Gypsum in Grand Valley Soil, and What It Does to Concrete

Most foundation content treats soil as a mechanical problem: it moves, the house moves with it. In the Grand Valley there is a second, quieter problem that has nothing to do with movement, and it eats the foundation rather than shifting it.

The soil data

The Natural Resources Conservation Service records gypsum content for mapped soil series. Under and around Grand Junction, the figures include roughly 35 percent gypsum in the Fruita series, 33 percent in Gyprockmesa, 16 percent in Gypimack, 10 percent in Killpack, 8 percent in Fruitvale and 6 percent in Persayo. Several of these are classified taxonomically as Argigypsids or Gypsiargids, which are formal designations for gypsum-accumulating soils.

The US Geological Survey separately notes abundant secondary gypsum occurring in shale partings and fractures within Mancos Shale residuum, which is the source. Marine shale deposits sulfate; weathering concentrates it near the surface in an arid climate where there is not enough rainfall to leach it away.

Salinity comes with it. The same soil data records high electrical conductivity in several series, and Homko and Uffens are sodium-affected soils classified as Natrargids, with sodium adsorption ratios high enough to change how water moves through the profile.

What sulfate does to concrete

Gypsum is calcium sulfate. When sulfate in soil or groundwater comes into contact with hardened concrete, it reacts with compounds in the cement paste. The reaction products occupy more volume than the material they replace, so the concrete expands internally. The result is cracking, loss of strength, and progressive softening of the paste until the aggregate is no longer bound.

The visible signs are distinct from movement damage:

• Spalling. Surface layers flaking or scaling off, particularly near grade level.
• Softening. Concrete that can be scratched or crumbled with a screwdriver where it should be hard.
• Exposed rebar. Reinforcement showing through a degraded face, often with corrosion because it has lost its cover.
• Pattern cracking without displacement. A network of cracks in the concrete itself, with no offset between the faces and no accompanying movement elsewhere in the house.

None of that responds to piering, because the house is not moving. The concrete is dissolving.

Where it shows up first

At the stem wall, at grade level, on the side of the house with the most consistent soil moisture. Sulfate attack needs water to transport the sulfate, so it progresses fastest where the ground stays damp: near a downspout discharge, along an irrigated bed, on the shaded north side where evaporation is slower, or where a lateral runs close to the house.

That gives it an unhelpful similarity to freeze-thaw spalling, which occurs in the same places for related reasons. Distinguishing them matters less than people assume, because the remedy for both starts with the same step, which is getting the water away from the concrete.

The standard mitigation

Sulfate-resistant cement. Cement chemistry can be adjusted to reduce the compounds that react with sulfate, and where sulfate exposure is known, specifying a resistant type is normal practice rather than an upgrade.

On repair work this matters for anything newly poured into the ground: footings under new piers, pier caps, replacement stem wall sections, and any concrete backfill. A repair specification here may therefore call out a cement type rather than only a compressive strength, and a homeowner reading a quote in a gypsum-bearing area is entitled to ask which one is being used.

Lower water-to-cement ratios and denser concrete also help, simply because less permeable concrete admits less sulfate.

What a homeowner can do

Keep water away from concrete in contact with the ground. Same drainage work that reduces soil movement also slows sulfate transport. It is the highest-return action available and it addresses both problems at once.

Do not assume a crumbling stem wall is a movement symptom. Check whether the floor is out of level and whether there is diagonal or stair-step cracking elsewhere. Deterioration on its own, with a level floor and no other cracking, points at the concrete rather than at the ground.

Ask what the repair concrete is. If new concrete is going into gypsum-bearing ground, cement type is a legitimate line item.

Do not seal over it and hope. A coating applied to softened concrete traps moisture behind it and accelerates the problem. Deteriorated sections need cutting out and replacing, not covering.

Why this rarely comes up

National foundation content is written for markets where sulfate soils are uncommon and the whole conversation is about movement. Grand Junction sits on weathered marine shale in a climate too dry to leach the sulfate out, which is a combination that produces gypsum content in the tens of percent in some mapped series. It is a local condition, and it is one of the reasons a repair specification written for this valley should not look identical to one written for a wetter state.

The full soils picture is in the Mancos Shale and expansive soil guide. The drainage work that slows it down is in the drainage and grading guide.