(118c) Fabrication of High-Dispersed Strawberry-like P(St-co-MAA)/SiO2 Composite Microspheres and Their Applications in Mortar

Silica nanoparticles (SiO₂ NPs) can be used in cementitious materials to improve their workability and mechanical properties. But avoiding the agglomeration of SiO₂ NPs within the cementitious matrix are still the challenges to play the role of them effectively. In this work, the poly(styrene-co-methyl acrylic acid) (P(St-co-MAA)) latex particles with hydrophilic carboxylic chains grafted from the surface were first fabricated via soap-free emulsion polymerization, and then high-dispersed strawberry-like composite microspheres were prepared with the P(St-co-MAA) latex partially covered by SiO₂ NPs through lay-by-layer (LBL) assembly process. The effects of such composite microspheres on the properties of cementitious materials were investigated afterwards.

The surface carboxylic content of P(St-co-MAA) latex particles was measured by conductometric titration. The morphologies of the latex particles and P(St-co-MAA) / SiO₂ composite microspheres were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thermos gravimetric analyzer (TGA) and dynamic light scattering (DLS) were employed to characterize the composition and the dispersibility of the composite microspheres, respectively. After adding the composite microspheres into cementitious matrix, the hydration process of the fresh mixture was monitored by cement hydration heat analyzer. Its workability and mechanical properties were tested via conventional methods. The porosity of the hardened specimens was characterized by mercury injection method.

The results show that the surface carboxylic content of P(St-co-MAA) latex was about 1.5×10^-4 mol/g. For the P(St-co-MAA) / SiO₂ composite microspheres, SiO₂ NPs with the mass ratio about 5.0 wt% were anchored on the latex surface, leaving above 50 % area of the surface uncovered. Such strawberry-like composite microspheres could be easily dispersed in aqueous phase without further treatment where the poly-dispersity index (PDI) was about 0.3. The composite microspheres had no obvious effects on the fluidity of the mortar and the water absorption of the hardened specimen. But it could promote the condensation of the mortar and improve their early compressive strength and flexural strength. Compared to the blank control, the 3d compressive strength of the specimens in which the mass of composite microspheres was 4 wt % of the cement was 5 MPa higher, and the flexural strength of those was 1 MPa higher. The modified mechanism of the composite microspheres for mortar was proposed.