Table 1.

Effect indexes and critical properties of biochar used for 7-day compressive strength measurements, including means and numbers of records

Biochar variables	Effect index (%)		C content (%)		Molar O/C ratio		Specific surface area (m2 g−1)		Si concentration (%)		SiO2 concentration (%)
	Mean	n	Mean	n	Mean	n	Mean	n	Mean	n	Mean	n
Feedstock
Corn	17	35	–	–	–	–	483	24	–	–	4.6	24
Rice	− 4	16	43	11	1.02	7	17	9	5.67	7	66.4	9
Hardwood	− 22	16	66.7	9	0.24	1	60	8	–	–	–	–
Softwood	12	39	76	29	0.17	29	147	9	0.42	29	15.4	3
O.P.	− 2	115	68	72	0.29	50	62	49	0.36	28	23.2	23
Manure	− 26	5	19	5	3.06	5	–	–	0.03	5	–	–
Sludge	− 4	28	47	12	0.89	12	250	5	0.34	6	–	–
Pyrolysis temperature (ºC)
< 350	6	31	60	17	0.33	17	403	9	0.40	16	4.5	6
350–450	− 8	46	56	24	1.19	16	221	15	0.15	6	19.8	12
450–550	5	113	69	71	0.35	59	164	61	1.17	46	22.7	30
> 550	− 5	48	67	17	0.69	11	27	18	0.21	6	34.6	9
Pyrolysis heating rate (ºC min-1)
5–10	6	146	70	85	0.28	79	217	75	0.37	58	13.2	32
> 10	2	29	51	6	0.94	6	118	4	9.70	4	44.2	6
Pyrolysis residence time (min)
< 60	1	122	64	94	0.55	74	84	47	0.95	62	34.1	26
60–180	1	69	65	22	0.37	22	288	45	0.40	5	4.6	24
> 180	− 16	6	–	–	–	–	142	2	–	–	–	–

The Si concentration represents the Si content to the total weight of biochar, with the Si content typically determined through inductively coupled plasma spectroscopy. The SiO₂ concentration represents the SiO₂ content to the total weight of oxides, with the SiO₂ content typically determined through X-ray fluorescence. The term “Effect index” is defined by Eq. (5)

O.P. other plant materials. n numbers of records