Recycle sludge incineration ash for efficient preparation of foam concrete: Performance, microstructure, and mechanisms

In this paper, the performance of producing foamed concrete by sludge incineration ash(SIA), blast furnace slag(BS) and Portland cement(PO) was investigated. The mixture design in Minitab software was employed to conduct three-component experimental design for SIA, BS and PO and to optimize and determine the optimal component ratio. The influence of two admixtures on the compressive strength and dry density of foamed concrete at different ages was explored. The hardening mechanism of foamed concrete was analyzed by XRD and SEM. With the amount of reactant as the variable, the compressive strength of foam concrete was the expected response. The results of Minitab software showed that the regression coefficient between the variable and the response was very high. The R-sq value of the 7-day compressive strength was 95.03 %, and the 28-day compressive strength was 96.34 % (where R-sq represents the accuracy of the model data fitting; the closer to 100 %, the higher the fitting accuracy). The difference between the measured values and the fitting value was small, indicating that the fitting model performed well. The optimal mix ratio was 30.87 % SIA, 28.65 % BS and 40.48 % PO. Under this condition, the maximum compressive strength of foam concrete were 3.60 MPa (7d) and 9.53 MPa (28d), with corresponding dry densities of 1235 kg/m³ and 1252 kg/m³ . Sludge incineration ash exhibits pozzolanic activity. The reactive SiO₂ and active Fe₂O₃ contained in it can undergo hydration reactions with cement at room temperature, generating C-S-H gel and insoluble AFt phases. These substances interlocked with each other, forming a dense microstructure that provided early strength to the foam concrete. Considering the effect of admixtures on the performance of foamed concrete, the experimental data demonstrated that the addition of polycarboxylic superplasticizer and sodium sulfate-based early strength agent significantly improved the mechanical properties of foamed concrete.