Inhibiting effect of styrene-butadiene copolymer on the whitening of calcium aluminate cement mortar

Abstract

This paper was based on the results of the research group's previous research on early whitening of mortars. The inhibition of whitening and changes of white deposit on the surface of calcium aluminate cement (CAC) mortar with styrene-butadiene copolymer (SB) were studied using a long-term accelerated whitening method under conditions of (20 ± 2) ℃ and (60 ± 5)% RH. The whitening phenomenon and white deposits of CAC mortars were studied using digital processing of images, optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate that the white deposit on the surface of CAC mortar primarily consists of newly formed carbonation products, calcium carboaluminate hydrate (C4AC¯H11) and calcium carbonate (CaCO3), and with the extension of this exposure time, C4AC¯H11 will be carbonized to generate CaCO3 and Al(OH)3 (AH3). It is interesting to note that SB reduces the production of C4AC¯H11 and CaCO3 and that at SB dosing levels above 12% the C4AC¯H11 disappears and there is hardly whitening of mortar at this time. The inhibition mechanism of SB on the whitening of CAC mortar was investigated using X-ray diffraction (XRD), capillary water absorption and water contact angle tests. The analysis reveals that hydration product of CAC is mainly CAH10 after curing at 20 ℃ for 1 d, and the addition of SB did not change the type of CAC hydration products. Furthermore, incorporating SB enhances the hydrophobicity and reduces the capillary water absorption in CAC mortar, and forms a polymer film in CAC mortar. They prevent the transfer of ions from the interior of CAC mortar to the exterior and the contact of CO2 from the air with the cement hydrate on the surface of CAC mortar, thus reduces the production of white deposit on the mortar surface and inhibits the whitening of CAC mortar.