Alkali-aggregate reaction in concrete is a long-term deleterious reaction between alkaline pore solution of concrete and amorphous/poorly crystallized silica phase from aggregates. This reaction produces sodium potassium calcium silicate hydrate gels, which swell upon water absorption and cause cracking and deterioration of concrete. Presently, several techniques for mitigating this reaction are available. Majority of which involve the use of chemical admixtures and supplementary cementitious materials to minimize the effect of the named reaction. Preceding studies have already highlighted that addition of minerals rich in pozzolanic properties greatly reduces the alkali-silica reaction (ASR).
Existing reports have also highlighted alternative means of mitigating ASR, unfortunately, the right method for mitigating internal damage consequent to this reaction is yet to be established. For instance, significant differences in alkali uptake between calcium silicate hydrate and silicate hydrate phase containing aluminum phases have been reported.
Recently, Dr. Justyna Zapała-Sławeta from the Faculty of Civil Engineering and Architecture, Kielce University of Technology presented a paper on the inhibitory role of meta-halloysite in ASR and the minimum meta-halloysite content capable of mitigating the ASR distress. In particular, She focused on evaluating the effectiveness of calcined halloysite (meta-halloysite) in improving the resistance to alkali-silica reaction. Her work is currently published in the research journal, Materials and Structures.
Dr. Justyna Zapała-Sławeta started by carrying out mix design where cement, meta-halloysite and reactive aggregate were proportioned. Samples were prepared as per the specifications of design codes and later subjected to mortar test results. The scholar then carried out thermo gravimetric analysis where the calcium hydroxide content was evaluated using TGA on the pastes. Thermal analysis was performed in the nitrogen environment using a specific thermogravimetric analyzer. Lastly, scanning electron microscopy and energy dispersive X-ray spectral analysis were carried out on the samples. The two procedure were performed on polished sections from mortar bars without mineral addition containing a 20% meta-halloysite dosage.
The author observed that satisfactory levels of pozzolanic reactivity were obtained when cement was partially replaced by meta-halloysite. Most importantly, he noted that a 20% addition of meta-halloysite was able to mitigate ASR and lower expansion of mortar bars with reactive aggregate to a safe level of not more than 0.1% at 14 days. Interestingly, compared with the control mortars, the quantity of ASR products in the mortars prepared with meta-halloysite was seen to be smaller. Moreover, the calcium silicate hydrate phase in the mortars containing meta-halloysite showed increased Si/Ca and Al/Ca ratios, which resulted in the improved sodium ion sorption capacity. The reduced content of calcium ions in the ASR gel of the mortars with meta-halloysite confirmed pozzolanic activity of meta-halloysite.
In summary, the study by Dr. Justyna Zapała-Sławeta presented an alternative means of mitigating the ASR reaction by use of meta-halloysite. Microstructural observations of the specimens containing meta-halloysite indicated the presence of a calcium–alkali–silicate– hydrate gel; but fewer reaction products and with different composition than those forming in the pastes without mineral additives were present.
J. Zapała-Sławeta. The effect of meta-halloysite on alkali–aggregate reaction in concrete. Materials and Structures (2017) 50:217Go To Materials and Structures