Sulphate-bearing ground and concrete
Sulphate-bearing ground can attack ordinary Portland cement concrete if groundwater and soil chemistry are aggressive enough. UK ground investigation often includes water-soluble sulphate and related tests so the structural engineer and concrete specifier can select an appropriate concrete quality for buried foundations — commonly discussed with reference to BRE Special Digest guidance and BS 8500 concrete practice, applied by the designer to site results.
This page explains why sulphate appears on GI reports and what it means for foundation concrete on site. It does not assign ACEC or design chemical class numbers to your plot — that is a design output. Contaminated-ground investigation covers wider brownfield chemistry. Flagship ground-conditions-risk-explained frames residual ground risk.
Aggressive ground classification can vary with groundwater mobility — a “dry” trench on the day of pour does not prove low long-term exposure. The specification already accounts for the assessed environment.
review Groundworks Contractors in Wandsworth and Groundworks Contractors in Kingston.
When this ground condition becomes a design issue
When GI reports elevated sulphates or aggressive ground conditions for concrete; when specifications call for sulphate-resisting or otherwise classified concrete in foundations; when brownfield fills or gypsum-bearing strata are present; or when Building Control queries concrete designation for buried elements.
It also arises when groundwater is mobile, increasing exposure of concrete to dissolved sulphates.
Some fills contain gypsum demolition waste that elevates sulphate risk even when natural geology is benign. Brownfield desk studies should inform the chemical testing suite.
What the ground is doing
The designer selects concrete designation, cover, and sometimes protective measures from the chemical assessment. Groundworks must order and place the specified mix — substituting a generic “structural mix” is a compliance failure.
Keep tickets and delivery records. Do not add water on site in ways that breach the specification. Where pyritic or other specialist chemistries are flagged, follow the engineer’s additional notes — these are not DIY chemistry problems.
Mainline places concrete to the issued specification. We do not reinterpret laboratory sulphate results into a mix design.
Precast piles and buried concrete products must also meet the chemical design class where specified — not only in-situ strip concrete. Keep curing water and site-mixed mortars from undermining the specification approach. If GI chemistry varies across the plot, do not assume the mildest result applies everywhere.
Keep reinforcement free of mud and standing sulphate-bearing water before concrete where the specification requires clean conditions — washdown water must not create a worse chemical environment in the dig.
Think you might have sulphate-bearing ground and concrete? A professional inspection will confirm the diagnosis.
Get Project QuoteSite and investigation signals
- Sulphate or aggressive ground notes in GI
- Concrete designation for sulphate resistance on drawings/spec
- Brownfield or gypsum-bearing geology flagged
- Mobile groundwater increasing exposure risk
- BC / warranty checks on foundation concrete quality
How foundation strategy usually responds
Supply and place the correct concrete, maintain cover and curing as specified, and file tickets for the QA pack. Escalate if the wrong mix arrives before placing.
Reject loads that arrive with the wrong designation before discharge into the pour; remediation of non-compliant buried concrete is disproportionately expensive.
Investigation and design checklist
- GI sulphate / chemical assessment summary
- Structural concrete specification for foundations
- Supplier confirmation of mix designation
- Cover and curing requirements
- Ticket filing process on site
Risk and response summary
| Role | Responsibility |
|---|---|
| GI laboratory testing | Report sulphate / related chemistry |
| Designer / specifier | Select concrete designation and details |
| Groundworks contractor | Order, place, and record the specified concrete |
Programme and cost implications
Special designations and any protective membranes add material cost; ripping out non-compliant concrete is far worse. Rate intent: /groundworks-costs — no invented sulphate mix premiums listed here.
From investigation to construction
Step 1: Confirm concrete designation
What this step delivers: Spec matches GI chemistry advice.
Step 2: Order correct mix
What this step delivers: Supplier briefed on designation.
Step 3: Place and protect
What this step delivers: Cover, compaction, curing as specified.
Step 4: Retain tickets
What this step delivers: Evidence for inspections and warranty.
Where this condition is commonly encountered
- Foundations in sulphate-bearing clays or fills
- Brownfield plots with aggressive groundwater
- Specs calling for designated foundation concrete
- Warranty reviews of buried concrete quality
Frequently asked questions
Can I use ordinary concrete if the trench looks dry?
No. Follow the specification derived from GI chemistry. Dry weather at pour does not remove aggressive ground classification.
Is sulphate the same as contamination?
Sulphate aggression is a concrete durability issue. Broader contamination (human health, controlled waters) is a separate assessment — see contaminated-ground-investigation.
Where does wider ground risk sit?
See /guides/ground-conditions-risk-explained and foundation-design-strategy.
Does sulphate-resisting cement fix all aggressive ground?
Not always. Designations consider sulphate and related aggressivity together. Follow the specified concrete quality — do not substitute based on cement type name alone.
How This Issue Is Normally Diagnosed and Repaired
Follow the typical path from problem identification through to resolution:
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Related Topics
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Get Project QuoteSulphate-bearing ground and concrete FAQ
Can I use ordinary concrete if the trench looks dry?
No. Follow the specification derived from GI chemistry. Dry weather at pour does not remove aggressive ground classification.
Is sulphate the same as contamination?
Sulphate aggression is a concrete durability issue. Broader contamination (human health, controlled waters) is a separate assessment — see contaminated-ground-investigation.
Where does wider ground risk sit?
See /guides/ground-conditions-risk-explained and foundation-design-strategy.
Does sulphate-resisting cement fix all aggressive ground?
Not always. Designations consider sulphate and related aggressivity together. Follow the specified concrete quality — do not substitute based on cement type name alone.
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