Choosing the rock set the ceiling: how much CO₂ a tonne of feedstock could ever capture. This module is about the journey. Each tonne has to be dug or collected, ground, stored, trucked, spread and worked into the soil. None of that is free in carbon terms, and the removal itself arrives slowly, over years, as the rock weathers by the chemistry in The chemistry of enhanced weathering.

You saw in Weathering, erosion & soil that halving a grain's size doubles its surface area, and weathering happens at surfaces. So a fine powder reacts much faster than coarse grit. The catch is energy. Breaking rock means breaking chemical bonds and overcoming friction in the mill, and every further halving of grain size takes more energy than the last.

The process of crushing and grinding is called comminution. A powder is never one size, so mill operators describe it by its d80: the size that 80% of the powder (by weight) passes through. Engineers size mills with an empirical rule called Bond's law: the energy needed per tonne rises roughly with one over the square root of the final particle size. Go ten times finer and the energy goes up by roughly three times (√10 ≈ 3.2). For a fairly hard rock fed in at about 10 mm, that means roughly 4 kilowatt-hours (kWh) per tonne to reach 1 mm, about 18 kWh to reach 100 µm and about 60 kWh to reach 10 µm. Below a few tens of micrometres, real mills become less efficient still, so the cost rises even faster than the rule suggests.

This is why quarry fines are so attractive. Quarries crushing basalt for road stone and concrete aggregate already produce dust-sized material as a by-product, often in stockpiles they struggle to sell. The energy to make it has been spent for another purpose, so little or no extra grinding is needed. The trade-off is that you take the grain size you are given, which is usually coarser and more variable than a purpose-ground powder.

Rock is heavy and cheap, so moving it is often the biggest cost and a large share of the emissions. Freight emissions are counted per tonne-kilometre: one tonne moved one kilometre. As rough round numbers, road freight emits about 0.06–0.12 kg of CO₂ per tonne-kilometre depending on the truck and how full it is; rail and ships emit several times less.

Put that next to the removal. If a tonne of basalt removes, say, 90 kg of CO₂ within the period you can measure and claim (more on that below), and grinding and spreading cost 16 kg, then 74 kg is left to "spend" on transport. At 0.1 kg per tonne-kilometre, a truck uses it all up in about 740 km. Beyond that, the project emits more than it removes. And that assumes the truck's emissions cover only the loaded trip: if it drives back empty, the real figure per tonne is higher.

Trials typically apply tens of tonnes of rock per hectare (a hectare is 100 m × 100 m, about two and a half acres). That is far more than a fertiliser dose, and closer to how farmers apply lime or manure, so the same kinds of spreaders are often used. Most Indian farms are small, so on many plots it is spread by hand or with small equipment.

After spreading, the rock can be incorporated, mixed into the topsoil by ploughing or other tillage, or left on the surface. Mixing puts grains in contact with moist, acidic soil water and roots, which can speed weathering, and it keeps dust and runoff down. Surface application suits fields that are not tilled, but grains may stay drier and wash or blow away more easily. Either way, it is cheapest to fit spreading into work the farmer is already doing.

Timing follows the crop calendar. Much of India farms two main seasons: kharif crops such as rice are sown with the monsoon rains in about June–July and harvested in autumn, and rabi crops such as wheat are sown in about October–November and harvested in spring. The windows between them, when fields are being prepared, are when spreading disrupts least.

The rate controls from Weathering, erosion & soil apply directly. Weathering is fastest where soils are warm (reaction rates roughly double for every 10 °C), wet (water must flow through to carry the products away), and acidic (more hydrogen ions to attack mineral surfaces), with plenty of roots and microbes adding CO₂ to the soil air. Tropical and subtropical farmland, including much of India, scores well on warmth and, in the monsoon, on water. Dry soils and already alkaline soils slow the reaction down. Calcareous soils (rich in solid calcium carbonate) may offer less removal, because added calcium tends to re-precipitate as carbonate, which stores half as much. Many of the black soils on India's Deccan basalts are neutral to alkaline and calcareous, so there soil inorganic carbon has to be measured as part of MRV.

Water matters in a second way. If too little water passes through the soil, it fills up with dissolved products and gets close to saturation, the point where it cannot hold any more; dissolution then slows almost to a stop until fresh rain flushes it out. That is one reason why piling on more rock per hectare does not raise removal in proportion: the soil water can only carry away so much each year. The climate setting in the calculator below uses these levers.

A life cycle assessment (LCA) adds up the emissions from every step of a product's life, so they can be subtracted from the benefit. For enhanced weathering, the core sum is simple:

Start with the CO₂ drawn down by the rock that actually weathers: the theoretical ceiling multiplied by the fraction that dissolves within the crediting window, the period over which removal is measured and claimed. Because rock weathers over years to decades, that fraction is a scenario, not a constant: for example 20–30% over the first few years, and highly uncertain.

Dissolved is not the same as removed. Some of the calcium and magnesium is held on clay surfaces or taken up by crops instead of travelling on as bicarbonate; some rock is dissolved by strong acids from fertiliser, which removes no CO₂; and roughly 10–20% of the bicarbonate's carbon goes back to the air when it reaches the ocean (the chain is traced in The chemistry of enhanced weathering). These losses are subtracted next. Finally the emissions, and those happen up front, on the day the rock is ground and trucked. So every project starts with a small carbon debt that weathering then has to pay back.

A full LCA goes further than this calculator. It asks whether quarrying emissions should be charged to a by-product at all, includes the trucks and machinery themselves, counts storage and any repeated handling, and considers changes in soil emissions or fertiliser and lime use. Different methods give different answers, which is why standards for carbon credits spell out exactly what must be counted.