Investigation of the Mechanical Strengths of Concrete with Phosphogypsum from a Fertilizer Plant
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The production of phosphoric acid faces a challenge in disposing of its by-product, phosphogypsum (PG). If not properly managed, the PG and its contaminants can leak into soil, causing environmental damage. As there is a large volume of PG that needs to be disposed of, an effective waste management method could be to use it in mass applications such as concrete construction. PG waste is mostly gypsum and can therefore be valorized as a material for concrete production, as cement requires gypsum to control its setting time. Using PG in concrete, particularly at high volume, could result in lower strength. To mitigate this strength reduction, particularly its long-term strength, rice hull ash (RHA) can be used as a supplementary cementitious material. While concrete is the preeminent construction material globally, the production of cement, its main ingredient, accounts for 8% of global carbon emissions annually. It is therefore vital to reduce cement use while maintaining concrete's inherent advantages. This study investigates the use of PG as a partial cement replacement with RHA and evaluates its mechanical properties. Concrete specimens containing 0-15% PG in increments of five were prepared and cured for 28 days. Additional specimens for 5-15% PG were prepared, but each contained 10% RHA. Curing for the PG-RHA-concrete specimens was set at 28 and 90 days. Results showed that at 5% PG, there was no significant decrease in compressive or splitting tensile strength, whereas flexural strength decreased slightly. The overall strength of PG-concrete decreased markedly when the PG content was increased to 10% and 15%. The addition of 10% RHA did not result in a significant improvement in the overall strength of concrete at 28 days of curing for all PG replacement percentages. However, after 90 days of curing, concrete with 5% PG and 10% RHA exhibited mechanical properties similar to those of standard concrete. The 10% and 15% PG specimens modified with RHA also showed significant improvements in their mechanical properties, but they were still not on par with standard concrete. SEM-EDS testing showed significant voids and ettringite in PG-concrete, while PG-RHA-concrete exhibited fewer voids and more C-S-H gel formation. The overall results indicate that RHA can significantly mitigate the negative effects of PG, provided sufficient curing time is allowed.
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