Deterioration and mitigation of efflorescence of alkali-activated phosphorus slag

Abstract

To achieve the efficient application of alkali-activated phosphorus slag (AAPS), this study revealed the efflorescence mechanisms and found a feasible way to mitigate the efflorescence deterioration. The efflorescence formations were mainly constituted of Na2CO3·H2O, Na3H(CO3)2·2H2O and NaHCO3. The order of the efflorescence ability of Na species was Naf (free Na in unreacted alkali activator) > Naw (weakly bonded Na in calcium-sodium aluminosilicate hydrate (CNASH)) > Nas (strongly bonded Na in CNASH). CNASH with lower Na/Si ratios and higher polymerization degrees was harder to form efflorescence. The accelerated efflorescence not only caused the dealkalization, dealumination and polymerization shrinkage of CNASH, but also deteriorated the homogeneity of CNASH distribution in pastes, due to the uneven carbonization of the upper and lower parts of pastes. The growth of efflorescence formation and the polymerization shrinkage of CNASH caused the upper parts of pastes to crack, which deteriorated the integrity of pastes. The accelerated efflorescence deteriorated the homogeneity and integrity of pastes. An effective strategy to mitigate the efflorescence and crack of pastes resulted from the addition of aggregates, which assisted in the development of compressive strength. AAPS mortars with low Na2O doses or moderate silicate moduli had better resistance to efflorescence.