Gaudí drew the central towers of the Sagrada Familia more than a century ago, but even then he knew they could not be built with the construction techniques of the time. "Gaudí proposed a project for the future," trusting that future generations would be able to provide new technologies to solve the construction challenges posed by his most colossal work, as Jordi Faulí, the basilica's chief architect, likes to recall.
"At that time, Gaudí thought of incorporating floors within the towers to support the structure, but when it came time to start building the six central ones, we wondered if it was possible to eliminate the floors, recovering the structural function of the stone," Faulí recounted to a different audience than usual, including some thirty international journalists from around the world, drawn by the promise of discovering one of the secrets that have made possible the construction of the world's tallest temple.
That secret is imperceptible to the eye, but indispensable for shaping the construction system that would allow the towers to be raised without floors: tensioned stone, which combines stone and steel to create lighter and more resistant structures. But stone and steel are not enough: something must unite them so that the two pieces fuse and behave as a whole. And that is where the secret kept by the temple for ten years is revealed: the one responsible for joining the large stones of the towers has been Loctite. Yes, the Henkel adhesive present in many homes, like the one many use to repair the handle of a broken cup or a worn-out shoe.
But it's not just any Loctite: it's specifically Loctite EA 9497, which Henkel presents as a high-performance adhesive. When the Sagrada Familia approached the German company in 2015, through its headquarters in Catalonia, to explore materials for joining stone to steel for the tower panels, Henkel identified this adhesive as its ideal candidate, but that was not enough. A testing process then began that lasted eight months, as Begoña Cantera, chief engineer of the project from Henkel, recalls.
"The complexity of the project was very high", with trials and tests even from external laboratories, to guarantee that it was the ideal adhesive for the basilica: "The construction with panels has been possible because this adhesive was found that allows stone and steel to work together", highlighted Faulí, in an event alongside the general director of the Sagrada Família Construction Board, Xavier Martínez.
Thus, through tests and trials, not only the use of the adhesive to assemble the panel-based towers was established, but also the process for assembling them: the injection of the stones and the curing process, during which it is kept under stable thermal conditions, were carried out in the quarry, and then the modules were brought to the temple ready to be placed one on top of the other to scale skyward.
"Once cured, the adhesive is inert, and stays there forever, everything becomes one, like a modular structure", pointed out the president of Henkel in Spain and Portugal, Adrian Orbea, who declined to specify economic details of the project. The result of all this? Six towers with 24 tons of Loctite to join their 826 panels and more than 2,100 stone elements. On average, 30 kilos of adhesive have been used per panel.
The system has not only allowed saving part of the structure's weight, but, above all, significantly accelerating the pace of construction of the six central towers: "It has been possible to accelerate because it has not gone floor by floor, but panel by panel. It has reduced construction time by 40 or 50 years", Cantera emphasized. All this, with a constructive solution capable of supporting large loads of magnitudes difficult to imagine, which can be illustrated with examples such as the weight of 100,000 people or 2,400 cars in a single square meter. "Or 1,600 elephants, which illustrate well the robustness of this system", he added.
Now, this system will foreseeably be the one used to erect the towers of the Glory façade, essential to consider the temple finished. "The Pope's visit has been another step towards completion," Martínez highlighted a week after Leo XIV blessed the Tower of Jesus and lit up the Sagrada Familia with a spectacle of lights, drones, and pyrotechnics. "It has had a global impact beyond what we expected," Martínez assured.
Now, the basilica continues its path towards its culmination, seeking current techniques for a dream that Gaudí drew last century thinking about the future. He would not have suspected then that its construction would be linked to the project that businessman Fritz Henkel —born only four years before Gaudí— was just then launching in Germany, like two contemporaries destined to leave a legacy that is hybridized in a basilica now converted into an icon.