I thought this was common knowledge? Even anarchists acknowledge this

Finnbol goes over Makhnos peasant slave labour

https://www.youtube.com/watch?v=ZiSM8SkE4mo

Anarchists argue that they could have done better [in post-revolution USSR]. But, when anarchist activists introduced workplace self-management during the Spanish Civil War, scarcity, military pressures and workers’ indiscipline pushed these activists in the same authoritarian direction as the Bolsheviks. The anarchist Justice Minister, Garcia Oliver, initiated the setting up of ‘concentration camps’ and even the most principled anarchists, the Friends of Durutti, advocated ‘forced labour’.[12]

https://libcom.org/history/revolution-back-agenda-mark-kosman

The Spanish Revolution, like the Russian, also had its labor camps (campos de trabajo), initiated at the end of 1936 by Juan Garc¡a Oliver, the CNT Minister of Justice in the central government of Largo Caballero. As we have noted, Garc¡a Oliver was a very influential fa¡sta and the most important figure in the Central Committee of Antifascist Militias, the de facto government of Catalonia in the first months of the Revolution. In no way could this promoter of Spanish labor camps be considered marginal to the Spanish Left in general and to Spanish anarchosyndicalism in particular. According to his supporters, Garc¡a Oliver had established the principle of equal justice under law that the Spanish bourgeoisie had previously ignored. The work camps were considered an integral part of the “constructive work of the Spanish Revolution,” and many anarchosyndicalists took pride in the “progressive” character of the reforms by the CNT Minister of Justice. The CNT recruited guards for the “concentration camps,” as they were also called, from within its own ranks. Certain militants feared that the CNT’s resignation from the government after May 1937 might delay this “very important project” of labor camps.72

[…]

[…] Understandable resentment against a bourgeoisie, a clergy, and a military whom workers considered unproductive and parasitic crystallized into a demand to reform these groups through productive labor. Anarchosyndicalists endowed work with great moral value; the bourgeoisie, the military, and the clergy were immoral precisely because they did not produce. Thus penal reform meant forcing these classes to labor, to rid them of their sins through work. The Spanish Revolution was, in part, a crusade to convert, by force if necessary, both enemies and friends to the values of work and development.

The ministry of the fa¡sta was proud of its “advanced” ideas and considered its camps more progressive than those in the Soviet Union.74 Garc¡a Oliver promised humanized detention, and CNT representatives investigated complaints of gross negligence, in the L"šrida prison, for example.75 Sometimes, however, the tone of the reformers shifted:

The weeds must be torn out by their roots. There cannot be and must not be pity for the enemies of the people, but . . . their rehabilitation through work and that is precisely what the new ministerial order creating “work camps” seeks. In Spain great irrigation canals, roads, and public works must be built immediately. The trains must be electrified, and all these things should be accomplished by those who conceive of work as a derisive activity or a crime, by those who have never worked. . . . The prisons and penitentiaries will be replaced by beehives of labor, and offenders against the people will have the chance to dignify themselves with tools in hand, and they will see that a pick and a shovel will be much more valuable in the future society than the placid, parasitic life of idleness that had no other aim than to perpetuate the irritating inequality of classes.76

According to a CNT historian, "delinquents, reactionaries, subversives, and suspects were judged by popular tribunals composed of CNT militants and, if found guilty, jailed or condemned to forced labor. Fascists, soldiers who looted, drunkards, criminals, and even syndicalists who abused their power were put behind bars or in work camps where they were forced to build roads."77 Inmates of the work camps reported that they also dug trenches and built railroads. One avid franquista lamented that “duchesses, marchionesses, countesses, wives and daughters of military officers” were forced to harvest grain.78

Most who were sent to prisons and work camps were convicted on political charges-which included violating public order, possessing arms, and engaging in fascist activities.79 A much smaller number received sentences for robbery, murder, hoarding, and black marketeering. This last category increased markedly in 1938 when, for example, revenue guards arrested a mason with 2,200 pesetas or another individual carrying 179 eggs.80 The number of prisoners in Catalonia multiplied fivefold during the war. In November 1936, 535 were in Catalan jails; in November 1938 the figure was 2,601. The greatest increase was of women inmates, whose numbers jumped from 18 in November 1936 to 535 two years later. Deserters from the Republican army (more numerous than those from the Nationalist army) filled their own camps, and their numbers increased dramatically in Catalonia during 1938.81

https://libcom.org/library/workers-against-work-michael-seidman-1