Phosphogypsum as a construction material precursor

A review of phosphogypsum generation, physicochemical constraints, purification approaches, and deployment routes in cementitious and gypsum-based binders.

The review argues that phosphogypsum, despite impurity and radionuclide constraints, can serve as a viable raw material for binder systems when adequately purified and formulated.

By-product valorization

Purification-dependent usability

Cementitious & gypsum binders

Background and objective

The article situates phosphogypsum (PG) within the context of rising demand for gypsum-based materials and constraints on natural gypsum availability. PG is described as a waste by-product from phosphate rock processing for phosphoric acid production. The review’s objective is to consolidate (i) generation magnitude, (ii) salient physicochemical properties and impurity profiles, (iii) environmental implications of unmanaged storage, and (iv) technically plausible pathways for using PG as a construction-material input.

Material properties and constraints

PG properties are presented as dependent on phosphate ore characteristics and process/disposal variables, including plant operational efficiency, disposal method, and stack/landfill conditions. The material is described as acidic (reported pH < 3) owing to residual acids (phosphoric, sulfuric, and hydrofluoric), with free-water content that varies with drainage time and local weather. Solubility is reported to depend on pH, and PG is noted as highly soluble in saline water environments. The presence of impurities and radioactive constituents is highlighted as a central barrier to broad adoption, primarily through effects on setting behavior and mechanical performance.

Generation intensity and scale

Origin By-product of phosphoric acid production

Reported intensity ~5 tons PG per ton of phosphoric acid

Reported scale ~280 million tons globally (annual); >6 million tons/year in India

Core function: Establishes the waste magnitude motivating valorization into construction materials.

Acidic character and residual acids

Reported pH < 3

Residual species Phosphoric, sulfuric, hydrofluoric acids (as stated)

Implication Influences setting behavior, handling, and compatibility with binder chemistries

Core function: Defines chemical constraints requiring neutralization/purification prior to use.

Composition and impurity elements

Major constituents CaO and sulfate phases (reported as dominant)

Trace/toxic elements (cited as present in untreated PG) As, Ag, Ba, Cd, Cr, Pb, Hg, Se

Implication Environmental risk and performance variability

Core function: Motivates purification and controlled utilization routes.

Environmental burden of unmanaged PG

Primary issue Storage and disposal without treatment

Mechanisms of impact Land occupation; contamination risk to soil/water/air

Core function Frames PG management as both a waste-control and resource opportunity problem.

Purification strategies described

The review characterizes purification as the primary constraint on commercial deployment of PG in construction. Methods summarized include physical separation (washing, wet sieving) and chemical treatments intended to remove or neutralize residual acids and reduce impurity burden. The text further notes combined treatments (e.g., solution-based treatments followed by neutralization using lime, and thermal treatment) as approaches to improve suitability for use as raw material in binder systems.

Utilization in binders and construction products

PG utilization is presented across several binder families: as a mineralizing admixture in Portland cement manufacture, as a setting-time regulator substituting natural gypsum, as a sulfate activator in low-clinker/slag-containing systems, and as a constituent of supersulfated cement-type formulations. For gypsum-based binders, elevated water demand is emphasized as a limitation, with superplasticizer-assisted modification proposed to restore workability and mechanical performance.

PG as mineralizing admixture (Portland cement)

Addition level (reported) 2–3% PG (containing ~1% SO₃)

Reported outcomes ~1% reduction in fuel consumption; improved grindability

Process note Calcination endpoint referenced at ~1400 °C

Core function Positions PG as a process modifier influencing clinker formation and energy demand.

PG as setting-time regulator (gypsum replacement)

Use role Alternative to natural gypsum for setting control

Workability note Workability decreases at ~5% PG (relative to conventional), while cohesiveness improves and bleeding/segregation decrease (as described)

Strength note Compressive strength reported as comparable to conventional mixtures in cited contexts

Core function: Defines PG as a functional set-control input with mix-design sensitivity.

Sulfate activator for low-clinker binders

Role Sulfate activation in slag/low-clinker systems

SO₃ range (reported) ~3–5.5% in referenced gypsum-stone/PG context; performance penalties discussed at higher sulfate content (e.g., >7.5%)

Core function: Summarizes PG’s role in activation chemistry with performance dependence on sulfate level.

Modified gypsum binders and SSC-type systems

Constraint High water demand (reported water/PG ratio ~0.9 vs ~0.6 for natural gypsum)

Mitigation Superplasticizers (e.g., polycarboxylate-based) proposed to reduce water demand and improve applicability

Core function: Frames PG incorporation as feasible with admixture-enabled rheology control.

Technical findings and conditional performance points

PG is described as strongly acidic (reported pH < 3) due to residual acids. This acidity is treated as a primary driver for conditioning (neutralization and/or removal) before use in binder systems, both for environmental safety and to reduce adverse impacts on setting and strength development.

The review identifies impurities (including toxic trace elements) and radioactive constituents as core limitations for unmanaged PG use. These constituents are linked to environmental risk and to deleterious effects on binder kinetics and mechanical performance, motivating purification as a prerequisite for broad deployment.

PG addition (reported 2–3%, with ~1% SO₃) is associated with improved grindability and an approximate 1% reduction in fuel consumption in a cited context. The text also references clinker formation at ~1400 °C and reports changes in lime content with and without PG addition, indicating an effect on clinker chemistry.

Mineralizing admixture effects in clinker production

As a substitute for natural gypsum, PG is presented as capable of setting-time control; however, mix consistency and workability exhibit sensitivity to PG content, with noted workability reduction at ~5% PG and simultaneous improvements in cohesiveness and reduced bleeding/segregation.

Setting-time regulation and fresh-property sensitivity

Portland cement manufacture

PG as mineralizing admixture affecting clinker processing and energy demand.

Setting-time regulation

PG as partial/alternative gypsum source for cement set control.

Low-clinker/slag binders

PG as sulfate activator in blast-furnace-slag-containing systems.

Supersulfated cement-type systems (SSC)

PG as calcium sulfate source combined with slag and activators.

Gypsum binder products

Modified gypsum binders using PG with admixture-enabled water-demand management.

Masonry units and blocks

PG with fly ash/lime systems for building blocks in cited contexts.

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