Please always post complete documents.
The comment about not being able to use \multicolumn
with tabularx
is wrong as shown in the first table below, but tabularx
is really for adjusting the widths of paragraphs in p
columns. Using it for numeric columns doesn't really produce a good result, it just spaces the table out, making it harder to read, and if you use \centering
the numeric data isn't aligned. I'd use a setting more like the second table where the data aligns on the ,

\documentclass{article}
\usepackage{dcolumn,tabularx,booktabs}
\begin{document}
\begin{table}[htbp]
% \centering does nothing if table is full width
\caption{Add caption}
\begin{tabularx}{\textwidth}{*{3}{>{\centering\arraybackslash}X}}
\toprule
\textbf{Aditivo} & \multicolumn{2}{c}{\textbf{Quantidade (g)}}\\
\cmidrule{2-3}
& \multicolumn{2}{c}{Carga Mineral (\%)} \\
& 20 & 30 \\
\midrule
Fibra & 1,266 & 252 \\
GCC & 0,316 & 52 \\
Amido & 0,016 & 25 \\
ASA & 0,002 & 252 \\
Percol & 0,00032 & 25 \\
\bottomrule
\end{tabularx}
\label{tab_formacao_folhas}%
\end{table}
\begin{table}[htbp]
\centering
\caption{Add caption}
\begin{tabular}{cD{,}{,}{3.7}D{,}{,}{5.1}}
\toprule
\textbf{Aditivo} & \multicolumn{2}{c}{\textbf{Quantidade (g)}}\\
\cmidrule{2-3}
& \multicolumn{2}{c}{Carga Mineral (\%)} \\
& \multicolumn{1}{c}{20} & \multicolumn{1}{c}{30} \\
\midrule
Fibra & 1,266 & 252 \\
GCC & 0,316 & 52 \\
Amido & 0,016 & 25 \\
ASA & 0,002 & 252 \\
Percol & 0,00032 & 25 \\
\bottomrule
\end{tabular}
\label{tab_formacao_folhas2}%
\end{table}
\end{document}
\multicolumn{2}{c}{Quantidade (g}
– David Carlisle May 31 '14 at 23:20