Adhesion characteristics of geopolymer mortar to concrete and rebars

In recent years, geopolymers have gained a wide attention as highly ecological-friendly building materials, having a capability to cut down 70% of CO2 emissions in comparison to the ordinary cement concrete. In this study, geopolymer mortars are proposed as repair materials for reinforced concrete structures, due to their superior acid resistance, heat resistance and high strength in comparison to the existing repair materials. The objective of this study is to investigate the adhesion properties of geopolymer mortars to concrete substrates with different surface treatments, steel plates and rebars. As a result, the geopolymer mortars are found to have excellent adhesion properties to dry concrete substrates, steel plates and rebars. Concrete substrates treated with grinder, further enhanced the adhesion properties of geopolymer mortars. On the other hand, poor adhesion of geopolymer mortars to wet concrete substrates was observed due to the presence of water on the interfacial zone, which decreased the alkali concentration of the geopolymer, resulting in lower adhesion strength. In general, geopolymer mortars are found to have suitable adhesion properties to the concrete substrates, steel plates and rebars and can be applied as repair materials for reinforced concrete structures.

. shows the materials used to prepare GP mortar.
The FA used is the type II ash specified in JIS A 6201-2008. Table 2. shows the chemical composition and physical property of FA, Quartz sand (water absorption rate: 0.28%, particle size : 0.5 mm) was used as fine aggregate material. An alkali activator, 7 M sodium hydroxide solution (NA) and water glass (SiO2: 35 to 38%, Na2O: 17 to 19%, referred to as WG) mixed solution was used. Lignin sulfonic acid and polycarboxylic acid ether type high performance AE water reducing agent were used as admixture. Table 3 shows the materials used for making the concrete substrates. Ordinary Portland cement (referred to as OPC) specified in JIS R 5210-2009 was used for cement. Table 4. shows the properties of cement used. The fine aggregate used was crushed sand from Sukagawa-shi and Shirakawa-shi, Fukushima prefecture (Particle size : less than 5 mm in all), coarse aggregate used was crushed stone from Iwaki-shi, Fukushima prefecture (Particle size : 5 -20 mm). The mixing water used for all mixes was tap water and a high performance polycarboxylic acid ether type AE water reducing agent was used.

Concrete
In this paper, one of the objectives is to examine the adhesion properties of GP mortar using concrete substrate cured for long term. Therefore, concrete cured for two years after casting was used as substrates. The mix proportions of concrete are shown in for two years before using as substrates for adhesion test.

Formwork steel plate and steel plate
Surface treatments of formwork steel plate (SPF) and steel plate (SP) was done in accordance with JIS K 6848-1987.
The surface was cleaned with n-hexane and abraded with No. 240 sand paper as specified in JIS R 6252-2006, and finally cleaned with acetone to remove all the dust particles. and have not been clarified. Therefore, concrete substrates immersed in water for 48h prior to bonding GP mortar and were tested for the influence of water on adhesion.

Concrete substrates
Furthermore, to improve the adhesion of GP mortars to wet concrete substrates, an alkali activator (WG + NA) was coated on the bonding surface of concrete substrates.
The coating amount of alkali activator was controlled by visual observation.    Specimens were prepared by casting GP mortar which was mixed by the above-mentioned method into the molds (150 mm ×150 mm × 150 mm) in which rebars (D19-SD345) were installed at the center, and subjected to heat curing after casting.

Pull-out test of rebars
The tensile load and slip of the rebar at each loading interval was measured using displacement meter in accordance to JCI-SPC15. Three specimens were tested, and the bond stress was calculated from the measured maximum load by using eq. (1).

Adhesion strength test in tension of GP mortar
to concrete and steel plate

Microstructure analysis of adhesion interface
The microstructure analysis of adhesion interface between the concrete substrate and the bonded GP mortar was observed using a field emission scanning electron microscope (FE-SEM).

Conclusions
In this paper, the proposed GP mortar as repair material for RC structures have been tested for adhesion properties to concrete substrates, formwork steel plates, steel plates and rebars and the following conclusion can be derived as below.