Efficacy of Aluminum Hydroxides as Inhibitors of Alkali-Silica Reactions ()
1. Introduction
As it is well known, the use of highly active mineral additives—fly ash, silica fume, metakaolin—is the most effective way to suppress the alkali-silica reactions applied in practice [1-7]. High efficiency of active mineral additives as inhibitors is due to their pozzolanic properties (an ability to bind Ca(OH)2 into C-S-H) and the ability to bind alkaline compounds. Several studies have showed a high inhibitory activity of aluminosilicate additive compared to their siliceous analogs (e.g., metakaolin over silica fume superiority) [7,8]. It is assumed that the incorporation of aluminum into the Si-O chains of C-S-H promotes binding of alkali ions from pore solution [4,9]. According to other point of view, aluminum adsorbs on particles of reactive aggregates to form inactive aluminosilicate complexes, which guarantee an advanced inhibitory effect [10].
At the same time, some of the aluminum compounds are used as alkali-free accelerators for mortars and concretes [11,12]. One component of the accelerating admixtures of this type is highly dispersed amorphous modifications of aluminum hydroxides [13]. In the presence of aluminum hydroxides, the decrease in setting time of cement paste is due to rapid formation of ettringite with participation of Ca(OH)2 and gypsum [13,14].
The effect of highly dispersed amorphous aluminum hydroxides on the hydration of Portland cement was studied in [15]. It was established that the small dosages (1% of the cement weight) of amorphous aluminum hydroxides have no adverse effect on the cement hardening in the early period (1 day), whereas, on the contrary, they cause somewhat increase in the 1 day strength as well as in the following period of the hydration. With dosage of amorphous aluminum hydroxides 3% - 6%, 1 day strength is mainly decreased several fold in comparison with the reference sample. The strength of the cement paste in later age decreases too, although differences become lesser with increasing age; in this case, the higher the dose, the greater the loss of the strength by the stone.
An ability of amorphous aluminum hydroxides to bind intensively free Ca(OH)2 allows the idea that these substances could be effective in inhibiting alkali-silica reaction. This idea is based on the role of calcium in the alkali expansion processes [16]. Alkali silica gel itself, not containing calcium, has high mobility and may be therefore easily and rapidly removed from the formation zone. The presence of calcium, which forms bridging bonds between single silicate ions, makes gel immobile. As a result, the gel accumulates in the formation zones with the appearance of dangerous inner stresses. As known, the increase in the Ca(OH)2 content in the cement compositions containing a reactive filler which facilitates alkali expansion [17,18]. Therefore, the ability of active mineral additives to bind Ca(OH)2 is considered by some researchers as one of the main reasons responsible for their inhibition effect [19]. So, it would be interesting to investigate the efficiency of aluminum hydroxides, and further—of other aluminum compounds as inhibitors of alkali silicate reactions and alkali corrosion of Portland cement compositions.
An objective of this work is to carry out a study of amorphous aluminum hydroxides as inhibitors of alkali expansion of Portland cement mortars and concretes. To find out more clearly the action of amorphous Al(OH)3, crystalline forms of Al(OH)3 were also investigated; moreover, different forms of Al(OH)3 were compared in sense of their activity to bind CaO from saturated Ca(OH)2 solution.
The relevance of this study is due to the prospect of using multi-functional additives that would give an optimal solution to solve several tasks in concrete technology.
2. Experimental Part
2.1. Materials
As the research subjects, the following types of comercially available aluminum hydroxides were used: amorphous Al(OH)3 Geloxal (Industrias Químicas del Ebro, Spain), amorphous Al(OH)3 SiTau (P & J Cretechem (P) Ltd, India), the crystalline Al(OH)3 (hydrargillite) GD-18 (“BaselCementPikalyovo”, Russia). Properties of aluminum hydroxides are shown in Table 1. Aluminum hydroxide GD18 additionally grinded in vibro-grinder was also tested in experiments.
Portland cement CEM 1 42.5 N was used. Phase composition according to petrographic analysis is, wt%: alite 52-53, belite 18-20, intermediate phase 20-22, gypsum (CaSO4∙2H2O) 3-4, anhydrite 1, CaCO3 2.
As aggregate, a quartz-feldspar sand of the following fractional composition, wt%, was used: 1.25 - 2.5 mm— 5.27, 0.63 - 1.25 mm—27.5; 0.315 - 0.63 mm—27.5; 0.16 - 0.315 mm—17.5. In the initial aggregate, the content of SiO2 dissolvable in NaOH and determined by the method described in GOST 8269.0-97 specification is equal to 0, and aggregate is not reactive to alkali environment. Therefore, sand was previously ignited for 4 hrs at 1080˚C followed by a rapid cooling to ambient temperature. After this procedure, soluble SiO2 content has reached 80 mmol/l.
2.2. Testing Methods
Pozzolanic activity of aluminum hydroxides was determined by absorption of CaO from saturated solution of Ca(OH)2 [20].
The alkali-silica expansion of cement-sand mortars with the addition of aluminum hydroxides and control samples (without additives) was investigated by the accelerated test in accordance with GOST 8269.0 specification (analog to mortar-bar test method ASTM C 1260).
A reference mortar mix were prepared by mixing sand with cement at a ratio of 2.25:1 (by weight), water-tosolid ratio was 0.125. Mixtures with Al(OH)3 additives in amount of 1 and 3 wt% of cement weight were similarly prepared. Dry blends were mixed with water at the same water-to-solid ratio of 0.125. In case of amorphous Al(OH)3, a plasticizing agent, Melflux 2651, was used (0.05% by the cement weight).
Mortar mixes were put into molds (20 × 20 × 100) mm. In accordance with the procedure after 1 day storage at 100% RH and 20˚C, samples were demolded and put in water at 80˚C for one day. Samples were then cooled in a sealed box to 20˚C and samples’ lengths were measured. During the test period, samples were being stored in 1 M NaOH at 80˚C, daily measurements of samples were performed (total test duration was 2 weeks).
3. Results and Discussion
Results of investigation of the binding of CaO from saturated Ca(OH)2 solution are shown in Figure 1.
Figure 1 shows that the highest absorption of CaO is