The Effects of Sodium Silicate and Sodium Citrated on the Properties and Structure of Alkali-Activated Foamed Concrete

Abstract

Alkali-activated slag cementitious (AASC) foamed concrete (FC) has presented challenges such as rapid setting time and poor working performance. The use of sodium citrate (Na3Cit) as a retarding agent can improve the workability and microstructure of AASC foamed concrete. The effects of the dosage, modulus of water glass (WG, the main component is Na2O·nSiO2), and retarding agent on the properties and structure of FC were studied in this paper. The results indicated that using a water binder ratio of 0.4, WG with a modulus of 1.2, and an additional amount of 15% and 0.5% of Na3Cit, the prepared FC had a flowability of 190 mm. Its initial and final setting times were 3.7 h and 35.3 h. Its 7 d and 28 d compressive strengths reached 1.1 MPa and 1.5 MPa, respectively. After hardening, the pore walls were dense and consistent in size, with few larger pores and nearly spherical shapes. The addition of Na3Cit resulted in the formation of calcium citrate, which adsorbed onto the slag surface. This hindered the initial dissolution of the slag, reduced the number of hydration products produced, and decreased the early strength. With increasing curing time, the slag in the FC mixture dissolved further. This led to the decomposition of a portion of calcium citrate and the release of Ca2+. The Ca2+ reacted with [Si(OH)4]4− and [Al(OH)4]−, creating more C-(A)-S-H gel. This gel filled the voids in the FC and repaired any defects on the pore walls. Ultimately, this process increased the compressive strength of the FC in the later stages.