The objective of this research was to study the cellulose addition effect on the geopolymerization of heated clay. The clay, composed of illite, plagioclase and kaolinite, was heated at 700°C for 2 h and mixed with cellulose (up to 10 mass%). The mixtures were NaOH-activated, and shaped samples were aged at 83°C for 30 days. The cured samples were investigated by using X-ray diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. The influence of the cellulose addition on the mechanical/physical properties was also evaluated. The results showed that zeolite ZK-14, hydrosodalite, sodium carbonate and a geopolymer composed of poly(sialate) units were formed in all cured samples. The relative amounts of zeolite and metakaolin evolved antagonistically, whereas that of illite slightly decreased with increasing cellulose content. Metakaolin and illite were involved in the geopolymerization process. Cellulose addition led to the improvement of the flexural strength of the samples and to porosity reduction. By contrast, water absorption was increased. The positive effect of cellulose on sample performance is explained on the basis of hydrogen bonding between the functional moieties of cellulose and the active sites of sample constituents, namely zeolites, metakaolin, illite and the geopolymer. As a filler, zeolite probably contributed to sample strengthening, and the detrimental impact of Na-carbonate was insignificant. Based on the results obtained, the composites could be used as binders for brick manufacturing or as lightweight mortars.