Investigation into the properties of concrete modified with biomass combustion fly ash

Concrete is the most widely used construction material obtained after the setting of the mix composed of coarse and fine aggregates, cement and water. The main properties of concrete are determined by the quality and characteristics of aggregates, W/C ratio, and the uniformity of mix compaction. Compressive strength is one of the key characteristics of concrete. Materials used: Portland cement CEM I 42.5 R, 0/4 fraction sand, 4/16 fraction gravel, biomass combustion fly ash, superplasticizer, and water. Seven batches of specimens were made with different biomass fly ash content: 0%, 5%, 10%, 15%, 20%, 25%, and 30% (replacing cement in the mix, %). The following properties of modified concretes were tested: compressive strength, water absorption, density, ultrasonic pulse velocity, porosity (open and closed), and predicted resistance to freezing and thawing cycles. Concrete, where 15% of cement is replaced with biomass combustion fly ash, has higher density (2360 kg/m3), compressive strength after 28 days (38.3 MPa), ultrasonic pulse velocity (4466 m/s), lower water absorption rate (3.72%) and higher closed porosity (2.54%). All these characteristics improve the freeze-thaw resistance of concrete. The tests revealed that concrete modified with 15% of biomass fly ash had better durability and freeze-thaw resistance characteristics and can be used in construction works. The obtained linear regression equations illustrate the relationships between the density, compressive strength after 28 days of curing, ultrasonic pulse velocity after 28 days of curing, and predicted freeze-thaw resistance of modified concrete.