Commercial lime based grouts: Are they all the same?
Lime-based grouting is a well-known consolidation technique for ancient masonry. It is efficient because it improves the mechanical characteristics of stone, brick and even, earth masonry walls, while preserving the historical character of the existing materials. This technique recovers the continuity of the existing structure, providing a more homogeneous material and increasing the cohesion and strength of the damaged structural elements, with minimal changes in their morphology and in the load-bearing system. The grout is introduced manually or with some pressure, into the internal and non-visible part of the masonry and, because of this, it does not damage the aesthetical outlook of the wall.
How to choose the right grout?
But it is difficult to choose the right grout for each intervention. Some investigators said that a mixture of hydrated lime and pozzolans with a few percentage of cement results in a grout with a good performance in terms of mechanical, chemical and rheological properties. Alternatively, hydraulic grouts with natural hydraulic lime have been proposed; moreover, the effect of the addition of other materials like fly ashes, silica fume, bentonite, plasticizers and superplasticizers among others have been recently studied. Despite the fact that several formulations are proposed, many commercial ready-mix grouts are available in the market and have been either frequently prescribed by designers or proposed by specialized companies in the area, mostly because of their easy preparation, quality control and guaranteed performance.
The preparation of these premixed grouts requires only water and no special equipment, so using commercial grouts is, in fact, very easy and the injection process becomes very attractive.
Are the commercial lime based grouts all the same?
They show no significant differences, so, is it indifferent to choose one brand from another? We don´t think so!
The composition of commercial grouts is varied and the description of their composition in technical data sheets is vague. Technical information is usually scarce and it remains unclear which procedures were done for quality control and which requirements are applicable.
Four commercially available grouts were tested in laboratory, to compare their performance, providing a range of properties found and alerting for the adequate selection of the injection materials. The tests demonstrated a good stability with low bleeding and segregation for all grouts, but they presented different characteristics, both in terms of other measurable properties using selected tests, as well as in terms of tone, texture, workability and quantity of necessary water.
It is possible to have products with a greyish colour and texture with small dark grains, and products with a very smooth texture and a light beige colour. Alternatively, a very white product with a powdery texture was also found. The workability and fluidity of these grouts is also quite different, and this aspect makes the injection process, even under pressure, quite variable. Regarding the mechanical properties of the four commercially available grouts, testing revealed some similarities in two of them, in axial compression tests, with similar values between 31-32 MPa, at 360days of age. But for the other grouts, values of 4MPa and 16MPa were obtained, also in compression tests, at 360 days of age.
But the main property affecting the behaviour of grouted walls is the shear bond strength of the support (masonry units as stones or bricks) and the grout interface. The bond mechanism in stone-to-grout interfaces was characterized using pull-off tests. It is curious that significant differences were obtained between the products evaluated, both in terms of stone type and humidity conditions.
The best commercial grout found revealed generally good results with an average tensile bond strength of 0.8MPa at 28 days of age for three types of stone, but the other grout, with very similar mechanical characteristics in compression tests, revealed weak tensile bond strength values. This result confirms that a higher compression strength does not necessarily mean also higher values of bond strength capacity.
Now we know that the performance of the commercial products is rather different and careful selection of injection materials in practical applications is recommended.