This paper examines the behavior of cement paste under constant and changing relative humidity (RH) conditions to evaluate the contribution of cement paste to the dimensional instability of cement-bonded particleboard (CBPB). It was found that the trend of changes in cement paste was very similar to, but the degree of changes was different from, that of CBPB at various exposures. The comparison of the results of cement paste with those of CBPB indicated that the inclusion of wood chips accelerated the carbonation reaction, and that carbonation of the cement paste exerted additional stresses on the wood chips in CBPB; this resulted in a slightly higher increase in mass but an appreciably greater decrease in the dimension of CBPB under constant 20°C/65% RH. The cement paste had considerably lower changes in mass and dimension with a single change in RH between 35 and 90% RH (except for the increase in mass on adsorption at 90% RH) compared to CBPB. The inflection in the relationship between mass and dimensional changes of cement paste was more distinct than that of CBPB with the change of mass per unit length change after the "inflection point" being about eight times higher than that of CBPB on desorption. Under cyclic RH, the response to the level of RH and the history of sorption was different between cement paste and CBPB, with the difference in dimensional change between adsorption and desorption being more significant, while the adsorption at 90% RH for the cement paste was considerably higher. Fitting of models previously developed to the data permitted the prediction of accumulated change of the cement paste with a good degree of fit and established the suitability of using these formulae for modelling CBPB as a composite to be described in a further paper in this series
