Research on the mechanical properties and freeze resistance of geopolymer concrete

To investigate the mechanical properties and freeze resistance of fiber-reinforced geopolymer concrete, this paper uses kaolin, slag powder, and fly ash as the silicate-aluminate cementitious materials, sodium silicate and sodium hydroxide as the alkaline activators, The alkali activator dosage was adjusted to optimize the performance of different silica-alumina cementitious materials. Polypropylene fibers with lengths of 9 mm and 18 mm were blended as reinforcement to prepare geopolymer concrete. strength tests, rapid freeze-thaw tests, and salt freeze resistance tests were carried out. Combined with pore structure analysis and the establishment of a damage model, key conclusions were drawn. In terms of mechanical properties, fly ash is beneficial for the subsequent strength development of concrete, while slag promotes early strength development. Polypropylene fibers can enhance the flexural and compressive strengths. Among them, the P60F20S20M1.2N12 mix ratio has the best performance, with a 28-day flexural strength of 6.5 MPa and a compressive strength of 45.3 MPa. In terms of freeze resistance, concrete’s resistance decreases as porosity and the proportion of harmful pores increase. The incorporation of fibers can inhibit deterioration of the pore structure, reduce the quality loss and relative dynamic elastic modulus caused by freeze-thaw, and increase the freeze resistance cycles of the four mix ratios by 25 times, 25 times, 25 times, and 50 times, respectively. The anti-salt freeze test shows that deicing salt aggravates freeze-thaw damage, while fibers can effectively resist the temperature stress and permeation pressure induced by salt freeze, thereby improving the anti-salt freeze effect. Based on the Lemaitre effect of strain variation, a freeze-damage equation for P60F20S20M1.2N12 fiber geopolymer concrete was established and can be used for freeze-damage evaluation. This research achievement provides an excellent fiber geopolymer concrete mix ratio scheme and theoretical support for pavement repair in seasonally frozen areas and new construction projects.