01/09/2018
Sewer manhole, spin-coated with one-part alkali-activated mortar.

Sewer manhole, spin-coated with one-part alkali-activated mortar.

Source: BAM, Division Technology of Construction Materials

Sewer infrastructures are of major importance for any modern society. In such sewers, bacteria can produce up to 4 g of sulfuric acid from 1 g of sulfur, contained in the wastewater. This leads to a decrease of the pH on the surfaces of the sewer pipes and eventually to the biogenic sulfuric acid corrosion of Portland cement-based concretes, which are usually employed to produce the pipes. This has led to a repair need worth about 60,000,000,000 Euro in Germany alone.

Alkali-activated materials, because of their high acid resistance, have the potential to contribute to the solution of this problem. However, possible complications that arise from the need to employ highly alkaline activator solutions, have hampered the application of these materials until now. A recent approach, currently studied at BAM, are one-part formulations in which the activator is present as a solid in the binder, so that only water has to be added to initiate hardening. The mixing process of mortars based on these materials is therefore comparable to the process for conventional, Portland cement-based mortars.

One-part alkali-activated mortars developed at BAM, based on silica-rich feedstocks (microsilica, rice husk ash) and sodium aluminate, conform to the demanding requirements of German standard DIN 19573:2016-03 for mortars for the repair of sewers regarding the resistance against sulfuric acid attack. Microstructural analyses show that the acid attack leads to the dissolution of the products of the hardening reactions in the outer layer of the mortars and subsequent precipitation of acid-resistant silica gel in the acid-mortar interface. The precipitated silica gel imparts mechanical strength to the corroded layer, meaning that the subjacent regions are protected and, thus, the corrosion rate is strongly reduced.

The deduced mechanism is probably effective in conventional alkali-activated materials too, and its elucidation opens up possibilities to tune and improve the engineering properties of alkali-activated mortars. The described results thus point out news ways to a technically feasible, sustainable maintenance, not only of sewer infrastructures.

Sulfuric acid resistance of one-part alkali-activated mortars
P. Sturm, G.J.G. Gluth, C. Jäger, H.J.H. Brouwers, H.-C. Kühne
Cement and Concrete Research, Vol. 109, pp. 54–63, July 2018
BAM Department Safety of Structures, Division Technology of Construction Materials and Department Analytical Chemistry; Reference Materials, Division Structure Analysis