicle, clothing, or tool can be placed against the void it left in a wall or floor, the complementary fit is decisive evidence. The fracture path, exposed aggregate faces, surface texture, and matrix colour must all align. Unlike chemical comparisons, fracture matching is a physical re-assembly; courts in multiple jurisdictions have accepted it as highly probative evidence.

Fracture matching requires that both surfaces be documented before any cleaning or handling. Photography at controlled angles under raking light reveals surface texture. Digital photogrammetry or structured-light scanning can produce 3D models that allow remote comparison. Where fragments are too small or degraded for a direct fit, the petrographic and chemical comparison methods described in earlier sections take over as the primary evidence.

Historic masonry mortars used air-lime (calcium hydroxide) or hydraulic lime (which contains reactive silicates and hardens under water). Roman builders added volcanic pozzolana. Gypsum mortars appear in dry climates and in internal plasterwork. Each binder type leaves a diagnostic phase assemblage in XRD and a characteristic petrographic appearance. When masonry from an illegal excavation, a looted archaeological site, or a damaged historic building is involved in a case, the mortar composition can place the material within a specific construction tradition or period.

• Bodies encased in concrete: the concrete can be sourced to a batch or building site, providing investigative leads about where the crime occurred.
• Tool and boot trace: concrete dust on a crowbar or the sole of a boot can be compared to the mix at a burglarised construction site.
• Building collapse investigation: poor aggregate quality, inadequate cement content, or incorrect water-to-cement ratio identified in post-failure core samples becomes central evidence in a structural negligence or manslaughter prosecution.
• Historic building damage: in arson or vandalism cases involving listed buildings, mortar analysis dates the masonry and establishes what was destroyed.