H. Cardenas, D. Sumy, H. Zhong
Louisiana Tech University,
Keywords: nanoparticle, pozzolan, transport, stability, strength, porosity
Summary:Electrokinetic pozzolanic nanoparticle treatments have been used to achieve rapid porosity reductions and deeply penetrating strength enhancement of cement and concrete. Particle cost and instability have tended to limit the feasibility of this approach despite the radical porosity reductions and strength enhancements that had been achieved. The high electric fields required to achieve these results have tended to be accompanied by particle suspension instability. Coagulation can limit the efficiency and effectiveness of a treatment by the loss of these costly particles from suspension. The current study examined the use of silica fume as a low cost particle alternative and the role of electro-coagulation in impacting the outcomes and efficiencies of an electrokinetic treatment. This work characterized the particle size distribution of silica fume (SF) and the behavior of a liquid suspension of SF that could support electrokinetic treatment. As a benchmark, these electrokinetic treatment outcomes were compared with similar treatments using the Nalco 1056 alumina-coated silica (ACS) colloid which consists of uniform, 24-nm particles. Key treatment parameters (pH, voltage and time) as well as the feedback parameters (including suspension stability, clarity changes and pH changes) were obtained from preliminary testing that enabled identification of threshold electric field strength levels that helped avoid electrocoagulation instability. The ACS stability threshold was found to be 40 V/m. The SF stability threshold was found to be x5 this level with a value 200 V/m. Scanning Electron Microscopy (SEM) revealed that the SF particles that remained stable for electrokinetic treatment were less than 800 nm in size. In a treatment period of 7 days, the 200 V/m threshold electric field value provided significant delivery of SF particles. The impacts included a 19% increase in tensile strength and a reduction of porosity from 24% to 18.6%. The benchmark treatment utilizing the ACS nanoparticle at 20 V/m provided a 25% tensile strength increase and a reduction of porosity from 24% to 18.5%. During these treatments, the current drop was monitored as an indicator of the effective delivery of particles into the HCP pores. A trend was observed that predicts a minimum current drop of 19% that apparently corresponds to when a given treatment stops having an effective impact on HCP properties. This constituted what may be the first measurement of such a current drop threshold. It compared well with anecdotal findings from the literature. The maximum threshold particle size that was able to enter the pores of the HCP (w/c = 0.5, Type I Ordinary Portland cement) was in the range of 75 – 100 nm. The cost benefit of utilizing suspended silica fume as compared to the ACS colloid was found to be a factor of 24. Consistently reaching this level of benefit will depend upon the availability of low-cost centrifugation that can provide particle size distributions that fit well within the pore size distribution of the HCP. This study found that electro-coagulation was influenced by the electric field strength, through a combination of particle crowding at pore entrances and pH shifts as driven by electrolysis.